By: Ahmad Riahi
Applications of thermocouples
Thermocouple Suffering: Thermocouples use two different metals to produce a phenomenon called the “thermoelectric effect”. This means that the sensor generates voltage when the temperature varies from one end of the thermocouple to the other. The device then translates that voltage into the numbers we can read.
(J) –Iron vs Constantan (Most Common)
May be used in vacuum, oxidizing, reducing and inert atmospheres. Heavier wire gauges are recommended for longevity above 1000 ° F because the iron element oxidizes rapidly at these temperatures.
(T) –Copper vs Constantan (Most Common Cold)
May be used in vacuum, oxidizing, reducing and inert atmospheres. It is corrosion resistant in most atmospheres. High stability at sub-zero temperatures and its error rate at cryogenic temperatures are guaranteed.
(K) –Chromel vs Alumel (Most Common Real Hot)
Recommended for continuous use in oxidizing or inert atmospheres up to 2300 degrees Fahrenheit (especially above 1000 degrees Fahrenheit). Below 1800 degrees Fahrenheit (1000 degrees Celsius), it is not recommended due to the change in EMF from the effects of hysteresis. It should not be used in sulfur reducing or oxidizing atmospheres unless it is protected by protective tubes. It is very reliable and accurate at high temperatures.
E) –Chromel vs Constantan)
It may be used in oxidizing or inert atmospheres, but is not recommended for intermittent oxidizing or inert atmospheres. It is not exposed to corrosion in most weather conditions. It has the highest EMF in any degree compared to other standard thermocouples and must protect the sulfur atmosphere.
(S, R) –Platinum vs Platinum Rhodium (Most Common Real, Real Hot)
Recommended for use in oxidizing or inert atmospheres. Barley depletion can cause grains to overgrow and scatter in the calibration.
(N) –Nicrosil vs Nisil (New… Better Than “K”)
May be used in oxidizing atmospheres, dry reduction or inert atmospheres. Must be protected in sulfur atmospheres. Very reliable and accurate at high temperatures. You can replace T-type thermocouples in many applications.
W) –Tungsten vs Rhenium)
For use in vacuum, high purity hydrogen or pure inert atmospheres are recommended. It may be used at very high temperatures (2316 ° C), yet it is inherently brittle.
Properties of thermocouples
TYPE E-negative wire has less resistance in ohms per foot than positive element for wire of the same size.
TYPE J-Positive element is often rusty and magnetic. For wires of the same size, it has less resistance in ohms per foot.
TYPE K- The negative element is slightly magnetic. For equally positive wire, it has less resistance in ohms per foot.
TYPE R or S-negative wire is softer. Positive wire has less resistance in ohms per foot for wires of the same size.
TYPE T-negative wire is silver in appearance. Positive wire has less resistance in ohms per foot for wires of the same size and usually has a copper color.
TYPE The N-positive foot is more resistant to ohm wire in the same foot.
Note: When in doubt, wind the wire and connect the opposite end to the voltmeter. Heat the twisted end with a cigarette lighter. If the voltage goes up – the polarity is correct
Note: To determine the error limit in degrees C, multiply the error limit in degrees F x 9.5.
:Construction details of thermocouple element
For high temperature applications from 1000 degrees Fahrenheit to 2300 degrees Fahrenheit, new proprietary materials have been developed to perform better than previously used alloys. U = HOSKINS 2300:
Maintains special accuracy up to 10 times higher than probes made from other cables.
“V = NICROBELL:” Sheathed Type N can be used to replace platinum / rhodium sensors up to a maximum continuous temperature of 2280 ° F … “
Thermocouple temperature limit
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