In photographic processing, the chemicals used for the development, bleaching and fixing stages need to be replenished periodically in order to produce optimum results. The rate at which the chemistry needs to be replenished depends mainly on the area and the exposure of the photographic material being processed. In Known replenishment systems, replenishment is generally carried out using chemicals in liquid form and pumping volumetrically metered quantities to the appropriate stage or stages of the processing apparatus as required. Accurate metering pumps can be used to dispense the replenisher in such systems, but these tend to be very expensive. A cheaper alternative to the metering pump is a bellows pump which is not very accurate, especially when only small quantities of replenishment material needs to be added.

US Patent Specification 4605297 discloses an arrangement in which a peristaltic pump is used to dispense fluid from a bottle during the processing of photographic film material. The pump also operates to return the excess fluid from the film surface to the bottle after use. A weight sensor is used to determine the amount of fluid in the bottle. The weight sensor is not used to meter fluid from the bottle but only to determine if there is sufficient fluid in the bottle for a processing operation before it is started. However, this type of arrangement tends to be inaccurate as there is no allowance made for any fluid which is retained in pipework connecting the bottle to its dispensing station.

Another problem associated with pump—based replenishment systems is that air can be trapped in the system causing air locks. As a consequence, when a certain volume is pumped through the system to effect replenishment, there is no allowance for trapped air and less replenisher material is dispensed than desired.

It is therefore an object of the present invention to provide a replenishment system which is both accurate and relatively inexpensive.

According to one aspect of the present invention, there is provided photographic processing apparatus comprising a plurality of processing stations, and replenishment means for replenishing the chemistry at each of the processing stations, characterized in that the replenishment means comprises a weighing system.

By this arrangement, replenishment of the chemistry in photographic processing apparatus can be easily and accurately controlled.

Advantageously, the weighing system comprises weighing means, at least one container carried by the weighing means and containing replenishment material, and dispensing means for allowing replenishment material to be dispensed from the or each container.

The replenishment system according to the invention has the advantage that replenishment materials may be dispensed in both liquid and solid form without the need to utilise separate metering equipment. Furthermore, dispensing according to weight overcomes the major problem associated with volumetric systems, namely, that even if there is an air lock in the replishment system, the desired amount of material will still be dispensed. Furthermore, changes in ambient temperature do not adversely affect accuracy. In volumetric replenishment systems, temperature affects the density

of replenisher solutions and herze the volumes of solution dispensed. This may lt^d to too little or too much replenisher material being added to the processing stage. Replenishment by weight is not 5 affected by temperature in the same way as replenishment by volume.

In a preferred embodiment, control means are provided for monitoring the chemistry at each processing station and for controlling the operation 0 of the dispensing means to effect replenishment. The weighing means may be a weighing platform, in which case, the container stands on the platform. Alternatively, the weighing means may be a suspension system, and the container is suspended ₅ therefrom.

A reservoir may be provided for refilling the container as replenisher material is dispensed.

For a better understanding of the present invention, reference will now be made, by way of o example only, to the accompanying drawings in which:—

Figure 1 is a schematic diagram of a replenishment system in accordance with the present invention;

Figure 2 is a schematic diagram of the 5 replenishment system shown in Figure 1 showing the use of a reservoir for refilling the dispensing container; and

Figure 3 is a graph showing a plot of measured weight against elapsed time. _Q Figure 1 illustrates a weighing platform 10 on which is placed a container 11 containing replenishment material for a photographic processing system. The container 11 is connected to one of the processing stations 12 by means of line 13. A valve 14 in the line 13 controls the flow of replenishment material from the container 11 to the processing station 12. Once all the replenishment material is

dispensed from the container 11, it is simply removed from the weighing platform 10 and replaced with a full container. A computer 18 is used to control the operation of the valve 14 in order to correctly dispense the replenishment material.

The computer 18 is also connected to the weighing platform 10 so that the weight thereon can be monitored. This ensures that the weight of replenishment material is monitored before, during and after a replenishment cycle.

Although only one container 11 is shown on the weighing platform 10 in Figure 1, more than one container may be present. In most processing apparatus, there are three processing stations which need replenishment. Ideally, one container would be provided for each of these processing stations. Alternatively, as shown in Figure 2, a reservoir 15 may be connected to the container 11 by a line 16 so that the container can be refilled after a quantity of the replenisher material is used. Another valve 17 in line 16 controls the flow of material from the reservoir 15 to the container 11. As with valve 14, the computer 18 controls valve 17 so that the required amount of replenishment material flows from the reservoir 15 to the container 11.

Valve 14 is used to control individual replenishment cycles. When the container 11 is nearly empty, as determined by the computer 18, valve 17 is opened to refill the container 11. There is no need to use level sensors in the container 11 to determine its fullness.

