Processors often have questions when it comes to measuring temperature in their facilities. This article aims to answer several of the most frequently asked questions heard by our engineers.
Q: Why can’t I use any multimeter for measuring temperature with thermocouples? What errors will
result if I do not use a thermocouple temperature meter?
A: The magnitude of the thermoelectric voltage depends on the closed (sensing) end as well as the open (measuring) end of the particular thermocouple alloy leads. Temperature sensing instruments that use thermocouples take into account the temperature of the measuring end to determine the temperature at the sensing end.
Most millivoltmeters do not have this capability, nor do they have the ability to do nonlinear scaling to convert a millivoltage measurement to a temperature value. It is possible to use lookup tables to correct a particular millivoltage reading and calculate the temperature being sensed. However, the correction value needs to be recalculated continuously as it generally is not constant over time. Small changes in temperature at the measuring instrument and the sensing end will change the correction value.
Q: How do I choose between thermocouples, resistance temperature detectors (RTDs), thermistors and infrared devices when measuring temperature?
A: You have to consider the characteristics and costs of the various sensors as well as the available instrumentation. In addition, thermocouples generally can measure temperatures over wide temperature ranges inexpensively and are rugged, but they are not as accurate or stable as RTDs and thermistors.
RTDs are stable and have a fairly wide temperature range, but they are not as rugged and inexpensive as thermocouples. Because they require the use of electric current to make measurements, RTDs are subject to inaccuracies from self-heating.
Thermistors tend to be more accurate than RTDs or thermocouples, but they have a much more limited temperature range. They also are subject to self-heating.
Infrared sensors can be used to measure temperatures higher than any of the other devices and do so without direct contact with the surfaces being measured. However, they generally are not as accurate, and they are sensitive to surface-radiation efficiency (or more precisely, surface emissivity). Using fiber optic cables, they can measure surfaces that are not within a direct line of sight.
Q: What are the two most often overlooked considerations in selecting an infrared temperature-measuring device?
A: The two most often overlooked considerations when selecting an infrared temperature-measuring device are:
• The surface being measured must fill the field of view.
• The surface emissivity must be taken into account.
Measurement errors can result if these factors are not taken into account.
Q: What are the best ways to overcome electrical noise problems?
A: There are several methods to overcome electrical noise problems.
• Use low noise, shielded leads, connectors and probes.
• Use instruments and connectors that suppress electromagnetic interference (EMI) and radio frequency (RF) radiation.
• Consider using analog signal transmitters, especially current transmitters.
• Evaluate the possibility of using digitized signals.
Your sensors and controls supplier also may be able to suggest others once he evaluates your specific conditions.
Q: If a part is moving, can I still measure temperature?
A: Yes. Use infrared devices or direct-contacting sensors, plus a slip-ring assembly, to measure the temperature of moving parts.
Q: Can a two-color infrared system be used to measure low emissivity surfaces?
A: A two-color infrared system can only be used to measure low emissivity surfaces if you are measuring at high temperatures ― say, above 1,300°F (700°C).
Q: What error will result if the spot size of the infrared pyrometer is larger than the target size?
A: It would be indeterminate. The value would be a weighted average that would not necessarily be repeatable.
To learn more about temperature sensing, consult a reputable sensors supplier.