This series of blogs was first published on the late, lamented forum, Microcontroller Central. I have modified it very slightly. In my opinion the best place to start whenever you discuss temperature measurement is an Agilent (actually it dates back to HP) application note, which gives a great background to the whole subject. So take time to read it before continuing here because I don’t think I could say it any better.
Finished? Let me say that I disagree with some of the statements in the app note. On page 3 the list of advantages and disadvantages for each approach differentiates between the RTD and the thermistor when both are simply temperature dependent resistors. You can get 2, 3, or 4 wire RTDs and you can even connect a 4 wire as a 2 or 3 wire. Nothing is stopping you from connecting 3 or 4 wires to a thermistor either just as long as your input circuitry caters to it.
The RTD can also suffer from self-heating depending on how you drive it and we will get to that as well. Another minor quibble is that Agilent talks about IC Sensors, when you could just use a diode junction instead. I am not saying that it is better, but it implies that you can only use ICs.
There are one or two other techniques. The first is a pyrometer which measures radiation (normally infra-red) and deduces the temperature from that. Often this is nothing more than a lens that focuses the radiation onto a thermocouple and so it has a familiar interface. A thermopile is made up of several thermocouples normally connected in series to increase the voltage output. It is commonly used in natural gas burners to detect the flame as part of the safety arrangement so that no gas flows if there is no flame.
My impression of the pyrometer technique is that it is rather inaccurate based on the fact that the radiation is affected by the color, size, shape and distance from the object. I got this impression from use of instruments like the Fluke 65. I had thought that this approach was quite esoteric. I came across the TMP006 from TI which appears to integrate a thermopile into a PCB mounting package. The user’s guide provides some more information. Our own Victor Lorenzo pointed out that Melixis makes a similar device MLX90614 as well as GE. Seems there may be a bigger market than I anticipated.
A variation on the temperature dependent resistor is the bolometer which is a blackened metal plate connected in a Wheatstone bridge configuration. It absorbs heat which unbalances the bridge in relation to the radiation. Apparently it can be quite accurate.
Omega is a well known supplier of sensors for the process industry and produces some great catalogues with plenty of information. The temperature sensor you end up choosing is often determined by the application. For instance, if you are measuring the temperature of a furnace, you will probably be limited to a thermocouple because of the temperature range. The cost evaluation should be based on a system approach and not just on the sensor itself.
As a case in point, the thermocouple normally will need a second temperature sensor for its cold junction compensation. Also if you have a narrow enough temperature range you may not have to linearize your readings. Transferring the heat from the source to the temperature sensor can also be challenging. How do you affix it? Do you need to improve the thermal conductivity with thermal compound and how long will it take to reach thermal equilibrium? All are non-trivial factors that must be considered even if not readily apparent.
Temperature is one of those physical properties that held up as proof that the real world is analog and is also a measurement that is made very often. Temperature normally changes very slowly and so there are no high speed demands on the electronics. The first stage of any temperature measurement design is to decide at which point to convert to digital, if at all. Dedicated hardware for thermocouple and RTDs with an analog output abounds and has been around for many years. Before you dismiss the approach as passé, consider this: Sometimes the sensor can be in rather an inhospitable environment and it would really be nice to have the digital electronics in a friendlier place.
Of course you can convert the signal to digital and send it by some protocol, but there are single chips that will condition the temperature to an analog signal like a 4-20mA analog current loop. Many, if not most, industrial plants use this technique extensively and is my employer’s bread and butter. See Figure 1.
This is an extensive topic and I am going to drag it out over several parts, so this seems like a good place to stop. Next blog I will deal with analog conversion techniques before moving on to ADCs and micros along with digital sensors. Watch this space for more! In the meantime, any comments?