The container 11 can be either a bag-in-box container, as described above, or a drip—feed bag. Each type of container having suitable means to provide a drip-free connection to the line 13. If the container is a drip—feed bag, the weighing platform is replaced by a load beam, the bag being suspended from the beam. In both cases, however, the replenisher

material stored in the containers is gravity fed to the processing station 12, or alternatively sucked into the processing station by an existing recirculation pump associated with that station. 5 Preferably, line 13 is a small bore tube which restricts the flow of replenisher material, and reduces the 'dead weight' presented to the weighing system.

As mentioned previously, the flow of material 0 from the container 11 is controlled by the computer 18 through valve 14. During a replenishment cycle, the computer 18 opens the valve 14 and monitors the loss in weight of the container 11 as material flows into the processing station 12. When the required weight 5 loss has been attained, the computer 18 shuts off valve 14.

Any over— or under—replenishment is monitored by the computer 18. The computer is programmed with software which contains self-learning algorithms which 0 can be used to correct for over— or under—replenishment by shutting off the valve 14 earlier or later to compensate as required in subsequent replenishment cycles. These algorithms also allow for the correction of the absolute quantity 5 required in order to maintain a zero cumulative error. This results in an overall replenishment error which depends only on the absolute accuracy of the weighing system used. The system can be precisely calibrated at intervals. _Q Any drift in the electronics, or creep in the mechanical parts of the weighing system can be removed by the computer 18. This is done by storing in memory the absolute weight recorded at the end of a replenishment cycle and checking this weight at the beginning of the next replenishment cycle.

As the weight at the end of one replenishment cycle is checked before the start of the next cycle,

any large change in weight can indicate that there are leaks present in the system. Valve failure and/or system blockage can also be detected in a similar way. A replenishment cycle commences with a calibration of the weighing platform 10, and then the computer 18 monitors the current weight on the platform 10 before calculating the end-weight value from the exact amount of replenishment material required and initiating the cycle. As the replenishment material is dispensed, the computer 18 monitors the weight decrement and once the weight has dropped below the end—weight, then the cycle is terminated. However, the accuracy and precision of the amount of replenishment material dispensed depends, to a certain extent, on the flow rate during the cycle and the maximum rate at which the computer 18 can perform weighings.

If the flow rate is high, then there are fewer weighings required during the cycle and the precision falls. If the flow rate is low, therby giving good precision, the time taken per cycle may be inconveniently long.

This problem may be overcome by utilising time-interpolation techniques. Once the dispensing valve 14 has been opened, the material will start to flow and after a very short time the flow rate will stabilise and remain constant. If the weighings carried out by the computer 18 are at strictly regular time intervals then the amount of the replenishment material dispensed during each time interval will be constant.

Figure 3 shows graphically the decrement of measured weight against elapsed time. If the flow rate is arranged to be constant, then triangles indicated by ABC and CDE are similar, and the following relationship holds:—

AB/CD = BC/DE

Therefore, in terms of parameters on Figure 3:-

W _n -1 - W _n t _n - t _n _ι

^n W _a i _m t _a i _m — t _n where tn is the time at which the nth shot is dispensed; tn—,l is the time at which the (n-l)th shot is dispensed; t . is the interval between dispensed shots; „n is the weight at time tn; n_—,1 is the weight at time tn—l, ; and

W . is the weight at time t .

Therefore, to determine t . for a desired replenishment shot, W . :—

taim = tn ⁺ C*n "" t _n _ )(W _n - W _aim ) (W _n -1-W _n ⁾

As all of these values are readily available to the computer 18, and provided the time resolution is better than the time interval between weighings then an improvement in precision will result. B way of example, in the system described above, the weighing interval was found to be about 400ms. The time resolution of the computer was 5ms so an improvement of up to 80 times was possible. This improvement can be realised by permitting a faster flow rate for a given precision or by improving the precision for an acceptable flow rate.

Replenishment systems which use weighing techniques to meter out the material have several advantages over volumetric metering arrangements. First, more than one component can be dispensed from the same weighing apparatus. Secondly, solid replenisher materials can be used either alone or in

combination with materials in liquid form. Thirdly, dynamic weight measurement is easier and more precise than dynamic volumetric measurement. Fourthly, as the weight change is monitored during replenishment, closed loop control can be used. Fifthly, no extra pumps are required as the material is dispensed by gravity feed or sucked into the appropriate stations by existing equipment. This reduces the overall cost of the system. Finally, ratiometric measurements can be accurately carried out allowing 'in—line' mixing of replenisher components. Several components or solutions can be weighed on the same weighing system, for example a single weighing platform can be used for all of the processing stations. This also reduces the overall cost of the system.