GB2062238A - An impedance measuring system - Google Patents

Abstract

In an impedance measuring system e.g. for detecting the presence of water, ice or other conductive material or liquid the probes are isolated from the operating supply. The system comprises an alternating voltage supply which feeds a winding on an impedance transformer, the voltage supply being of a high impedance such that when a secondary winding of the transformer is loaded by say the presence of water between the probes then the alternating voltage supply is reduced. This reduction in voltage is measured and, when at a preset level, an output circuit is activated. This system allows for the electrical supply to the control circuit to be operated directly from mains electric supply without a mains isolating transformer, and to have isolated detector probes that can be earthed. Instead of using a transformer the probes may be connected to the supply via capacitors. <IMAGE>

Description

SPECIFICATION An impedance measuring system This invention relates to an impedance measuring system and in particular to one which provides isolation between the operating supply and the detector probes.

It is common practice to detect the presence of water by measuring the reduction in impedance across detector probes. This change is typically from several Meg ohms (no water) to several Kil ohms (water present). Sometimes two probes are zsed and other times a metal tank is used as one probe. Often more than two probes are used to detect different levels. Most systems operate with A.C. on the probes in order to minimise the effects of electrolysis. In most systems it is most necessary to isolate the detector probes from the mains electric supply, this is commonly done by the use of a mains isolating transformer, and a further transformer or relay is needed if the load is to be driven by a triac direct on mains.

The present invention removes the need for a mains transformer and relay which are relatively expensive in that the isolation is provided by a small relatively cheap low power impedance transformer.

The invention will now be explained in more detail with reference to Fig. 1. The power supply for the circuit is operated directly from main without the use of a mains transformer. An oscillator which can run at any desired frequency or could be a tap from mains frequency normally runs continuously. The output from the oscillator is arranged such that the unloaded transformer does not significantly reduce the output voltage.

When the probes are covered with water the transformer is loaded and the output from the oscillator is reduced. A detector and amplifier circuit is then operated and the load switched via the triac Fig. 2 shows a possible variation which is satisfactory in some applications, in this case two capacitors are used in place of the transformer to provide isolation.

When water freezes its impedance increases, it is thus possible to apply the above system to the control of freezing equipment such as beer coolers. The applications of the invention can be extended to analogue measuring systems, and to the control of other solids and liquids that respond by lowering the impedance across the detector probes.

1. An ice detector or water level detector operating on the principle of measuring liquid impedance, comprising an impedance measuring system utilising an impedance transformer or one or more capacitors to provide isolation between the operating electric supply, which may or may not be directly connected to the mains electric supply, and the detector probes, such system allowing one detector probe to be directly connected to earth if required.

2. A system as in claim 1 where one of the detector probes is the metal or other electrically conductive container of the liquid.

3. A system as in claim 1 where there are two or more probes.

4. A system as in claims 1-3 designed to detect when water or other liquid or material freezes causing a change in impedance across one or more probes to a return probe.

5. A system as in claims 1-3 designed to detect the level of water or other liquid or material by measuring its impedance, there being one or more detected levels.

6. A claim as in any of the preceding claims where the output is of an analogue or digital form, the value relating to the level of water or substance above the end of the detector probe.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (6)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION An impedance measuring system This invention relates to an impedance measuring system and in particular to one which provides isolation between the operating supply and the detector probes. It is common practice to detect the presence of water by measuring the reduction in impedance across detector probes. This change is typically from several Meg ohms (no water) to several Kil ohms (water present). Sometimes two probes are zsed and other times a metal tank is used as one probe. Often more than two probes are used to detect different levels. Most systems operate with A.C. on the probes in order to minimise the effects of electrolysis. In most systems it is most necessary to isolate the detector probes from the mains electric supply, this is commonly done by the use of a mains isolating transformer, and a further transformer or relay is needed if the load is to be driven by a triac direct on mains. The present invention removes the need for a mains transformer and relay which are relatively expensive in that the isolation is provided by a small relatively cheap low power impedance transformer. The invention will now be explained in more detail with reference to Fig. 1. The power supply for the circuit is operated directly from main without the use of a mains transformer. An oscillator which can run at any desired frequency or could be a tap from mains frequency normally runs continuously. The output from the oscillator is arranged such that the unloaded transformer does not significantly reduce the output voltage. When the probes are covered with water the transformer is loaded and the output from the oscillator is reduced. A detector and amplifier circuit is then operated and the load switched via the triac Fig. 2 shows a possible variation which is satisfactory in some applications, in this case two capacitors are used in place of the transformer to provide isolation. When water freezes its impedance increases, it is thus possible to apply the above system to the control of freezing equipment such as beer coolers. The applications of the invention can be extended to analogue measuring systems, and to the control of other solids and liquids that respond by lowering the impedance across the detector probes. CLAIMS

1. An ice detector or water level detector operating on the principle of measuring liquid impedance, comprising an impedance measuring system utilising an impedance transformer or one or more capacitors to provide isolation between the operating electric supply, which may or may not be directly connected to the mains electric supply, and the detector probes, such system allowing one detector probe to be directly connected to earth if required.

2. A system as in claim 1 where one of the detector probes is the metal or other electrically conductive container of the liquid.

3. A system as in claim 1 where there are two or more probes.

4. A system as in claims 1-3 designed to detect when water or other liquid or material freezes causing a change in impedance across one or more probes to a return probe.

5. A system as in claims 1-3 designed to detect the level of water or other liquid or material by measuring its impedance, there being one or more detected levels.

6. A claim as in any of the preceding claims where the output is of an analogue or digital form, the value relating to the level of water or substance above the end of the detector probe.