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Step 4What is a Thermistor?
A thermistor is a resistor that changes resistance with the temperature. Using the techniques from the Voltage Divider to determine the Resistance, we can tell what temperature it is.
The problem is that I have some cheap thermistor in a casing from a 10 year old product. How was I suppose to create a function to go from a resistance to a temperature? Well, I have a Thermometer that it plugs into!
So I measured a lot. I copied the temperature down and then measured the resistance of the Thermistor. I placed it in the fridge, then I place it in warm water. Later I would grab room temperatures since I had time.
I supposed I could have read the Wikipedia plage can tried to guess the a and b factors, but I assumed I was using a non-linear, possible failing component that no longer matched it manufacturing specifications. Oh and I am lazy.
So I dumped all the values into Excel, and then graphed it.
I was originally worried that I had to remember the deep dark math of something like "least squared fit" when I discovered that Excell will do this for me!
Clearly I am missing gaps on the graph, but I got lots of good data around hot tub temperatures (100-105F).
While checking the room temperature ranges I noticed something, that almost makes my work worthless. The "Precise Temp" thermometer reported a 3-7 degree error between "indoor" and "outdoor" when the Thermistor was inches away! Now this might be because I mixed and matched thermistor between units, but I am betting it has more to due with quality of a 10 year old, $10 item&
Regardless of the "accuracy" of the temperature, I needed precision and multiple tests of the same range showed very close results over days. In the long run I will likely attach the thermistor to a pipe leading into the hot tub, so I will need an offset anyway.
So by getting Excel to display the equation, I then put it into the code, and so far it is "close."
The problem is that I have some cheap thermistor in a casing from a 10 year old product. How was I suppose to create a function to go from a resistance to a temperature? Well, I have a Thermometer that it plugs into!
So I measured a lot. I copied the temperature down and then measured the resistance of the Thermistor. I placed it in the fridge, then I place it in warm water. Later I would grab room temperatures since I had time.
I supposed I could have read the Wikipedia plage can tried to guess the a and b factors, but I assumed I was using a non-linear, possible failing component that no longer matched it manufacturing specifications. Oh and I am lazy.
So I dumped all the values into Excel, and then graphed it.
I was originally worried that I had to remember the deep dark math of something like "least squared fit" when I discovered that Excell will do this for me!
Clearly I am missing gaps on the graph, but I got lots of good data around hot tub temperatures (100-105F).
While checking the room temperature ranges I noticed something, that almost makes my work worthless. The "Precise Temp" thermometer reported a 3-7 degree error between "indoor" and "outdoor" when the Thermistor was inches away! Now this might be because I mixed and matched thermistor between units, but I am betting it has more to due with quality of a 10 year old, $10 item&
Regardless of the "accuracy" of the temperature, I needed precision and multiple tests of the same range showed very close results over days. In the long run I will likely attach the thermistor to a pipe leading into the hot tub, so I will need an offset anyway.
So by getting Excel to display the equation, I then put it into the code, and so far it is "close."
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