A thermocouple is a device that measures temperature. It comprises two different metal cables connected to the thermometer on one and joined at the other. Properly configuring the thermocouple enables you to attain accurate temperature measurements over a wide range.
S type Thermocouples act as versatile temperature sensors and are ideal in plenty of applications. For example, you can find them in regular utilities and appliances or even in industrial scenarios. However, the wide range of technical specifications and models requires you to select a thermocouple based on:
- How it works
- Basic structure
- Its range
How a Thermocouple Works
A thermocouple will function based on its differential calculations of known temperature points. The cold or reference junction plus the probe are connected to the unit you want to measure. Also, the voltages generated when you connect dissimilar metals are always constant and known.
Furthermore, the conditions often require a constant artificially generated temperature. This ensures that the reference points are founded on their relation to the measuring junction. However, any different metal combinations will always produce different temperatures.
Choosing a Thermocouple
Before buying the first thermocouple you come across, you need to consider how it functions. This enables you to select a one that will accurately measure temperatures. Below are a few considerations to make before picking a thermocouple:
Surface Probe
Many types of temperature sensors have a difficult time measuring temperatures on solid surfaces. For an accurate measurement, you need a sensor that will maintain a connection with the surface. This can be arduous, especially when dealing with a rigid surface and sensor.
However, because thermocouples are made from pliant metals, the junction should be thin and flat to offer the best contact. As a result, such thermocouples are ideal for surface measurement. More so, they can be built on a rotating mechanism to make them ideal for measuring moving surface temperatures.
Thermocouple Probe
The thermocouple probe contains a wire that’s housed in a metal tube. The tube’s wall acts as the probe’s sheath. The most common sheath materials include:
- Inconel
- Stainless steel
Inconel is ideal for higher temperature ranges compared to stainless steel. Stainless steel is better due to its broad chemical compatibility. Furthermore, other exotic sheath options work for high temperatures.
You can select the tip of a thermocouple probe depending on three styles. These are:
- Exposed
- Grounded
- Ungrounded
You should use exposed junction thermocouples when conducting air measurements. The grounded tips are in constant contact with the sheath wall. Therefore, having the grounded junction provides the fastest response time, but it’s susceptible to electrical ground loops.
The ungrounded junctions indicate that the thermocouple isn’t in contact with the sheath wall. Instead, it’s separated from the wall by an insulation layer. Therefore, the tip will have to protrude with an exposed junction outside of the sheath wall.
Beaded Wire Thermocouple
This is the most common form of a thermocouple. The beaded wire thermocouple consists of a welded bead that joins two wires. Unfortunately, the welded bead is always exposed, thus leading to a few limitations.
You should not use the beaded wire thermocouple in liquids. That’s because they can oxidize or corrode the alloy. Some metallic surfaces are problematic. These surfaces include pipes that ground electrical systems.
An indirect connection to electrical systems can also impact the measurement of the thermocouple. Therefore, beaded wire thermocouples are ideal for measuring gas temperature. Furthermore, due to their small size, they can provide you with a fast response time.
Types of Thermocouples
You will find lots of thermocouples in different calibrations or metal combinations. However, the base metal thermocouples are the most common, and they include:
- K
- J
- E
- T
- N
You will also come across noble metal thermocouples or high-temperature calibrations that include types:
- GB
- C
- S
- R
All calibrations have different temperature environments and ranges. However, the maximum temperature will vary depending on the thickness of the cable that’s present in the thermocouple. On the other hand, the thermocouple’s temperature range is affected by the calibration.
The thickness of the thermocouple cable will also impact the maximum temperature range. Therefore, a thin thermocouple might not get to the complete temperature range. K-type thermocouples are the ideal options for most applications because of their wide temperature range and low cost.
