BACKGROUND [0001] In some print apparatuses, printing fluid such as ink is selectively discharged from a printhead, positioned in a carriage of the print apparatus, toward an advancing substrate.

BRIEF DESCRIPTION OF DRAWINGS [0002] Examples will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

[0003] Figure 1 is a simplified schematic of an example print apparatus;

[0004] Figure 2 is a simplified schematic underside view of an example print apparatus; [0005] Figure 3 is a simplified schematic side view of the print apparatus of

Figure 2;

[0006] Figure 4 is a flowchart of an example method;

[0007] Figure 5 is a flowchart of an example method;

[0008] Figure 6 is a simplified schematic of an example machine-readable medium in association with a processor; and

[0009] Figure 7 is a simplified schematic graph of target flight times for different fluids.

DETAILED DESCRIPTION [0010] In some example print apparatuses, printheads discharge printing fluid

(for example, an ink such as a latex ink) toward a substrate to print an image on the substrate as the substrate advances underneath the printhead. In these examples a printhead may be retained by a carriage which may be movable from side to side in a direction perpendicular to the direction of movement of the substrate. Such a carriage may be to retain a number of printheads. Each printhead may comprise a fluidic die (for example, a printhead die) which may comprise a nozzle array. For example, some fluidic dies may comprise an array comprising two rows of nozzles per fluid that the fluidic die is to discharge (e.g. one example die to discharge two fluids may comprise two rows of nozzles per fluid, although other examples dies may be to discharge four fluids or even one fluid). For example, each nozzle array may respectively be to discharge black, cyan, magenta and yellow printing fluid. Each nozzle array may therefore comprise two rows of nozzles and a fluidic die may comprise a nozzle array or a plurality of nozzle arrays.

[0011] Each nozzle array may comprise a number of individual nozzles, each nozzle being to discharge printing fluid. The individual nozzles may be arranged in rows as described above and may be grouped in rows of two. Each individual nozzle may be associated with a resistor (or firing resistor). In use, according to print data describing an image to be printed on the substrate, a carriage (e.g. retaining the printhead) may be caused to move (e.g. under the control of a controller or processor) and the individual nozzles may be caused to selectively discharge (e.g. under the control of a controller or processor) so that the image according to the print data is printed to the substrate. This may involve the one-at-a-time ejection of individual printing fluid droplets from the individual nozzles at the correct weight, speed and direction to place a correctly-sized fluid droplet at the correct location on the substrate so that the image is correctly generated according to the print data describing the image. The printhead may comprise a drop generator for each nozzle, which may comprise the resistor. Each drop generator may comprise a chamber with a refill channel and the nozzle. To eject a droplet of printing fluid, an electrical signal, such as an electrical pulse, may heat the resistor (for example an electrical current may be caused to flow through the resistor) to cause printing fluid in the chamber to vaporise to form a bubble. This bubble expands to propel a droplet of printing fluid out of the nozzle and, in this way, the electrical pulse through the resistor causes the selective discharge of printing fluid from the nozzles in a nozzle array of a fluidic die of a printhead. Ceasing the electrical pulse causes the bubble to collapse and the pressure differential from this collapse may cause fresh printing fluid to be drawn into the chamber through the refill channel.

[0012] From time to time, individual nozzles may experience firing difficulties which, if not correctly identified or compensated for, can lead to an appreciable decline in the print quality. Some example print apparatuses therefore periodically test the performance of individual nozzles to maintain a dependable print quality. As part of such a test, individual nozzles that are not performing to within specifications or acceptable tolerances may be identified and any problems causing the poor performance may be fixed. One such problem is the phenomenon of kogation which may occur when printing fluid residue is deposited on top of the firing resistor for that nozzle. If printing fluid becomes deposited on the firing resistor in this way (and the nozzle becomes kogated) then this can reduce the “drive bubble strength” (the ability to form the bubble due to the pulse of current through the resistor causing rapid vaporization of the fluid) in that the strength and/or size of the bubble may be reduced. This, in turn, can cause problems in the ability of that nozzle to discharge the printing fluid droplet which can reduce the velocity of the expelled printing fluid droplet (“drop velocity”) or even prevent the nozzle from firing.

