Making Temperature Probes


The three most common temperature probes are made from thermocouples, thermistors and calibrated sensors.

We are all familiar with the basic alcohol and bimetallic spring thermometers commonly available at Wall Mart. They are great for displaying the localized temperature BUT how can we take temperature readings from remote locations? This is the job of the electronic temperature probes.



A temperature monitoring station uses electronic sensors to display probe readings from remote locations, but unfortunately they are not available at Wall Mart. A DVM could be used to interpret sensor readings, but a temperature monitoring device eliminates the need to translate electronic readings. The device on the left is used to monitor up to six remote readings from one location.

Thermocouple (Copper and Steel)
Joining a copper wire to a steel wire is one way of making a simple temperature probe. Of course solder will not adhere to steel but wrapping the copper around the steel before soldering supports the thermocouple junction.

Thermocouple ( Type K Chromel - Alumel) 
Type K is the most common commercially available thermocouple. It's made from Chromium, Copper and Nickel alloys. It's fairly inexpensive and it works in the range between −200 C to +1350 C. Generally speaking thermocouples are used to monitor high temperatures because their output voltage is so low at low temperatures below 150*F. Thermocouples may be used to sense temperature and trigger differential controllers but as I said the voltage generated with a thermocouple is so low that differential regulators must be specially designed to handle the subtle micro-volt differential.

Thermistor (5K NTC & 10K NTC)
The 5K and 10K negative temperature coefficient thermistors are very popular. They change resistance with temperature. The 5 K thermistor has a resistance of about 5,000 ohms at 75* F. and at 100*F the resistance changes to about 3,000 ohms.
By connecting a thermistor to a power supply in series with a resistor it�s possible to create a voltage that�s proportional to temperature. Although the voltage generated by a thermistor is not a linear representation of temperature the voltage differential that it generates may easily be used to control differential controllers. 

The LM34 Sensor generates a voltage that's directly proportional to temperature. As you can see the thermistor is not far off�

Sensors may be made into a probes that can be handled by attaching leads to the terminals. 

A sensor is held in position while solder is applied.
 Next 1/8" shrink sleeves are slid over the solder connections and heat is applied.
The sensor is then immersed in polyurethane
Next a 1/4" sleeve is slid over everything and heat is re-applied.

Calibrated temperature Probe (LM34)
If temperature accuracy is important the LM34 may be used to generate a Fahrenheit reading and the LM35 may be used to generate a Centigrade reading. The same method of transforming the sensor into a probe may be used for both the thermistor and for the calibrated sensor. If you are careful with the soldering and the sleeve insulation you'll make durable probes that will last a very long time.

New Fahrenheit Temperature Probe

A simple way to secure a probe to a collector involves taping the probe to the heat output pipe. The storage probe may then be secured to the input pipe of the pump in the same manner.


    new Fahrenheit Temperatures Probe...........................................................................$10

    10K Thermistor Temperature Probes............................................................................$10



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