A Practical Guide to Improving Temperature Measurement Accuracy and Sensor Life
It’s not just a matter of selecting a thermocouple based on the temperature range involved. Using the wrong sensor can result in inaccurate readings, a reduced service life, unexpected periods of downtime, and greater maintenance costs. Since each application has its own particular operating conditions, it is important to understand these before choosing a thermocouple if reliable performance and long-term value are to be achieved.
The guide looks at ten common mistakes made by engineers, maintenance crews, and OEM manufacturers when specifying sensors, and provides practical advice on how to avoid each of them — ranging from choosing the correct thermocouple types and sheath material to avoiding the installation errors and accuracy failures that reduce the service life.
1.Selecting the incorrect type of thermocouple
Thermocouples aren’t designed with every application in mind. Although type K is the most commonly used, it isn’t always the best choice. Better performance might be obtained with type J, T, E, or N, depending on the operating temperature, the atmosphere, and the required accuracy.
Points worth evaluating before specifying a type:
- Temperature range for the intended process
- Oxidizing versus reducing atmosphere
- Typical industrial applications for each thermocouple type
2.Disregarding the process environment
A thermocouple which works well in a clean laboratory can fail rapidly when used inside a furnace, chemical tank, or during a heat treatment process; the environment surrounding it usually causes more damage than the temperature does.
Environmental factors that deserve attention include:
- Moisture
- Corrosive chemicals
- Vibration
- Abrasion
- High pressure
- Steam and washdown conditions
3. Chosing the Incorrect Sheath Material
The sheath provides protection for the sensing element, but if the wrong material is chosen then the life of the sensor can be greatly reduced.
Common sheath materials include:
- SS304
- SS316
- SS321
- Inconel® 600
- Incoloy® 800
Different materials work best under different chemical and thermal conditions and it is essential to match the sheath material to the process media if long and reliable service is to be obtained from a probe.
4. Selecting the Incorrect Junction Style
While many buyers concentrate on the temperature range they tend to neglect the measuring junction, even though the type of junction has a direct effect on the sensor’s daily performance.
The three common styles are:
- Grounded junction
- Ungrounded junction
- Exposed junction
There is a compromise between response time, electrical isolation, and durability. With an exposed junction, the response time is the best but the protection is the least; in the case of an ungrounded junction, some speed is given up in order to achieve better isolation and durability in harsh or electrically noisy environments.