[0013] In some examples herein, a print apparatus comprises a drop detector. In these examples, the drop detector may comprise a device to measure individual drops in-flight (e.g. following ejection from the nozzle and prior to deposition on a surface, or collection in a container). According to some examples, a drop detector may comprise an energy source (e.g. a light source), which may comprise an emitter, and a detector, with the detector being to detect any reflected energy from a fluid droplet. Put another way, the energy source may emit energy and if a fluid droplet is present then a portion of that emitted energy may be reflected (or back-scattered) off of the fluid droplet, this reflected energy being detected by the detector. This may be referred to as backscatter drop detection and, in examples where the energy source comprises a light source for emitting light, the detector may be to detect any light that is backscattered (or reflected, etc.) from a fluid drop that is passing through a focused light beam created by the energy source. The drop detector may comprise a lens to focus the emitted energy beam and/or a lens to focus the backscattered energy. According to some examples, the health of a nozzle (e.g. a health parameter for a nozzle) may be determined in this way, and several hundred nozzles per second may be tested by some example drop detectors. For example, and according to some example’s herein, a nozzle’s fitness to print may be assessed (e.g. by a processor or controller) based on the backscattered signal received from the drop detector (e.g. received by the processor or controller). According to some examples presented herein, the backscattered signal may enable a time-of-flight to be determined for the nozzle that discharged the fluid droplet that scattered the energy. For example, the time-of-flight, being the time taken for the discharged printing fluid droplet from the nozzle to reach a drop detection zone (or sample zone) of the drop detector, as determined from the backscattered signal, may be determined and compared to a stored value (e.g. in a database, for example in a look-up table) to determine if that nozzle is firing correctly. This may be the case if the determined time of flight is within a tolerance of the stored value. In other examples, the stored value may comprise a range of stored values and therefore it may be determined if the time-of-flight is within an acceptable range. The value, or range of values, may be unique or bespoke to the type of printing fluid discharged by the nozzle since the properties of an individual type of printing fluid may affect its acceptable flight time, for example this may depend on the compositions and/or density of the printing fluid, etc. (for example, black printing fluid may be ‘faster’ than yellow printing fluid).

[0014] To test the health of an individual nozzle, the printhead (for example, a carriage comprising the printhead) may be moved so that the nozzle is positioned over the drop detection zone of the drop detector. This zone may comprise an area of the drop detector receiving the focused energy (e.g. light) beams from the energy source and therefore fluid droplets may be ejected toward this region for the backscattered signals as described above to be properly registered by the detector. As each printhead may comprise a number of fluidic dies in a staggered arrangement (for example, each die may be staggered such that they overlap by an amount of nozzles) drops may be emitted at different distances relative to the emitters and receivers of the drop detector. The position of the drop detector may therefore effect the reliability of the backscattered signal and any measurements or assumptions that are made based on that signal. For example, the drop detector may be movable in a direction perpendicular to both the carriage movement and substrate advance directions - for example, closer and/or further away from the nozzle/die/printhead. This movement may be intentional or unintentional (for example, due to movement during transport; the drop detector and its location relative to the carriage may be assembled and calibrated during manufacturing but its position may change as it is moved). A wrongly positioned drop detector may lead to flight times being recorded that incorrectly point to a nozzle being kogated, and therefore heath issues may not be reliably determined. As will now be described in more detail with reference to the figures, some examples herein are related to determining whether a nozzle is kogated and/or if the drop detector is not correctly positioned and may therefore require re-calibration (e.g. a positional adjustment in the direction perpendicular to the media advance and carriage movement directions). [0015] Figure 1 shows an example print apparatus 1. The print apparatus 1 comprises a fluidic die 2 and a drop detector 3. In this example the print apparatus 1 comprises a printhead 10 which comprises the fluidic die 2. The printhead 10 in this example comprises five fluidic dies but in other examples the printhead 10 may comprise another number of fluidic dies, e.g. one fluidic die 2. As shown in the exploded view of the fluidic die 2, each fluidic die 2 comprises a nozzle array 7 which in this example is depicted to comprise four rows of nozzles. As described above each nozzle in the nozzle array 7 of the die 2 may be associated with a resistor (not shown) to cause the nozzle to discharge printing fluid as described above. Each nozzle in the die 2 is therefore to fire droplets of printing fluid individually and sequentially to produce a printed image onto a substrate, as described above.

