METERED LIQUID DELIVERY SYSTEMS The present invention relates to liquid delivery systems in which it is desired to deliver small volumes of liquid reliably and accurately. Accurate metering pumps are known and are expensive; even so, known metering pumps are not able to deliver small varying amounts of liquid accurately and reproducibly.

An object of the present invention is the -provision of simple inexpensive apparatus for accurately and reliably delivering varying amounts of liquid.

According to the present invention, there is provided an apparatus for delivering a desired volume of liquid comprising delivery means arranged, upon actuation, to deliver the liquid, measuring means for measuring the rate of delivery of the liquid, means for producing a cumulative signal indicative of the volume of liquid delivered, and means for deactuating the delivery means when the signal indicates a volume of liquid delivered less by a predetermined amount than the volume of liquid desired to be delivered. The cumulative signal producing means is advantageously also arranged, after deactuation of the delivery means ; to produce a signal indicative of the total volume of the liquid delivered, comparator means being provided for comparing the cumulative signal with a stored signal corresponding to the desired volume of liquid and for producing an error signal, and memory means are provided for storing the error signal and for adjusting the stored signal and consequently the predetermined amount, on a next delivery, in accordance with the error signal.

In a preferred embodiment, the apparatus includes a microprocessor for receiving the cumulative signal, for comparing such signal with the stored

signal corresponding to the desired volume of liquid, for producing the error signal and for adjusting the stored signal, and consequently the predetermined amount, in readiness for a next liquid delivery. The delivery means may comprise a solenoid operated valve controlling a gravity feed of the liquid, or alternatively may comprise a pump.

Advantageously, the measuring means comprises a turbine flow meter arranged, on rotation, to produce a series of pulses corresponding to the volume of liquid flowing therethrough.

Using such an apparatus enables low cost components to be used. Initial calibration (determination of the predetermined amount) to cater for the inertia of the system, enables any error for a particular delivery to be kept at a very low level and, furthermore, the error is not cumulative, the feed back arrrangement ensuring that correction takes place upon each delivery of liquid. According to the present invention there is also provided a method of delivering a desired volume of liquid, comprising the steps of determining the inertia of a liquid delivery system and deactuating the delivery system when the delivery system has delivered less than the desired volume of liquid by an amount determined by the inertia.

The method can include the further step of determining the exact volume delivered and correcting the value allocated to inertia in dependence upon any difference between the desired volume and the exact volume of the liquid delivered.

The present invention will be described further, by way of example, with reference to the accompanying drawing in which the single figure is a diagrammatic representation of an apparatus for delivering liquid in accordance with the present invention.

As shown in the drawing, a liquid delivery apparatus comprises a tank 10 for containing liquid 11 to be delivered. The liquid 11, in the described example, is a concentrated solution of a developer replenisher which is to be delivered to and to replenish photographic developer in a bath 13.

The tank 10 is connected to the bath 13 by a gravity feed pipe 14 including a solenoid operated valve 15 and a turbine flow meter 16. The valve 15 is opened by control means 17 to allow replenisher 11 to flow under gravity through the flow meter 16 to the bath 13. As the replenisher passes through the flowmeter 16, the turbine of the latter rotates and produces a plurality of electrical pulses proportional to the quantity of liquid flowing therethrough. The electrical pulses are fed to the control means 17.

The control means 17 includes a microprocessor and an input unit 18, which may be manual, e.g. a keyboard, or automatic, e.g. sensing means (shown dotted) in the bath 13 which determines the depletion of developer 12 and the desired volume of developer replenisher 11 to be fed from the tank 10. The microprocessor converts the input (manual or automatic) into a number corresponding to an appropriate number of pulses generated by the turbine flowmeter 16 and in turn corresponding to a volume of liquid 11 flowing therethrough. However, a simple system as described above has inertia and liquid 11 continues to flow after deactivation of the valve 15 by the control means 17. The inertia of the system is due to the opening and closing times of the valve 15, the inertia of the flowmeter 16 per se, the gravity head under which the liquid 11 is fed which in itself varies as the level of liquid 11 in the tank 10 drops and any temporary changes in the resistance to flow of the liquid 11 from the tank 10 to the bath 13.

The inertia of the system is measurable by a count of pulses from the flowmeter 16 after deactuation of the valve 15. This count is stored in the memory of the microprocessor and is deductable from the number determined by the input (manual or automatic) and the microprocessor is arranged to deactuate the valve 15 after receiving a number of pulses from the flow meter 16 equal to the number determined by the input less the stored count. For example, in use, an operator (or automatic sensing means) determines that 60 mis of developer replenisher liquid 11 need be added to the bath 13. This figure 60 is input to the microprocessor through the input 18. The microprocessor converts this to an equivalent number of pulses to be received from the flowmeter 16. In this example the flowmeter produced 10 pulses per millilitre.

A count of 600 pulses from the flowmeter 16 would, were it not for the inertia of the system, indicate delivery of the correct volume (60 mis) of liquid 11. By calibration, it is found that the inertia of the system is equal (under set conditions) to 40 pulses and this number is stored in the memory of the microprocessor.

The microprocessor activates the valve 15, counts 560 (600-40) pulses from the flow meter 16 and then deactivates the valve 15. The flowmeter 16 should, due to inertia, then produce a further 40 pulses giving a total of 600 pulses and delivery of 60 mis of replenisher liquid 11.

The microprocessor counts the pulses from the flow meter 16 after deactuation of the valve 15. If 40 further pulses are generated, an exact 60 mis of the liquid has been delivered. It is unlikely that 40 further pulses will be generated due to the varying inertia of the system. The number of further pulses

generated is stored in the microprocessor memory and is utilised instead of the exemplified number 40. The system is self correcting due to this feedback. If too little liquid 11 was dispensed the first time, on a next delivery, an extra amount of liquid 11 is dispensed equal to the first delivery shortfall.

The present invention is not confined to the precise details of the foregoing example and variation may be made thereto. For instance, the valve 15 may be replaced by a simple pump providing a pumped feed instead of a gravity feed. Any kind of flowmeter can be used which provides an output indicative of the volume of liquid flowing therethrough. The microprocessor may be replaced by constituent electronic parts, counter, comparator, memory and trigger devices. Developer depletion may be sensed by level sensing means, ion-selective electrodes, photoresponsive devices examining processed material and/or on a time-dependent basis. The present invention is also applicable to apparatus, other than photographic processing apparatus, in which small quantities of liquid are to be delivered reliably and accurately.