Resistance thermometer, thermocouple, or bimetal thermometer
In a previous article, we explained the technical differences between a resistance thermometer and a thermocouple. But how do you make the right choice?
Various factors are at play in choosing the right measurement method, such as potential temperature range, required accuracy, vibration tolerance, response time, necessary calibration, expected lifespan, lifecycle costs, purchase costs, and more. To take the first step, consider the following three factors:
- What is the temperature range that needs to be measured?
Temperature sensors can provide unreliable data if used outside their specified operating range. If temperatures above 600-700°C are expected, a thermocouple is recommended. - What is the desired accuracy of the measurement?
Generally, resistance thermometers (RTD sensors) are more accurate than thermocouples. If the temperature remains below 600°C, a resistance thermometer provides a more accurate solution. - Are there vibrations in the process (from machinery, piping, etc.) that must be accounted for?
Thermocouples are more resistant to vibrations due to their thicker wiring. Thin-film resistance sensors are also quite vibration-resistant and may be an option as well. - Does the temperature need to be read directly at the measurement location?
Bimetal thermometers display the measurement analogically, using a needle on a round scale. This reading is available directly at the location without an external indicator, but the measurement cannot be transmitted to a temperature switch, PLC, or DCS.
Thermowell or protection tube?
In addition to choosing the correct temperature sensor, it’s essential to protect it from the often harsh process conditions. Common protections are the protection tube and the thermowell. A protection tube is a simple tube with a welded bottom on one side and a process connection on the other. A thermowell, however, is a protection made from solid material. This choice depends on the process conditions where the measurement is conducted, ensuring reliable readings, extending sensor lifespan, and making sensor replacement easier.
But what exactly should you pay attention to when choosing the right protection? To take the first step, you can go through the following questions:
- What are the process conditions?
What pressure, temperature, flow rate, and medium can be expected? These factors help decide between a protection tube or a thermowell. For example, high pressure requires a thermowell because its solid construction reduces the risk of cracking. - What process connection is needed?
The connection type largely depends on the process conditions. The three most common methods are:- Threaded connection
- Flange connection
- Welded connection
- What are the appropriate materials?
Several factors influence the choice of material. Temperature is a primary factor; for instance, stainless steel can be used up to 900°C, but higher temperatures require materials such as Inconel, Incoloy, chrome steel, Hastelloy, and ceramic. It’s also essential to know the medium in contact with the thermowell and ensure compatibility with the piping. - What is the best profile for the application?
The three most common profiles are tapered, straight, and stepped. A tapered profile provides quick response and strong construction but creates relatively high flow resistance. A straight profile is also strong but has a slower response and creates significant flow resistance. A stepped profile offers lower resistance and faster response but is less durable than tapered or straight profiles.
There are additional considerations in selecting the correct thermowell, such as preventing issues like cracking or breaking. Common causes of thermowell problems include:
- Heavy forces (e.g., from flow)
- Excessive static pressure
- High temperatures
- Erosion
- Vibrations from the flow
The importance of high measurement accuracy and reliability
Even the smallest measurement deviation can disrupt the process and lead to high costs. Accurate, repeatable, and reliable temperature measurements are critical. Three points to consider:
- Identify performance factors
Key factors for an accurate measurement are accuracy, repeatability, and stability. Repeatability, or precision, indicates the consistency of repeated measurements under unchanged conditions. Accuracy shows how close the measurement is to the exact temperature value, and stability shows how consistent measurements remain over time. - Calculate measurement accuracy
The most common way to calculate measurement accuracy is through a method called Total Probable Error (TPE), which predicts errors based on expected installation conditions. - Use proven transmitter techniques
Advanced transmitter techniques are available, and the correct choice depends on various factors, such as susceptibility to environmental noise, voltage spikes, or temperature fluctuations.