[0016] The drop detector, schematically indicated at 3, is to detect the presence of printing fluid, and may comprise a transmitter or emitter and a receiver or detector, for example the transmitter may be to emit energy (e.g. light) that may reflect (e.g. backscatter) off a printing fluid droplet, with the scattered energy being detectable by the receiver. The print apparatus 1 comprises a controller 5 which may be to control the print apparatus 1 and may comprise any of a processor, an associated memory, programming, electronic circuitry and/or components needed to control any of the elements of the print apparatus 1 for the print apparatus to print an image to a substrate (not shown in Figure 1). The controller 5 is to cause a nozzle of the fluidic die 2 to fire a droplet of printing fluid toward the drop detector 3. As described above, the fired (or discharged) droplet of printing fluid may be directed toward a drop detection zone of the drop detector 3 such that emitted energy may reach the discharged droplet and the backscattered energy may be detected by the detector. The controller 5 is to receive a signal from the drop detector 3 (e.g., the backscattered signal) and is to determine, based on this signal, a time of flight for the printing fluid droplet. This time of flight is the time taken for the discharged printing fluid from the nozzle to reach the drop detector 3. The controller 5 is further to compare this determined time of flight to a stored time of flight (for example, stored in a database or look-up table) for the type of printing fluid discharged by the fluidic die 2 to determine a condition of the nozzle based on the comparison. Therefore, the controller 5 may be to access a database or look-up table to retrieve a stored value or range for comparison with the determined time of flight.

[0017] The stored value may comprise a stored range of values, and the controller 5 may therefore be to determine whether the time of flight is within a stored range of values. In this way, for each printing fluid the controller 5 may have access to a target range of values (flight times) such that if a time of flight for a fluid discharged from a particular nozzle is within this range then it may be determined that the nozzle is firing correctly and therefore has good health (e.g. it is not kogated or otherwise impaired), and this may be performed for different nozzles discharging different types of fluids. The controller 5 may therefore be to determine whether the flight time is outside of, or within, a tolerance of a stored flight time or within a predetermined range of flight times. A flight time outside of a tolerance of a stored flight time may be below, or above, the tolerance (respectively representing a flight time that is faster than, or slower than, acceptable) and similarly a flight time outside of the predetermined range may be below a lower bound of the range (indicating a too fast flight time) or above an upper bound in the range (indicating a too slow flight time). In this way, the controller 5 is able to assess the performance of an individual nozzle and assign a health parameter to that nozzle. If the controller 5 determines that the time of flight is within a tolerance of the stored time or within a predetermined range of flight times, which may be unique to that fluid type, then the controller 5 may determine that the nozzle that discharged the droplet is in good health. However, if the controller 5 records a time of flight for the first nozzle that is above the first stored time and outside of the tolerance, or faster than (e.g. above) the upper time the first predetermined range, then the controller 5 may determine that the drop detector 3 is positioned too close to the carriage 2 and therefore requires adjustment (e.g. in the Z-direction). In this way, in this example, the controller 5 may determine that the nozzle firing the droplet is healthy but, due to the flight time being too quick, determines that the drop detector 3 is too close to the carriage 2.

[0018] Figures 2 and 3 show another example print apparatus 1 which may comprise the print apparatus of Figure 1. Figure 2 shows the drop detector 3 in more detail. The drop detector 3 comprises a number of transmitters and receivers, schematically indicated at 21, and a drop detection zone 11. The apparatus 1 of Figure 2 comprises a carriage 20 which is to retain a printhead 10, the printhead comprising a fluidic die 2 having a nozzle array as described above. The carriage 20 is movable along the direction X which allows the carriage 20 to be moved (e.g. under the control of the controller 5) to move to position a nozzle 7 over the drop detector zone 11 of the drop detector. The controller 5 may be to control the carriage 20 to move (e.g. along the direction X) and may be to cause the nozzles of the fluidic die 2 to selectively discharge printing fluid (e.g. ink) stored therein towards a substrate to print an image to the substrate according to print data operated on by, or at, the controller 5. To test the health of a nozzle 7, the controller 5 may cause the carriage 20 to be moved such that a nozzle 7 to be tested is positioned over the zone 11. The controller 5 may then cause the nozzle

7 to discharge a printing fluid droplet such that it may be struck by energy from one of the transmitters, the backscattered energy being received by one of the receivers and the signal being transmitted from the drop detector 3 to the controller 5.

