Measuring Temperatures

Given what QPID is supposed to do, measuring temperatures is a fundamental capability. There are a variety of ways to electronically measure temperatures, but the most common methods employ either thermocouples or thermistors.

Thermocouples can be extremely accurate, but given the tiny voltages involved, they require special amplifier circuitry in order to be read by a typical analog-to-digital converter. And to make things more complicated, you also need to measure the temperature where the probe is connected to the circuitry since the voltage is dependent on the difference in temperature. Hmm, seems like a lot of work.

Thermistors are much easier to use than thermocouples. Their resistance changes with temperature, so a simple voltage divider circuit with a second resistor provides an ideal signal for the analog-to-digital converter. Their response is quite non-linear, so a lookup table is generally needed to convert readings into actual temperatures, but that's just software (which of course is easy).

Turns out the probes on my old ET73 unit are thermistor probes, so that got me thinking. They're relatively inexpensive (replacement probes can be purchased for about $10), use a standard 2.5mm plug, and they're designed specifically to measure the temperature range of a smoker. Best part is, I already have some. So I decided to use them for Project QPID.

Of course, I still needed a lookup table for the probes in order to use them. One option would be to heat or cool the probe to a known temperature and measure its resistance, but that's not easy to do without special equipment.

I realized that the ET73 unit had to know the lookup table data, I just needed a way to get that information out. Using a variable resistor hooked up to the ET73, I adjusted the resistance so that a given temperature was read. The resistor was then disconnected from the ET73 and measured using a digital multimeter. This process was repeated in 10 degree increments from 50 to 370 F degrees, thus providing a table of temperatures and corresponding resistance values.

On a long shot, I decided to also email Maverick Industries to see if by chance they could provide me with a lookup table for their probes. Suprisingly, they were very accomodating and emailed me a complete lookup table provided by their probe vendor. Many thanks to Maverick Industries for being so cool! :-)

So, how did my reverse-engineered lookup table stack up against the probe vendor's? Pretty damn close (typically with a degree or two).