Re: What is the difference between a thermistor and a thermocouple thermometer?

Dear Noname:
You have to explain the difference between a thermistor and a thermocouple 
to a group of non-technical people.  It would be very helpful to know the 
purpose of this explanation being given to people who will not understand 
it.  Could you explain?

Meanwhile here is your answer:

A thermistor is an electrical resistance which can be wired into a 
circuit.  The thermistor is made of semiconducting material, usually a 
mixture of manganese oxides and nickel oxides.  The electrical resistance 
of this material changes with temperature.  Therefore if you have an 
instrument which measures electrical resistance, and connect it across the 
thermistor, you will measure a resistance change with temperature of the 
thermistor.  This is a means for measuring temperature, by referring to a 
table which lists the temperature versus resistance for the particular kind 
of thermistor which you have. You can determine the temperature from this 
table.

If you place the thermistor into an electrical circuit with a voltage 
source that forces current through the thermistor, this circuit can be 
used to automatically read out or control the temperature continuously by 
having connected to it a circuit which measures the resistance and which is 
calibrated according to the curve for this thermistor type to read out the 
appropriate temperature.  So a thermistor requires a voltage source, which 
forces current through the thermistor; a means for measuring the resistance 
across the thermistor while the current is flowing; and a circuit which 
converts this resistance reading into a temperature readout.  The readings 
of the thermistor are absolute in that the actual resistance at any time is 
a measure of the actual thermistor temperature.

A thermocouple is very different.  A thermocouple is made of two wires of 
different metals (e.g. copper and iron).  The wires are of equal length and 
are electrically connected together at one end (by soldering, for 
example).  If both ends of the wires are at the same temperature there is 
no voltage measured.  However if the open ends are at a fixed temperature 
and then you change the temperature at the other ends of the wires a 
voltage will be generated between the two the two wires at the open ends.  
If you have a meter or electrical circuit designed to measure the voltage 
between the two wires at the open end then you can measure the temperature 
difference between the two ends of the wires by measuring this voltage 
difference.  Remember that if you have a circuit to measure the voltage 
then there is an electrical junction between the 
wires at the other measurement end and this voltage is developed 
between the two wires because the electrical properties of the different 
metals are different, and when the temperature is different between the 
junctions the two metals respond differently and the result is the voltage 
which develops between the two junctions.

So a thermocouple is different from a thermistor in these principle ways: 
first, it is a measure of a voltage which is generated by a difference in 
temperature between both ends of the wires, not a difference in resistance 
which happens with a thermistor; second, you must have two different 
temperatures to make the measurement-the temperature where the end junction 
is, where you want to know the temperature; and the temperature where the 
measurement circuit is located. You must know the measurement-circuit 
temperature in order to get an absolute number for the temperature of the 
junction placed where you are making the measurement.  The voltage which is 
measured gives a measure of the temperature difference between the end 
junction and the measurement-circuit temperature; if you do not know the 
measurements-circuit temperature then you will only know the temperature 
dirrerence between the two ends of the thermocouple wires but you will not 
know the absolute temperature of the end of interest.

Whether a thermocouple or a thermistor is used for a particular application 
depends upon the desired accuracy, and the temperature range of the 
measurement.  Generally speaking thermocouple measurements will be more 
precise, providing, of course, that the measurement-circuit temperature is 
accurately known!

R. Bersin.......