Information on a common GM coolant temperature sensor.
This sensor is an NTC (negative temperature coefficient) thermistor, which means its resistance decreases as temperature increases. Here is a good article to get you started on thermistor theory: http://www.facstaff.bucknell.edu/mastascu/elessonsHTML/Sensors/TempR.html .
A thermistor has a very nonlinear response, so you will need to linearize it in order to make it useful. Here is an excellent article on how to do that: http://www.maxim-ic.com/appnotes.cfm/appnote_number/817/ .
I chose to use the "voltage mode" mentioned in the article, and used an R value of 220 ohm, which puts the center point of my linear region at about 96.6 degrees celcius (100 degrees celcius was my design goal). Here is some data I measured:
note: the upper (hot) part of that graph should be more linear than it is. I attribute this to my measurement setup (water in a saucepan on an electric range). As the temperature became higher and higher, the values were changing too quickly to measure accurately (turning the stove on and off caused a dramatic "porpoising" at higher temperatures). also, my digital thermometer and the coolant sensor have vastly different masses, so I imagine one should "lag" behind the other. additionally, some of the temperatures in that upper area were taken "on the way up" (stove on) and others "on the way down" (stove off). in short, I am surprised the data looks as good as it does.
the cold data points look much "better" for two reasons. In order to measure the cold data points, I chilled some water in a metal bowl in the freezer, then placed the bowl in the (emptied) sauce pan. this effectively isolated the water from conductive losses, which greatly slowed the temperature change. additionally, the cold water was much closer to room temperature than the hot water, and the rate of heat exchange is related to the difference in temperature between the water and its surrounding environment. these two factors resulted in the water changing temperature much, much more slowly which allowed for more accurate data to be taken.
note: I was able to reach temperatures slightly above the boiling point of water at my altitude by adding large quantities of salt to the water.
at some point i will go back and take hot data points using this isolation technique.
these appear to be the same part:
- GP Sorensen Part Number: TSU81
- $11.98, http://www.partsamerica.com, 2007/12/30
- STANDARD MOTOR PRODUCTS Part # TX3T
- $11.07, http://www.rockauto.com, 2007/12/30
- STANDARD MOTOR PRODUCTS Part # TX3
- $16.23, http://www.rockauto.com, 2007/12/30
- Pacific Industrial Components part # 5615C
- $11.99, http://www.repairconnector.com, 2007/12/30