# Can anyone identify this thermometer probe?

I've tried to use this probe to detect temperature from an arduino microcontroller.

Unfortunately, I can't seem to get a decent reading out of this. I need to know appropriate coefficients for steinharts equation if they are applicable to this type of probe. My attempts at finding these coefficients have failed with various measurements of resistance and temperature. It seems as though this isn't a NTC thermistor. Can anyone offer some guidance to interpret an analog reading from this probe?

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What resistance value did you measure between the tip and sleeve of the connector? – Theran May 7 '12 at 2:51
At room temperature the resistance was about 150k Ohms. When the temperature raised the resistance decreased to around 80k Ohms and lower. – Joe May 8 '12 at 19:12
The drastic change in resistance due to temperature made me believe this was a thermistor. However, using steinharts equation and plugging in the coefficients I found didn't yield realistic temperatures. – Joe May 8 '12 at 19:14
I think you'll be better off using the simpler B parameter equation. Since you're finding the calibration coefficients yourself, the fewer you need to find the better, especially since there are only two accurate temperatures that are easy to produce at home (ice bath and boiling water). – Theran May 8 '12 at 21:17

As Nathan mentions, it looks like a meat thermometer probe. Those probes are inexpensive, so it's probably a thermistor, or possibly a thermocouple, and not anything more exotic like a platinum RTD or a temperature sensing diode. If the resistance between the tip and sleeve of the connector is over a few ohms, it's almost certainly a thermistor. Otherwise it's either a thermocouple or an unusually low resistance thermistor.

If it's a thermistor, you'll need to use it as one half of a voltage divider. Since the change in resistance is usually only a few percent/degree C it's often necessary to rescale the value using an op-amp circuit before analog-to-digital conversion. To pick the gain and offset of that circuit, you need to consider the range of temperatures you wish to measure, and the input and output ranges of the op-amp.

For converting resistance to temperature, instead of Steinhart-Hart, you might be better off using the B parameter equation:

$\frac{1}{T} = \frac{1}{T_0} + \frac{1}{B}ln(\frac{R}{R_0})$

Note that $T$ and $T_0$ are absolute temperatures (Rankine or Kelvin). One of the calibration points will be $R_0$ and $T_0$, and you can find $B$ based on the other calibration point either algebraically or numerically (goal seek in Excel). $B$ tends to be around 3000 Kelvin for most NTC thermistors.

If it's a thermocouple, you need a high input impedance differential amplifier. Thermocouples create a small voltage and can't supply much current. For accuracy, you'll also need to compensate for the themocouple voltage created where it plugs into your device. Measuring the thermocouple signal is a bit more involved than a thermistor, but not out of the range of DIY. Many DMMs have a mode to read thermocouples, which could help verify that is indeed what you have.

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 Thanks Theran! I was able to get some legitimate values out of the probe using the B parameter equation. In order to get a few decent readings I used a glass of warm water and cold water with a second temperature probe. I put both probes in the water recorded the temperature and resistance. It worked great! – Joe May 12 '12 at 15:34

Could it be a thermocouple? If so, at a minimum you'll need to amplify its output ahead of the analog input. T'couples generate micro-volt levels. There are ICs designed specifically for thermocouple interfaces, from amplifier to complete temperature conversion devices. MAX6675 is one of the latter. Adafruit sells them on a breakout board ready to plug into your Arduino or breadboard.

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