GB2340246A

GB2340246A - Hot water monitor

Info

Publication number: GB2340246A
Application number: GB9919445A
Authority: GB
Inventors: John Alfred Temple
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-08-18
Filing date: 1999-08-18
Publication date: 2000-02-16
Anticipated expiration: 2019-08-18
Also published as: GB2340246B; GB9919445D0

Description

2340246 HOT WATER MONITOR

Background and General Description

This invention is a monitoring device which indicates the water temperature at various levels in a domestic hot water storage tank. For the purposes of this description it will be given the name HOT WATER MONITOR.

Before the days of integrated foam insulation on hot water tanks it was always possible to find out how hot the water was and how much hot water there was in a tank simply by feeling the tank. You could tell, for example, whether there was enough hot water for a bath. Unfortunately, modern ways of insulating tanks prevent this simple task. The HOT WATER MONITOR fulfils a useful function by doing it for you.

The HOT WATER MONITOR consists of a control,box, a connecting cable to a junction box and a number of flexible wires connected to small probes (see figure 1 on page 5). The control box contains a battery and on its face there is a push button and a column of small indicator lights.

The HOT WATER MONITOR uses electronic circuitry which is familiar, but in a way that is new. Probes are inserted through the tank insulation so as to come into physical contact with the tank itself at several different levels. Each probe is affected by the temperature of the water at its own level and the information thus found is fed to a control box containing an electronic circuit which can interpret and display this information.

The information display on the control box is a column of lights each of which lights up if the water is hot at its own particular level. If a light is on it indicates that the water temperature is above a certain value at that level in the tank. If a light is off the water is below that temperature.

Technical Descriptio

This description refers to the figures on pages 5, 6 and 7. Figure 1 shows the appearance of the control box, junction box, cable and probes.

Figure 2 is a circuit diagram.

Figure 3 shows the circuit layout on a printed circuit board.

Figure 4 is a wiring diagram for the junction box.

kigure 1 shows the control box 1, connecting cable 2, junction box 3, hot water tank 4 and probes 5. The control box contains an electronic circuit detailed in figures 2 and 3 and a battery compartment 6. On the front of the box is a push button switch 7 which switches the circuit on when pressed. An indicator 8 shows when the circuit is on. The vertical column of indicators 9 shows whether the water is hot in the tank at the levels corresponding to the probes.

The circuit shown in Figure 2 contains four operational amplifiers 11 with their associated circuitry. The OpAmps occupy a single integrated circuit chip 12, type LM348N. A 9 volt PP3 battery (housed in a separate compartment 6 inside the control box and connected between 11+9 volt" and 110 volt") drives the circuit. The switch 7 is a momentary push to make switch and turns the circuit on when required. When the switch is pressed, the light emitting diode 8 indicates that the battery is providing power. There is a 2.2 k resistor in series with the LED which limits the current to less than 5 mA. All LEDs in the circuit are low current types. All resistors have a power rating of 0.25 watt.

An operational amplifier switches on when the potential at the non inverting input (11+") exceeds that at the inverting input. Conversely it switches off when the potential at the non inverting is less than that at the inverting input. It is this function which is used to switch the LEDs on and off.

The outputs from the operational amplifiers 11 are each connected to a light emitting diode 9, 11LED111, 11LED211 etc, via a 2.2 k resistor connected in series. The purpose of the resistors is to limit the current in each LED to less than 5 mA. When one of these LEDs 9 lights up it indicates that the water in the tank at that level is above a certain predetermined temperature.

The non inverting inputs (11+") to the OpAmps are each taken to a potential divider consisting of a bead thermistor 5 in series with a 150 k resistor. These potential dividers are connected across the supply. The thermistors are the probes which measure the water temperature. They have a nominal room temperature resistance of 100 k which gets smaller as the temperature rises.

Thus, as the temperature rises the probe's resistance falls and the potential at the non inverting input rises. (It gets closer to the positive rail.) When the potential reaches the voltage applied to the inverting input, the operational amplifier 11 switches on and the appropriate LED lights up.

The inverting inputs to the operational amplifiers are all connected to a variable reference voltage provided by a 1 M "preset" potentiometer 10 connected across the supply. Altering the slide position of the preset alters the potential applied to the inverting inputs and hence the temperature at which switching occurs.

The low voltage silicon diode 14 ensures that reverse battery polarity is not accidentally connected.

The optional light emitting diode 15 if included, lights up (alone) if the battery polarity is reversed. It also lights up (together with the "circuit on" LED 8) if a 6 volt AC supply is used instead of the battery. if AC is used, a suitable smoothing capacitor should be connected across the rectified supply, ie in parallel with the 1 M preset potentiometer 10.

If it is required to leave the circuit on continuously (for example when powered by AC), it is helpful to provide positive feedback in each operational amplifier. This can be achieved by connecting a resistor of value 680 k between each output and non inverting input. The effect is to raise slightly the potential at which the OpAmp switches on, and correspondingly to lower the potential at which it switches off. The cleaner switching which results from this modification is not needed in the battery operated circuit of the main description, which is only switched on occasionally for a second or so.

Figure 3 shows a layout on a printed circuit board which places the light emitting diodes 8 and 9, the switch 7, the preset potentiometer 10, and optionally LED 15 in positions which are correct for their location on the main control box.

The positions Probe 1, Probe 2 etc and "Common" provide connecting points for a 6 way cable 2 to the probes 5. In this implementation the cable is colour coded as follows:

Pi white P2 red P3 blue P4 black Common yellow, green The other end of the cable terminates in a junction box 3 shown in Figure 4 in which the wires are connected:

white to terminal 1 blue to terminal 4 yellow to terminal 2 green to terminal 5 red to terminal 3 black to terminal 6.

The probes 5 are bead thermistors of nominal value 100 k connected via twin flexible leads as shown.

I

Claims

1 A temperature sensor for monitoring a domestic hot water storage tank in which a pluraliy of probes are attached to the tank at different heights. Indicators on a control box show whether the environment of the corresponding probe is hot or cold - i.e. above or below a certain temperature (the reference temperature).

2 A sensor as claimed in Claim 1 with a facility for adjusting the reference temperature.

3 A sensor as claimed in Claim 1 or Claim 2 using an electronic circuit for detection and display.

4 A sensor as claimed in Claim 3 for use in a domestic environment.

A sensor as claimed in Claim 3 or Claim 4 using four probes.