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High-Temperature Thermometer/Thermostat

Need to measure or control temperature over a very wide range? Now you can do it with this compact unit which hooks up to a K-type thermocouple. It drives a relay which can be used to precisely control the temperature in ovens, kilns, autoclaves, solder baths or at the cold end of the spectrum, fridges and freezers. It is based on an Analog Devices AD8495 precision instrumentation amplifier with thermocouple cold junction compensation.

By John Clarke

Now we know that some digital multimeters can measure temperatures with a K-type thermocouple but that’s all they can do; they cannot control the temperature in an oven etc. In other words, they do not provide an adjustable thermostat function. In all the above examples, our new High-Temperature Thermometer/Thermostat can be used to measure and control the temperature at the same time. That’s because it has a relay output that opens or closes at a preset temperature.

The switched output can be used directly or in conjunction with a higher-rated relay to control power to the element of a heater or the compressor of a refrigerator. For heating, the power can be switched on when the temperature is below the preset temperature and switched off when it is above. Alternatively, for cooling, power can be switched on when the temperature goes above the preset and off when it goes below. The preset temperature for this thermostat action can be adjusted between -50°C and 1200°C.

It is important that the thermostat function does not cause rapid on and off switching of the heater, compressor or whatever is being temperature-controlled. Hence the design incorporates adjustable hysteresis. This allows a preset temperature difference to apply between switching power on and off. The hysteresis is adjustable from less than 1°C to more than 9°C.

The temperature is displayed on a 3½ digit LCD and while the unit can display a temperature from -50°C to 1200°C the actual measurement range will depend on the particular probe. Some K-type probes will operate from -50°C to 250°C, while others will operate from -40°C to 1200°C.

The High-Temperature Thermometer/Thermostat is housed in a small instrument case and controls on the front include a power switch and a switch to select between measured temperature and the preset thermostat temperature. A LED indicator is for power indication and a second LED shows when the thermostat relay has switched on.

At the rear of the case is the power input socket for a 12V DC supply and a socket for the K-type thermocouple. Additionally, there is a terminal connector inside the case for connection to the thermostat relay contacts. The common (C), normally open (NO) and normally closed (NC) contacts are available for connection.

Inside the case there are jumper links to select whether the thermostat relay switches on above or below the preset temperature for the thermostat. There are also jumper selections to select whether the Thermometer/Thermostat is built pre-calibrated or where the temperature calibration can be accurately adjusted.

Main Features

Click for larger image

• K-Type thermocouple probe

• Ground referenced or insulated
probe can be used

• Measures -50°C to 1200°C (depending on probe)

• Pre-calibrated temperature measurement

• Optional calibration of span and offset adjustment

• Thermostat switching at a preset temperature with adjustable hysteresis

• High to low or low to high thermostat threshold

• Relay output for thermostatic control

• Relay contacts rated at 10A (30V AC/DC maximum recommended switching voltage)

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