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realpars.com/thermocouple
⌚Timestamps:
00:00 - Intro
00:15 - I) Range and accuracy
02:28 - II) Styles of thermocouples
04:41 - A) Cold junction compensation
06:20 - B) Remote temperature monitoring
08:10 - Examples
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There are many different types of thermocouples. Most manufacturers have selection guides to help you decide what to purchase.
In addition, most reputable manufacturers will also have technical support specialists available that will walk you through a series of questions to help select just the right type of thermocouple for your application.
You’ll need to consider things like the range of temperatures you are trying to measure, and the accuracy you would like to have. This will help narrow down the two different metal wires that you need to use. The type of thermocouple is based on the type of metals used in the sensing wires.
A Type-K thermocouple is made from Chromel and Alumel alloy wires. It can be used to measure temperature from around -330 °F to over +2460 °F.
You need to keep in mind that the accuracy of a Type-K thermocouple may only be about ±5 °F across the entire range. Sometimes that’s fine, but sometimes more accuracy is needed.
If you were looking at a cryogenic application that needed more accuracy, but you didn’t need to worry about high temperatures, you could consider a Type-T thermocouple.
A Type-T thermocouple is made from a copper wire and a Copper-Nickel wire. Type-T thermocouples are usually accurate to within a degree or two... so that makes them about twice as accurate as Type-K thermocouples.
Type-T thermocouples can typically measure even lower than -330 °F but the upper end of the range is usually just over 600 °F. They are more accurate but have a more limited measurement range.
Many applications for temperature measurement can get away with just using thermocouple wire with the ends twisted together.
In cases where a little more protection is desired, the wires are encased in a probe, which is just some kind of protective sheath and some insulating material to help protect the wires.
Thermocouple probes come in Ungrounded, Grounded, and Exposed Junction style. The style of thermocouple probe you select will also be based on your application.
The thermocouple signal is based on the difference in temperature between the hot and cold junctions. But we need to make a small signal correction to convert that temperature difference into an absolute temperature.
Fortunately, most measurement instruments can perform cold junction compensation... either with standard built-in equipment or with an optional cold junction compensation add-on.
Cold junction compensation components measure the temperature of the metal at the cold junction wiring connections and then make a signal calculation correction.
If you need to extend the wiring of the thermocouple, you must use something called a thermocouple extension wire in order to reduce the amount of error.
A thermocouple extension wire is constructed of the same types of metal as the thermocouple, so when you need to extend a circuit, a thermocouple extension wire of the same type as the thermocouple must be used.
Similarly, if you need to use terminal blocks to connect the wiring, the terminal blocks must be constructed of the same types of metal as the individual wires.
It is not recommended to run thermocouple extension wire over a long distance due to the sensitivity of the signal to electrical noise.
If you are trying to go more than 50 to 100 ft, you need to consider using a temperature transmitter that converts the millivolt signal over to another signal type such as 4-20 mA.
If you have multiple temperature measurements to make in a remote location, a remote I/O rack from your PLC might end up being a good option.
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#RealPars #Thermocouple #Industry
1 июл 2024
