Water purification apparatus for use in laboratories and healthcare facilities are well known. Generally they involve the reduction and/or removal of contaminants and impurities to low levels. They typically contain a variety of technologies that remove particles, colloids, ionic species and organic substances and/or molecules.
Water purification apparatus generally allow manual variation by a user of the volume of water to be dispensed, such as from drops to liters, and/or the speed (i.e. flow rate) of water dispense, again from drop-wise to a number of liters per minute.
However, as well as manual dispensing based on a user physically operating a tap and manually gauging the volume of water being dispensed, frequently water purification apparatus provide for ‘automatic dispensing’, such as on a time basis or measured basis; optionally following a pre-programmed or pre-stored dispense profile as discussed in our WO2007/015048A1.
In order to accommodate such a range of dispensing options, the control of the water dispense outlet of water purification apparatus is often ‘electronic’, to automate this activity. Typically, a simple on/off solenoid-controlled valve is used. However, solenoids are limited in that the internal flow rate of the water dispense apparatus, and hence the flow rate of the dispense of the purified water stream, is conventionally set at a single and high value, and this does not allow for the easy dispense of slow or small volumes, particularly into smaller water collecting vessels such as beakers and flasks.
U.S. Pat. No. 5,925,240 describes a water treatment system having dosing control for providing treated water that can be accurately dispensed. In one embodiment, its controller converts an input signal to a motor control signal that causes the motor to operate at a speed which drives the pump and causes the flow of water at the outlet valve to correspond to the desired flow rate. In another embodiment of U.S. Pat. No. 5,925,240, the controller controls the outflow cross-section of a valve and causes the flow of water at the valve to correspond to the desired flow rate. However, both such embodiments involve increasing costs for additional solenoids and additional hydraulic complexity.
In order to overcome such limitations, but retain a good degree of dispense control, voltage or frequency dependent solenoids can be used, whereby adjustment of the coil, voltage or frequency adjusts the position of the armature, enabling some flow adjustment through such solenoid-controlled valves. However, such solenoids are still limited at very low and/or very accurate (such as drop wise) flows that can be desired or required when very precise volumes are to be dispensed. This is because on initial lifting of the valve diaphragm to start a dispense, the change in pressure on the valve diaphragm affects the delicate electronically-floating armature, leading to its incorrect positioning and therefore incorrect flow control.
A further disadvantage of solenoids is that in energising the activation coil of a solenoid over a long period, (such as during drop wise dispense for very accurate volume dispense), the heated coil causes undesirable localised heating of the passing purified water stream to occur, affecting the properties of the purified water stream.