[0019] During a print job, a substrate is movable through the print apparatus 1 in the direction Y, the Y axis thereby denoting the direction of substrate (or print media) advance. Therefore, during a print operation the carriage 20 is movable in a back-and- forth, or reciprocating, manner along the direction X and the individual nozzles of each fluidic die 2 are caused to selectively fire printing fluid toward the substrate which is advancing in the direction Y. The carriage movement direction X and substrate advance direction Y are perpendicular. The drop detector 3 movable in the Z direction and therefore is movable in a direction that is perpendicular to both the media advance direction Y and the direction of movement of the carriage X. When a nozzle 7 of a fluidic die 2 retained in the carriage 20 is caused (e.g. by the controller 5) to discharge a printing fluid droplet, the droplet will fly in the Z direction toward the drop detector 3. [0020] A given nozzle 7 of a fluidic die 2 is to discharge a fluid of a given type.

Another nozzle (for example, of a different nozzle row or of a different fluidic die 2) may be to discharge printing fluid of a different type. For example, a fluidic die may comprise four sets (or arrays) of two rows of nozzles, with each nozzle of each of the four rows being to discharge a different one of black, cyan, magenta, or yellow printing fluid. Therefore, to discharge a printing fluid of a different type, the controller 5 may cause a nozzle of a different row, or of a different fluidic die, or of a different printhead to discharge printing fluid.

[0021] As state above, the controller 5 is to determine a nozzle health parameter based on a flight time for printing fluid discharged from that nozzle. In some examples, if the flight time for an individual nozzle alone is not enough to infer a health parameter for that nozzle (e.g. the flight time was outside of the predetermined range or outside of the tolerance of the stored time) then the controller 5 may determine the health parameter by way of comparison to the flight time for a printing fluid discharged from a different nozzle. For example, the controller 5 may be to cause a first nozzle to discharge printing fluid toward the drop detector 3 (e.g. the first nozzle may comprise the nozzle as described above) and to determine whether the first time of flight for the first nozzle is within a tolerance of a first stored time, or within a predetermined first range of flight times. If the controller 5 determines that the first flight time is not within the tolerance, or is outside of the predetermined range, then the controller 5 may test a second nozzle. In these examples, the controller 5 may be to cause a second nozzle to fire a droplet of printing fluid toward the drop detector, and to determine, based on a signal from the drop detector 3, a second time of flight for the printing fluid droplet fired from the second nozzle to reach the drop detector 3. The controller may then be to compare the determined second time of flight to a second stored time of flight, or to a second predetermined range of times, for the type of printing fluid discharged by the second nozzle to determine a condition of the first and/or second nozzle based on the comparison. In this way, the controller 5 may be to effectively determine a health parameter of the first nozzle with reference to the flight time of fluid from a second nozzle. A fluidic die may comprise the first and second nozzles or a different fluidic die may comprise the second nozzle. A printhead may comprise the first and second nozzles or a different printhead may comprise the second nozzle.

[0022] The second nozzle may be to discharge the same type of printing fluid to the printing fluid discharged by the first nozzle or may be to discharge a different type of printing fluid. In examples where the two nozzles discharge the same type of printing fluid then the first and second stored times and/or predetermined ranges may be the same. Otherwise, if the two printing fluids are different the first and second times/ranges may be different and may be unique to the particular fluid.

[0023] In examples where the printing fluid discharged by the first and second nozzle is the same type, if the controller determines that the second time of flight is within a tolerance of the second stored time, or within a second predetermined range of flight times, then the controller may be to issue an alert to a user of the print apparatus comprising an indication that the first nozzle is kogated. For example, the first time of flight being abnormally high, e.g. the time of flight was over the stored range or outside of the predetermined range (e.g. above an upper bound of the range) (as below, this is correlated to an abnormally low drop velocity), but the second time of flight being within acceptable limits, can lead to a reliable determination that the first nozzle is firing incorrectly (e.g. due to kogation) but that the second nozzle is firing reliably.

