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Temperature Sensors for data recording.

Because this site is called Create And Make the goal is to work to projects that give us the excuse to learn some new things.  The excuse to learn about Op-Amps was to develop a temperature measuring device.  We are still going down that path.  Today is a general sort of post where I don’t have to worry about a lot of charts, diagrams, and graphs so I decided to try out a podcast again.   Most of the information is in the podcast but I will supply a few notes here.

When measuring something often it is necessary to measure a secondary property that is affected by the thing you are measuring.   Even the simple thermometer shown in the picture does that by measuring how much the liquid expands with temperature.

One way that we could computerize that is to use a web cam and write a program to generate a number change from red to white in the picture.  That would be an interesting challenge but not one I would care to do.

There are other ways to measure temperature from expanding materials.  Bimetallic strips could be linked to a potentiometer (variable resistor) and the voltage could be measured.   We could measure the pressure within a closed container and obtain the temperature by knowing the pressure to temperature relationship of the gas in the container.

In the audio I come up with a few more hair-brained ideas for measuring temperature.

The primary ones used are the measurement of the resistance of some material (RTD and Thermistor) or the change of voltage with two different materials (Thermocouples).  All of these methods have problems that will have to be overcome.  The problem with RTD is expense, and I am a cheap-skate so that one will not be attempted.  The problem with thermocouples is the compensation for the copper junction at the connection point.  That can only be dealt with by taking an accurate temperature measurement at that point which gets us right back to the original problem.  This leaves us with thermistors.   The problem with thermistors is a very non-linear relationship of resistance and temperature.  That will be the subject of a future post.

Thermistors cannot handle the temperature range of the other two devices, but in general they can handle anything we will probably require.


Note:  The picture on this post is public domain and was from http://www.wpclipart.com/


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