In the majority of the circuits we put together for which we need to measure temperatures we use LM335Z temperature sensors. These are affordable and easy to use, but they are fragile, sensitive to damp conditions and static electricity, and not always easy to fit in place. In this article we will look at an alternative range of temperature sensors which can be clipped directly onto copper pipework, are much more robust, and have high moisture resistance.
ATC Semitec Pipe-Clip Temperature Sensors
Pictured above are pipe-clip thermal sensors from ATC Semitec. These devices are thermistors – resistors which have a resistance value which changes in a known way with changes in temperature.
By measuring the resistance of the thermistor it is possible to calculate the temperature accurately using the RT Curve (resistance/temperature) supplied by the manufacturer.
With these ATC Semitec sensors at 25 degrees Celcius, the resistance measured will be 9.5-10.5 kOhms, and at 85 degrees it will be between 1.04 to 1.11 kOhms. Temperature measurement accuracy is within plus or minus approximately 1 degree Celcius.
The change in resistance with temperature is non-linear (as pictured in the RT curve above) which makes things quite complicated if you are interested in measuring and recording temperature. However, two of these sensors can very easily be used to compare temperatures relative to one another – perfect for Solar Water Heating Pump Controllers where the pump should only be switched on when the water leaving the solar panel is hotter than the water already in the hot water tank.
NEW– Click here to view the full R/T data for these temperature sensors.
Using the Temperature Sensors
There are two pins which stick out of the underside of the temperature sensor. These are the two contacts for the thermistor. By measuring the resistance across these two pins you can find out the temperature of the pipe around which it is clipped. As copper is an excellent heat conductor, the temperature of the pipe should be very close to the temperature of the fluid it contains.
For comparator circuits in which one sensor is to be clipped onto copper pipe and the second must be stuck to the side of a hot water tank (e.g. solar water heating), it is very easy to pop one thermistor out from its metal clip and stick it to the tank using thermal adhesive (or heatsink compound). By doing so you get two near identical thermistors with the one stuck to the copper pipe far more securely than is possible with other sensors.
You can of course clip one sensor to the pipe where it leaves the solar hot water panel, and clip the second sensor onto the pipe where it leaves the bottom of the hot water tank (making sure to insulate it well to get accurate readings).
Using Thermistors in Comparator Circuits
Pictured above is the schematic of a sample comparator circuit used in a solar hot water system. With the two ATC Semitec CH-range pipe-clip thermistors (10K resistance @ 25 degrees Celcius) sitting side by side at the same temperature, the 10K potentiometer should be adjusted until voltage 1 and voltage 2 are identical.
The hotter the thermistors get, the lower the measured voltage will be. Therefore, if the solar panel voltage (voltage 1) is lower than the hot water tank voltage (voltage 2) then we know the solar panel is hotter than the water tank and so the pump can be switched on (circulating hot water from the panel into the tank, and cooler water from the tank into the panel to be heated).
The actual comparison can be done electronically using an LM393 op-amp circuit (see solar water heating pump controller for an example), or a programmable PIC chip (e.g. PICAXE).
Solar Heating Pump Controller
Pictured above is a solar heating pump controller we put together* which uses these pipe-clip thermistors. Note how leads have been soldered to the temperature sensor contacts with heat shrink tubing over the exposed wire to prevent short circuits.
This controller is built using a picaxe microcontroller which compares the temperature of the hot water tank and the solar heating panel, turning the pump on only if the solar heating panel has been hotter than the hot water tank continuously for 30 seconds.
* If you need something similar please contact neil@reuk.co.uk with details of your requirements – pipe diameters, pump voltage, pump power etc.
Buy Pipe-Clip Thermal Sensors
Three different sizes of pipe-clip temperature sensors are available for under £2 each. Both the 12-14mm and 16-18mm clips can be used for the common 15mm size of copper pipe, and the 20-22mm clips can be used for 22mm copper pipe. Click here to view the full ATC Semitec pipe-clip range for sale now at Rapid Online.