To ensure the longest possible life and correct readings, vacuum pressure gauges that have soft-soldered pressure joints mustn’t be exposed to processor close temperatures over 120˚F. This can be very true of Vacuum pressure gauges with liquid filled cases, because of the expansion of the case fill fluid.Long term exposure to temperatures in excess of 120˚ F may cause discolouration of dials and fill fluids, as well as hardening of the case seals and possible fill leakage.
Types of temperature sensor:
Today, two core types of temperature sensors are used across the industry.
- Thermocouples
Two metal wires are used in a thermocoupleto produce a voltage relative to the temperature present in the junction between them. There are many specialized kinds of thermocouples they can combine different metals to measure various characteristics and temperature ranges, and produce specialized calibrations.
- Resistance temperature detectors (RTDs)
AnRTD sensor measures temperature based on the resistance changes in a metal resistor inside. The most popular RTDs, called PT100 sensors, use platinum and have a resistance of 100 ohms at 0°C.
Thermocouples vs RTD
It is not practical to compare RTDs and thermocouplesgenerally. However, if we compare their performance in terms of specific criteria, we can see which is the best suited for specific applications.
Temperature range: For working at high temperatures thermocouples are best. New manufacturing techniques have improved the measurement range of RTD probes, but more than 90% of RTDs are designed for temperatures below 400°C. In contrast, some thermocouples can be used at up to 2500°C.
Cost: Thermocouples are generally cheaper than RTDs. An RTD can usually cost 2 or 3 times over a thermocouple junction with a similar temperature and style.
Savings can be made on RTDinstallation, which is cheaper as inexpensive copper wire is used. However, this saving isn’t enough to catch up on the higher device price.
Sensitivity: Whereas each sensor types respond quickly to temperature changes, thermocouples are quicker. A grounded thermocouple can respond nearly 3 times quicker than a PT100 RTD.
The fastest-possible temperature sensor is an exposed tip thermocouple. However, manufacturing enhancements have also greatly improved the response times of thin-film PT100 probes
Accuracy: RTDsare generally more accurate than thermocouples. RTDs have typically an accuracy of 0.1°C, compared to 1°C for most. However, some thermocouple models can match RTD accuracy. The many factors that may have an effect on sensor accuracy include linearity, repeatability or stability.
Stability:RTDprobe readings keep stable and repeatable for an extended time. Thermocouplereadings tend to drift due to chemical changes in the sensor (such as oxidation). RTDs’ linearity and lack of drift create them more stable in the long term.
Conclusion:
Thermocouples are more economical than RTDsbecause of their cheaper manufacturing method. Counting on the number of probes that you want for your application, this could be a serious factor. RTDs, on the other hand, gives a more reliable output. After carefully deciding the range and the performance needed, you can now choose the most suitable sensor type for your application.