[0024] In examples where the printing fluid discharged by the second nozzle is a different type of printing fluid, if the controller determines that the second time of flight is within a tolerance of the second stored time, or within the second predetermined range, then the controller may be to issue an alert to a user of the print apparatus comprising an indication that the first nozzle is kogated. As above, for the example where the printing fluids are the same, this occurrence also reliably indicates that the first nozzle is firing incorrectly (e.g. due to kogation) but that the second nozzle is firing reliably. However, if it is determined that the second time of flight is outside of the tolerance, or outside of the second predetermined range, then the controller 5 may be to issue an alert to a user of the print apparatus 1, the alert comprising an indication that the drop detector 3 requires a recalibration or positional adjustment. The recalibration or adjustment may be in the Z- direction (e.g. the direction perpendicular to the substrate advance direction and the carriage movement direction) since this is the direction in which positional errors could affect the flight time measurements. The controller 5 in this example may be to automatically adjust the position of the drop detector 3, e.g. in the Z-direction. For example the controller 5 may be to move the drop detector 3 further to, or closer away from, the nozzle and/or the printhead and/or the carriage 2. It these examples, two nozzles discharging two different fluids record unacceptable flight times, meaning that it can be reliably inferred that the cause of these errors are the drop detector 3 being positioned incorrectly (e.g. in the Z-direction), as opposed to a health condition of either nozzle. For example, if the drop detector 3 is too close to the nozzle discharging the printing fluid then this may mean that a healthy nozzle records a time of flight that is too quick. Conversely, if the drop detector 3 is too far away from the nozzle then this may mean that a healthy nozzle records a time of flight that is too slow (and this could wrongly indicate kogation without comparison to another nozzle). Therefore, if the controller 5 determines a time of flight for the first nozzle that is too slow (e.g. slower than the stored time and outside of the tolerance, or below the lower bound of the first predetermined range), then this may indicate that either the first nozzle is kogated or that the drop detector requires re-positioning and so in these examples the controller 5 is to test the second nozzle. Testing a second nozzle firing a different printing fluid can lead to the reliable determination that the first nozzle is kogated (if the second nozzle is firing correctly) or that the drop detector 3 is incorrectly positioned in the Z-axis. [0025] The controller 5 may comprise, or be associated with, a database storing a target flight time, or the range of flight times, for a number of different printing fluid types since each fluid type may have its own target flight time, or target range of flight times, that indicate a correctly firing nozzle. For example, a nozzle’s target flight time may be dependent on the viscosity and/or composition of the printing fluid it discharges. For example, yellow printing fluid may have a “slower” flight time than black printing fluid in the sense that a yellow printing fluid taking longer to reach the drop detector 3 than a black printing fluid does not indicate that the nozzle firing the yellow fluid is kogated. The flight time for a given printing fluid type may depend on the colorant (for example a dye or pigment) comprised in the printing fluid. A nozzle’s target flight time may also depend on the architecture of the printhead comprising the nozzle (for example different printheads in the same carriage may have different arrangements) as well as the architecture of the nozzle itself. For example, a target flight time may depend on the shape and/or size of the nozzle. The stored time may therefore comprise an average time for that type of particular fluid, the average time indicating a general acceptable time for that fluid type. [0026] Figure 7 shows a graph of acceptable flight times for different printing fluids, each type is listed in the key to the right of the graph (C = Cyan, M = Magenta, K = Black, Lc = Light cyan, Lm = Light magenta, OC = overcoat, and PT = pre-treatment), with the x-axis denoting different nozzles ejecting the fluid (for example, different nozzles of a nozzle array or fluidic die or different nozzles for different printheads; the different nozzles may also be part of different print apparatuses) and the y-axis denoting an average number of time-of-flights for the nozzle (or die or printhead or apparatus etc.) on the x-axis. All of the drop times in Figure 7 for each fluid may be regarded as acceptable and therefore Figure 7 illustrates how each fluid may have an associated range of acceptable drop times. The stored time as discussed above may comprise an average time of flight for that printing fluid type and therefore the controller 5 as discussed above may be to determine if a flight time is within a tolerance of an average time of flight.

[0027] In some examples the controller 5 may use the signal received from the drop detector 3 to determine a drop velocity rather than a time of flight. The controller 5 may calculate the drop velocity by dividing the distance between the nozzle and the drop detector 3 by the determined time of flight. Here, the distance may be a factory setting or may be obtained by the controller 5 or by another means. Therefore, the controller 5 may be to calculate a drop velocity for a nozzle from the signal received from the drop detector 3 and to compare the drop velocity to a stored drop velocity. In this way it may be determined if a drop velocity is within a tolerance of a stored velocity or within a predetermined range of velocities. Therefore the above description relating to determining whether a nozzle is kogated, or the drop detector requiring adjustment in the z-direction, may also follow from a comparison of velocities.

