There are several types of thermocouples on the market, each best suited for a specific environment. Your choice should be informed by the desired characteristics and the area where you wish to use the thermocouple. This article looks at some popular types and characteristics to help you make the right selection.
The temperature Range for the Thermocouple
The gadget sensor may give unreliable readings if it is used outside the manufacturer’s specified temperature range. Here is an example. If the temperatures of the environment where the device is to be used are above 700 degrees Celsius, you are required to use a thermocouple.
There are several types of thermocouple range and each has their recommended working temperatures. Type D can work at temperatures above 1700 degrees Celsius, while types K and N have a maximum working temperature of 1200 degrees Celsius. On the other hand, RTD sensors work best at temperatures under 600 degrees Celsius.
Do You Need the Thermocouple to Be Grounded?
A grounded thermocouple has a welded junction to the sheath wall of its sensor tip. This enables better heat transfer, improving the response time when the temperature changes. However, the grounding can cause an electrical loop that may damage the equipment.
On the other hand, ungrounded thermocouples have insulation between the sheath wall and junction. The insulation material is non-conductive and reduces the change in the ground loop. This makes its response time slower.
Do You Need to Have an Exposed Junction?
Exposed junctions are found outside the sheaths. This allows RTDs and thermocouples to provide fast response times when making measurements. However, the junctions can be oxidized by water, given that the metals used are iron alloys. These devices are used in applications that pose a minimal risk of damage to the measuring tip.
Where Do You Want to Use the Sensor?
Are there any vibrations in the process? If the vibrations are significant, you need sensors with better resistance to the conditions. Opt for the thin film resistance ones or pick devices with thick wires. On the other hand, if the environment is highly corrosive, go for devices with non-exposed probes. Grounded and ungrounded devices can be used in high-pressure and corrosive environments. In addition, check the specifications of the equipment where you wish to install the thermocouple. Determine the best size and wire thickness of the temperature probe to use.
How About the Accuracy?
Some applications require a high level of accuracy, while others will do with an error of a few degrees. Applications with accuracies of between 0.5 and 5.0 degrees Celsius can work with thermocouples. However, a platinum resistance sensor is ideal if you need high accuracy below 1.0 degrees Celsius.
What about the Surface Temperature?
Thermocouples are basically tip sensing tools, which enables them to be used in surface and immersion applications. When measuring temperature changes in immersed applications, you should always take care to prevent false readings from the heat collected by the probe metals. Such devices need a thicker, non-conductive sheath.
On the other hand, for surface temperature measurement, ensure that the probe is exposed or uses a welded junction form with a small mass to conduct heat. Check the construction of the thermocouple to ensure contact with the required surface.
Cold Junction Compensation
As you make temperature measurements or during calibration, be sure to account for cold compensation. This is where voltage is subtracted or added from the output of your device. This creates a reference point of 0 degrees Celsius, even if this is not the case.
It is difficult to reach 0 degrees Celsius in a normal operating environment and room. This may cause you to calculate the output temperatures rather than read them off the display. Buy a device that can be calibrated accordingly.
Sheath Materials
The choice of sheath materials depends on the working environment and temperatures where you will be using the device. Popular materials include a recrystallized refractory oxide, silicon carbide, mild steel, stainless steel, and Inconel. The first three materials are good for high temperatures.
A thermocouple’s choice depends on the working environment and temperature range where it will be used. Besides, its construction, the process where it is used, metal choice, covering material, and measurement range also determine the best device for your application.
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