April 8, 2020

theatrus' Perfect Temperature Controller - Temperature Sensors

Let’s talk temperature sensors, at least let’s talk about the guts of a temperature sensor and less about the housing, cables, etc.

There are three main varieties you’ll find in your typical “probe” style sensor, and a fourth I’ll mention because its common for integrated circuits.

I’m considering “relevant reefing temperatures” to be about 75F to 83F - sorry, I haven’t switched to metric on temperature just yet, even though I reference them below. Its a hard habit.

Thermocouple

Thermocouples are a sensor made of two dissimilar metals welded at one point. The dissimilar junction produces a small voltage due to the thermoelectric effect. However, you won’t see thermocouples used for temperatures for reefing jobs, as the sensors are hard to make precise under a few degrees F. Thermocouples excel at high temperature measurements (1000C or higher) - and since they are actually a voltage source can be used in stack arrangements called a thermopile, commonly used as a safety valve sensor for natural gas appliances with pilot lights. We won’t discuss them further, as they’re not an ideal sensor type.

Diode sensors and digital sensors

Most digital sensors, such as the DS18B20 type or even the sensors on your CPU are of the diode band-gap kind. These are effectively voltage references which means the voltage drop (not resistance, its a diode) which has been selected for temperature sensitivity. They’re common on digital electronics for the sole reason that they are effectively free to fabricate on a silicon die and require no special extra processes to do so. With careful calibration, they can be accurate, but are designed around digital circuits.

I also suspect a large number of Chinese probes with the DS18B20 sensors are actually using a compatible but non-Maxim part, and in my experience do not meet the stated accuracy.

As its a digital sensor and requires logic and code to read, I’m going to rule out using this sensor family.

RTD / Platinum RTD

This is one of the two main classes of resistive sensors, in that the resistance of the sensor will change in response to the temperature change. Unlike the other class of sensor (the thermistor), an RTD is mostly linear - a 1 ohm resistance change is nearly the same across all of the same sensor type (Platinum) and nearly across the entire temperature range. RTDs also have a wide operating range from -50C to 500C.

RTDs are usually considered the more expensive choice, and if you get a Class A sensor, the reference accuracy choice due to its stability - the element is an encapsulated platinum wire and there isn’t much that can go wrong with it, except for mechanical damage (or melting).

The Achilles heel of a platinum RTD sensor is the usual low resistance - at 25C, its a mere 100 ohms for a typical sensor. This means system inaccuracies such as the wire and the connections can matter for accuracy, and long wire lengths are usually ruled out. There are compensation schemes available for this, including three and four wire RTD cabling, where one or both legs of the sensor has an extra wire run in parallel, and the measuring circuit also measures the resistance of this wire to compensate for the cable resistance. This provides an added benefit of being able to detect an open or shorted RTD (or more likely to) by checking for conductivity on the 3rd/4th wire pair.

Thermistor / NTC Thermistor

Thermistors are the other kind of resistive temperature sensor, made up of a proprietary mix of a various metal oxides, ceramics and polymers in a powder form, baked or sintered together to produce a material with large resistance swings over temperature. Thermistors have the lowest temperature range (up to about 130C), but for our application that is more than adequate. Thermistors also come in two flavors, PTC (Positive Temperature Coefficient) and NTC (Negative Temperature Coefficient). Most temperature sensors are made of the NTC material: as temperature goes up, resistance goes down.

Thermistors have a few problems of course. Every thermistor material is different, so they are actually hard to interchange as the curves the temperature follows are different from one type to the next. There are some common values, but its hard to know which you have without further information. The response is also not linear, and can change from over 15% per-degree resistance change to about 5%, depending on how far away from the center of the curve you are. One saving grace thermistors have for our application is that their curves are usually much flatter around 20-25C than near freezing, so circuitry does not need as extensive of compensation in the small span we consider the sweet spot.

What to choose?

So, thats a lot of words. Which to choose? Due to the differences in the sensors (a thermistor is nominally 10k ohm, an RTD 100 ohm), its often not feasible to simply support both.

My initial circuit design I’m going to focus on using platinum RTDs. Why? - Linear response (though thermistors would be well suited since we only require operation in a very narrow band of temperatures) - Grade A RTDs are considered a reference standard once calibrated, and are very stable over time. - Common availability of three/four wire RTDs opens an interesting possibility of determining sensor health and connectivity.

PRTDs are going to be more expensive, and I have no ruled out a thermistor design, or simply making a board with both analog front ends if the cost is not prohibitive.

Using PRTDs does require some special measurement setups, which I will get into next. If you want to do some “light reading” this guide is very well done over the nuances of the subject:

http://www.ti.com/lit/an/sbaa275/sbaa275.pdf

© 2020 Yann Ramin