[0028] In some examples, the controller 5 is to cause each nozzle (e.g. the first and/or second) to discharge a plurality of droplets and to determine an average time of flight for each nozzle. In this way an average firing time, or rate, for each nozzle may be determined based on a number of discharged samples, and the comparison may be between an average flight time for a nozzle to the stored time or range.

[0029] If the controller 5 determines that a nozzle is kogated then the controller 5 may be to trigger, e.g. automatically, a mitigation action. For example, the controller 5 may be to recommend that the fluidic die or printhead be changed and/or increase the energy through the resistor to form the printing fluid bubble to compensate for the long flight time/velocity loss due to kogation. In some examples this may comprise increasing the “over energy” through the resistor. In these examples, a minimum energy (which translates into heat in the nozzle) may be used to form the printing fluid bubble, this energy being a predetermined value to deliver a good quality of printing fluid bubble and therefore droplet from the nozzle, with the actual energy through the resistor being this value increased by an “over energy”. This “over energy” represents an energy increase beyond the minimum to ensure that the printing fluid droplets will be fired properly while keeping the energy (and therefore the heat) as low as possible. Therefore, in some examples where kogation is found, the controller 5 is to increase the over energy (and in this way this increased energy may “fight” the residue on the resistor such that the nozzle can still fire a droplet properly despite kogation).

[0030] In some examples, there may be a threshold time of flight and if the determined time of flight is over the threshold then the controller 5 may be to issue a recommendation to change the die and/or the printhead. In these examples, the threshold may be set such that if the flight time is over this threshold then it may be taken that the kogation problem is severe enough to recommend replacement of a component of the print apparatus. [0031] Figure 4 shows a method 400. The method may comprise a computer- implemented method. The method may comprise determining a health parameter of a nozzle (e.g. of a print apparatus). The controller 5 as described above may be to perform the method 400, e.g. any blocks thereof, as will now be described.

[0032] At block 402 the method comprises causing, by a processor, a nozzle of a printhead to discharge a printing fluid droplet toward a drop detector, for example as described above with reference to the print apparatus 1.

[0033] At block 404, the method comprises receiving, from the drop detector, by a processor, a signal indicative of the printing fluid droplet reaching a drop detection zone. [0034] At block 406, the method comprises determining, by a processor, a flight time for the nozzle, the flight time being the time taken for the printing fluid droplet to reach the drop detection zone. Block 406 may comprise determining the flight time based on the signal from the drop detector, received at block 404.

[0035] At block 408, the method comprises comparing, by a processor, the flight time to a predetermined flight time for the printing fluid (e.g. the stored time as above). For example, block 408 may comprise accessing a database to retrieve the predetermined flight time for the type of printing fluid. Block 408 may also comprise retrieving a stored range of flight times for the type of printing fluid. [0036] At block 410, the method comprises determining, by a processor, a health parameter of the nozzle based on the comparison. For example, if, at block 408, the comparison results in the determined flight time (at block 406) being within an acceptable level of the predetermined flight time (e.g. a tolerance) or within an acceptable range of flight times, then at block 410 it may be determined that the nozzle is healthy (and therefore not subject to kogation).

[0037] Figure 5 shows a method 500. The method may comprise a computer- implemented method. The method may comprise determining a health parameter of a nozzle (e.g. of a print apparatus). The controller 5 as described above may be to perform the method 500, e.g. any blocks thereof, as will now be described. The method 500 may comprise the method 400.

[0038] At block 502, the method comprises causing, by a processor, a first nozzle of a printhead to discharge a printing fluid droplet toward a drop detector. At block 504, the method comprises receiving, from the drop detector, by a processor, a signal indicative of the printing fluid droplet reaching a drop detection zone. At block 506, the method comprises determining, by a processor, a first flight time being the time taken from the droplet discharged by the first nozzle to reach the drop detection zone. For example, blocks 502-506 of the method may comprise blocks 402-406, respectively, of the method 400 as described above where the nozzle (of the method 400) is a first nozzle since, as will now be explained, the method 500 comprises performing these blocks again for another (second) nozzle, discharging the same or a different printing fluid, to determine a health parameter of one of the nozzles.

