From choosing the right sensors for your particular process application to the periodic recalibration of the sensor, these tips can help you ensure temperature measurement accuracy and get the most out of your process.
1) Select the Correct Sensor Assembly for the Application
If the wrong type of thermocouple is purchased, its suprising how often the process efficiency is hampered by using the wrong thermocouple type. Factors such as time at temperature, degree of accuracy, thermal cycling rate and environemnt may impact the selection of the best thermocouple type to produce accurate, reliable and long-term performance.
For most applications with operating temperatures of 760°C or less, any of the base metal calibrations (J, E, T and K) will function, but all are not created equal.
Type J offers the widest range of environments, including vacuum, oxidising, reducing and inert atmospheres over the temperature range of 0 to 760°C. Above 538°C, however, the iron leg is susceptible to rapid oxidation, and below 0, the iron experiences rusting and embrittlement.
Type T is preferred for lower temperatures, including subzero -201°C, and is resistant to corrosion in moist atmospheres. In air or oxidising atmospheres, however, oxidation of the copper thermoelement occurs at temperatures above 370°C.
Type K is a good choice for oxidising and inert atmospheres up to 1,260°C. With better oxidation resistance than type E, J and T, Type K thermocouples are often used at temperatures above 537°C, although they are subject to green rot corrosion at
temperatures above 815°C in low oxygen environments. Type K thermocouples are not recommended for use in reducing
atmospheres or atmospheres that alternate between oxidising and reducing, in sulfurous atmospheres or under vacuum conditions.
Type N is becoming more popular over time as a replacement for Type K. Type N has the same temperature range, is comparable
in price, but does not suffer from the order-disorder temperature drift or green rot that can affect Type K thermocouples.
Type E exhibits the highest sensitivity of the base metal thermocouples and often is selected for this feature. These thermocouples function well in a broad temperature range from -330 to 1,600°F (-201 to 870°C) in oxidising or inert atmospheres. They also successfully resist corrosion in high moisture environments. Type E thermocouples are not recommended for reducing or vacuum atmospheres.
The noble metal thermocouples (S, R and B) and Type C are the best choice for very high temperatures or to increase long-term accuracy and repeatability, though they are more expensive. Selecting the thermal element is not the only consideration. Careful selection of insulation materials and sheath materials, being certain of how these materials may interact with each other, also is critical. Consult with your sensor manufacturer, providing all the details about your application environment, to ensure that you select the best assembly for your needs.
2) Ensure Adequate Immersion Length
One of the most common errors made in configuring a temperature sensor is not providing adequate immersion length. In a process pipe application, the sensor should extend into the process a minimum length equal to one third the inside diameter of the pipe, with the optimum location being the centre of the pipe diameter. When calculating this distance, be sure to add enough length to account for pipe and mounting flange thickness.
Another consideration is stem conduction. This refers to unwanted heat transfer up the metallic sheath of the thermocouple. To compensate for this, use a minimum immersion length of 7 to 10 times the diameter of the probe. Keep in mind that use of a thermowell will magnify the stem conduction effect.
3) Maximise Sensor Response Time by Reducing Mass
Sensor material mass equates to sensor speed of response. Lower mass yields faster sensor response. Most oven and furnace applications do not require meaty-sized sensor elements as the environments generally are benign and sensor sheathing is not needed. Use of thermowells and pipewells offers physical strength but increases mass, slowing sensor reaction time. In addition, bare junction thermoelements respond faster than closed-sheath elements of equal size. This is particularly important to consider during thermal ramp and cool time, as excessive material mass will hamper the sensor’s ability to keep track of the rapidly changing temperature.
4) Lower than Expected Readouts May Indicate Poor Construction
Another common problem in sensor performance is seeing a lower-than-expected output from the sensor. At low temperatures less than 121°C, this may indicate thermal shunting due to degradation of the cold-end seal. To check for this, measure the insulation resistance of the ungrounded thermocouple between the leadwires and the metallic sheath. The insulation resistance should be greater than 1 x 109 Ω at 500 VDC. Thermal shunting at lower temperatures also may be caused by moisture penetration of the cold-end seal.
Poor termination practices are another source of low readings. Stripping the leadwire too far or improperly applying insulation can permit moisture and contaminants to shunt the thermoelements, creating low insulation resistance. This can be corrected easily by cleaning the terminal blocks and ensuring that the insulation material covers all exposed wires except the portion immediately under the terminal screw heads.
5) Use the Proper Leadwire
Extension-grade wire offers a more restricted temperature range than thermocouple-grade wire. The extension wire generally follows the EMF/temperature curve as the thermocouple wire to a certain temperature, then veers significantly. If you use extension-grade wire beyond its limitations, you may introduce a significantly large error into your readings. For reliable readings, the thermocouple wire in the terminal head must be connected to the same thermocouple type wire all the way to the instrumentation that performs the cold junction compensation (readout, PLC, transmitter, etc.). Simply twisting signal wires together in the terminal head should be avoided, and use of a proper screw-type terminal block will provide long-term interconnection and avoid signal degradation over time due to poor connection and corrosion issues. For Types C, R, S and B, running thermocouple-grade wires would be prohibitively expensive, so proprietary alloy wires have been developed for use with these materials.
A common cost-saving scenario is to use copper wire rather than extension wire, but copper does not follow the EMF/temperature curve of thermocouples at all and therefore introduces a significant degree of error. The exception to this is Type B thermocouples, which may use copper/copper wire up to 212°F (100°C).
