Temperature measurement is a core part of most process industries. Broadly speaking, there are three major tools: thermistors, resistance temperature detectors and thermocouples. All have their own uses and potential pitfalls.
Thermistors are generally metal oxide resistors who's resistance is a function of temperature. It is not a linear response, not a wide response (-176 to 392F generally) nor is it a standardized one (aka different manufacturers will vary, whereas most PT100 RTDs will be the same). They will rarely be involved in industrial control systems, except at the component level.
These come as NTC and PTC devices. NTC thermistors have resistance that decreases with temperature. You may run into NTC thermistors when you look inside of your thermostat, or in the head of your 3D printer. In contrast, PTC thermistors have resistance that increases with temperature. These can be used as replacements for fuses in smaller, current limiting applications, in motor windings and in dry-type transformers.
A Resistance Temperature Detector (or RTD) is similar, in that it is a better version. The response is more linear, with a wider range (-200 to 1475 F) and very well standardized. A PT100 is a platinum based RTD with a resistance of 100 ohms at 0 C. A PT1000 is a platinum based RTD with a resistance of 1000 ohms at 0 C. (Titillating, I know.) There are articles on when to use a PT100 vs a PT1000 and the difference between the 2/3/4 wire RTDs.
Thermocouples act a little differently. Rather than change resistance, these work by connecting two dissimilar metals, which generates a small voltage across them. For low-accuracy applications, and/or those with vibration, they may be acceptable. You have a "reference" or "cold" junction, which adds voltage differences to each of the bimetal leads to the sensor in question. (More details can be found here or here.)
Common bimetallic pairs are iron and constantan (type J), chromal and alumel (K) and copper and constantan. If you have a quality installation, you should be able to tell the type without opening a thermocouple manual, as the wires are color coded. For example, J should be white/red, K should be yellow/red and T should be blue/red.
The selection of the type of thermocouple depends on the temperature range, accuracy requirements and the redox potential of the environment.
Integrated circuit sensors are very limited in their temperature range (typically not going above 250 C), but they do allow for very linear measurement, as well as your choice of signal output (4-20 mA, 0-5VDC, etc.)
If you have to select between the types of temperature sensors, check the tables in the PE exam book! In the 2022 edition, this would be pages 5 & 6.
I have a few practice questions. Scroll down for answers...
Practice question 1: What temperature would you have if you had a resistance of 106 ohms on a PT100?
Practice question 2: You have purchased a type K thermocouple from Omega. What temperature measurement would you expect if you measured 11.919V between the leads? (Use this chart.)
Practice question 3: Having reviewed your instrument, you now read 11.919 mV. What temperature do you expect?
Practice question 4: The system you are looking at requires the kiln to stay around 2500F. What sensor should you use?
Practice question 5: You are not using a PLC to control the system, but a small "smart" relay that accepts a 4-20 mA signal. The temperature range is 80-120 C. What sensor should you use?
Answers...
Between 15 and 16 C.
Something is clearly wrong with your setup. The voltage difference should be in mV.
293 C, assuming your reference junction is at 0 C.
A thermocouple, as it is the only one that can make that range.
An intergrated circuit sensor would be your best option (for the exam). In real life, you could use another sensor and a signal conditioner.
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