[0039] At block 508 the method comprises comparing, by a processor, the determined first flight time to a first predetermined time. At block 508 it is determined whether the first flight time is outside of a predetermined range of a first predetermined time. At block 509, if, at block 508, it is determined that the first flight time is within the predetermined range of the predetermined first time, then the method determines that the first nozzle is firing correctly. Put another way, block 509 may comprise determining that the first nozzle is not kogated. [0040] If, at block 508, it is determined that the first flight time is outside of the predetermined range, then the method comprises, at block 510, causing, by a processor, a second nozzle of a printhead to discharge a printing fluid droplet toward a drop detector. At block 512, the method comprises receiving, from the drop detector, by a processor, a signal indicative of the printing fluid droplet reaching a drop detection zone. At block 514, the method comprises determining, by a processor, a second flight time being the time taken from the droplet discharged by the second nozzle to reach the drop detection zone. For example, blocks 510-514 of the method may comprise blocks 402- 406, respectively, of the method 400 as described above, or blocks 502-506 of the method 500, where the nozzle is a second nozzle.

[0041] At block 516 the method comprises comparing, by a processor, the determined second flight time to a second predetermined time. At block 516 it is determined whether the second flight time is within a predetermined range of the second predetermined time. As stated above, the predetermined time may comprise a stored time, which may comprise a target time for the type of discharged printing fluid.

[0042] The second nozzle may discharge the same type of printing fluid or a different type of printing fluid to the first nozzle. In either example, if it is determined, at block 516, that the second flight time is within a predetermined range of the second predetermined time (in other words, the first nozzle has fired outside of the first range but the second nozzle has fired within the second range) then, at block 518, the method comprises determining that the first nozzle is kogated. In these examples, block 518 may also comprise issuing a recommendation to change the printhead comprising the first nozzle. In some examples, block 518 may comprise causing a status of the printhead comprising the nozzle to be displayed, e.g. to a user (for example, a green/yellow/red colour may be shown where green indicates healthy, red unhealthy and yellow somewhere in between a healthy and unhealthy state). In these examples, following the health display a user may decide to print a nozzle pattern or plot before commencing a print job to visually confirm the print quality. In yet other examples block 518 may comprise commencing, e.g. by a processor, e.g. automatically, a servicing operation (for example to replace individual kogated nozzles). Block 518 may, in some examples, comprise increasing the energy through the resistor associated with the nozzle that form the printing fluid bubble (for example, increasing the over energy as discussed above).

[0043] In examples where the second nozzle fires a printing fluid that is different to the type of printing fluid fired by the first nozzle and if, at block 516, it is determined that the second firing time is outside of the second predetermined range then both nozzles are recording flight times that are outside of acceptable bounds for different printing fluids. Then, the method comprises, at 520, determining that the drop detector requires adjustment (for example, recalibration). With reference to Figures 1-3 the drop detector may require adjustment in the z-direction (the direction perpendicular to both the media advance direction and the carriage movement direction) since the drop detector’s position in this direction may influence the recorded flight time for any given nozzle. Block 522 comprises issuing an alert that then drop detector requires adjustment. Block 522 may comprise changing, by a processor, the position of the drop detector. In other words, block 522 may comprise automatically repositioning the drop detector (e.g. in the z-direction).

[0044] Figure 6 shows an example non-transitory machine-readable, or computer-readable, medium 602 comprising a set of machine-readable instructions 606 stored thereon. The medium 602 is shown in Figure 6 in association with a processor 604. The controller 5 as described above may comprise the medium 602 and/or the processor 604. The instructions 606 when executed by the processor 604 are to cause the processor to perform a task. For example, the instructions 606, when executed by the processor 604, may be to cause the processor 604 to perform the method 400 or 500 as described above, e.g. any of the blocks thereof. The instructions 606, when executed by the processor 604 are to cause a nozzle of a fluidic die of a print apparatus to discharge a printing fluid droplet toward a drop detector. For example, the instructions 606 may cause a processor of the controller 5 of the print apparatus 1 as described above with reference to Figures 1-3 to cause a nozzle 7 of the fluidic die 2 to discharge a droplet toward the drop detector 3, etc.

[0045] The instructions 606 are also to cause the processor 604 to determine, based on information from the drop detector, a time of flight for the printing fluid droplet, the time of flight being the time taken for the discharged printing fluid droplet to reach the drop detector, to compare the time of flight to a stored time of flight for the type of printing fluid discharged by the nozzle, to determine a health parameter of the nozzle. For example, the instructions 606 may be to cause the processor 604 to perform the method 400 as described above with reference to Figure 4.

[0046] In one example, if the time of flight is not within a predetermined range of the stored time, then the instructions 606 are to cause the processor 604 to cause a further nozzle of the print apparatus to discharge a further printing fluid droplet of the same printing fluid type or of a different printing fluid type toward the drop detector, and to determine, based on information from the drop detector, a time of flight for the further printing fluid droplet, to compare the time of flight for the further printing fluid droplet to a stored time of flight for the type of printing fluid discharged by the nozzle. If the time of flight for the further printing fluid droplet is within a predetermined range of the stored time then the instructions are to cause the processor to issue an indication that the nozzle of the print apparatus is kogated. If the printing fluid discharged by the further nozzle is of a different type and if the time of flight for the further printing fluid droplet is not within a predetermined range of the stored time then the instructions are to cause the processor to issue an indication that the drop detector requires a positional adjustment. For example, the adjustment may be in the direction perpendicular to both the direction of movement of the carriage and the direction of movement of a substrate through the print apparatus when the print apparatus is to perform a print job (e.g. the z-direction as described above). In some examples the instructions ay be to cause the processor to adjust (e.g. automatically) the position of the drop detector in the aforementioned direction.

[0047] The examples presented herein are able to reliably detect kogation in nozzles early and without impacting the customer or the printed output from the apparatus. The drop detector may already be present in some printing systems and therefore the examples herein are able to determine nozzle health without the addition of new hardware and therefore without direct impact on the manufacturing costs or the addition of a failure rate associated with new components. Furthermore, the examples herein are able to determine whether the drop detector is incorrectly positioned, which may therefore ensure the correct determination of nozzle health and reduce the instance of error hiding. [0048] Examples in the present disclosure can be provided as methods, systems or machine readable instructions, such as any combination of software, hardware, firmware or the like. Such machine readable instructions may be included on a computer readable storage medium (including but is not limited to disc storage, CD-ROM, optical storage, etc.) having computer readable program codes therein or thereon. [0049] The present disclosure is described with reference to flow charts and/or block diagrams of the method, devices and systems according to examples of the present disclosure. Although the flow diagrams described above show a specific order of execution, the order of execution may differ from that which is depicted. Blocks described in relation to one flow chart may be combined with those of another flow chart. It shall be understood that each flow and/or block in the flow charts and/or block diagrams, as well as combinations of the flows and/or diagrams in the flow charts and/or block diagrams can be realized by machine readable instructions.

[0050] The machine readable instructions may, for example, be executed by a general purpose computer, a special purpose computer, an embedded processor or processors of other programmable data processing devices to realize the functions described in the description and diagrams. In particular, a processor or processing apparatus may execute the machine readable instructions. Thus functional modules of the apparatus and devices may be implemented by a processor executing machine readable instructions stored in a memory, or a processor operating in accordance with instructions embedded in logic circuitry. The term ‘processor’ is to be interpreted broadly to include a CPU, processing unit, ASIC, logic unit, or programmable gate array etc. The methods and functional modules may all be performed by a single processor or divided amongst several processors. [0051] Such machine readable instructions may also be stored in a computer readable storage that can guide the computer or other programmable data processing devices to operate in a specific mode.

[0052] Such machine readable instructions may also be loaded onto a computer or other programmable data processing devices, so that the computer or other programmable data processing devices perform a series of operations to produce computer-implemented processing, thus the instructions executed on the computer or other programmable devices realize functions specified by flow(s) in the flow charts and/or block(s) in the block diagrams.

[0053] Further, the teachings herein may be implemented in the form of a computer software product, the computer software product being stored in a storage medium and comprising a plurality of instructions for making a computer device implement the methods recited in the examples of the present disclosure.

[0054] While the method, apparatus and related aspects have been described with reference to certain examples, various modifications, changes, omissions, and substitutions can be made without departing from the spirit of the present disclosure. It is intended, therefore, that the method, apparatus and related aspects be limited only by the scope of the following claims and their equivalents. It should be noted that the above-mentioned examples illustrate rather than limit what is described herein, and that those skilled in the art will be able to design many alternative implementations without departing from the scope of the appended claims.

[0055] The word “comprising” does not exclude the presence of elements other than those listed in a claim, “a” or “an” does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. [0056] The features of any dependent claim may be combined with the features of any of the independent claims or other dependent claims.