Frequently Asked Questions about Temperature Measurement

Frequently Asked Questions about Temperature Measurement
Q: How many feet of thermocouple wire can I use?
Answer: For specific devices, please check their specifications to see if there are any limitations on input impedance. But based on experience, MAX limits the resistance to 100 Ω, and this depends on the wire gauge; The larger the diameter, the smaller the resistance, and the longer the distance for signal transmission. However, if there is electrical noise in the environment, a transmitter capable of transmitting 4-20 mA signals is needed, which can transmit over longer distances and has stronger noise resistance.

Question: I should useGrounding probestillNon grounded probe？
Answer: It depends on the instrument. If there is any opportunity for a grounded reference (usually in an uninsulated input controller), a non grounded probe is required. If the instrument is a handheld instrument, a grounding probe is almost always used.

Q: What type of relay do I need to use to control myheater？
Answer: This must be calculated using known parameters. Obtain the total power of the heater and divide this value (in watts) by the rated voltage of the heater (in volts). The answer obtained will be in amperes, and solid-state and mechanical relays will be calibrated with "rated current" in amperes.

Q: Can I send the 4-20 mA control output to the chart recorder to detect process inputs?
Answer: It's not possible. Control output is dedicated to controlling valves or some equivalent control devices. If you need to send analog signals to a recording device, please select a controller with the option of "retransmission or recorder output".

Q: Can I split a thermocouple signal into two independent instruments?
Answer: It's not possible. Thermocouple signals are very low-level millivolt voltage signals and should only be connected to one device. Dividing into two devices may result in reading errors or signal loss. The solution is to use a "dual" thermocouple probe, or use a transmitter or signal conditioner to convert one thermocouple output into a 4~20 mA signal; Then the new signal can be sent to multiple instruments.

Q: What are the precision and temperature range of various thermocouples?
Answer: These data have been summarized in the table on page 11. It is important to understand that accuracy and range depend on the following factors: thermocouple alloy material, measurement temperature, sensor structure, sheath material, measurement medium, medium state (liquid, solid, or gas), and thermocouple wire diameter (when exposed) or sheath diameter (when the thermocouple wire is not exposed and has been armored).

Q: Why can't I use any multimeter to measure the temperature of the thermocouple? What kind of error will it cause if I don't use a thermocouple thermometer?
Answer: The magnitude of thermoelectric voltage depends on the closed (sensing) end and open (measuring) end of a specific thermocouple alloy head. The temperature sensing instrument using thermocouples will consider the temperature at the measuring end to determine the temperature at the sensing end. Most millivolt meters do not have this function and cannot perform non-linear adjustments to convert millivolt voltage measurements into temperature values. You can use a lookup table to correct specific millivolt voltage readings and calculate the sensed temperature. However, the correction value needs to be constantly recalculated because it is usually not constant and will change over time. The small temperature changes at the measuring instrument end and sensing end will change the correction value.

Q: How do I measure temperaturethermocouple、Resistance Temperature Detector (RTD)Thermistors andInfrared equipmentBetween choices?
Answer: You must consider the characteristics and costs of various sensors and the instruments provided. In addition, it should be considered that thermocouples can usually measure temperatures over a wide temperature range, are low-cost and very durable, but not as stable as RTDs and thermistors. RTD is stable and capable of measuring temperatures over a wide range, but not as durable and affordable as thermocouples. Due to the fact that RTDs require current for measurement, they are prone to inaccuracies caused by self heating. Thermistors are more accurate than RTDs and thermocouples, but their temperature measurement range is more limited. Thermistors are also prone to self heating. Infrared sensors can be used to measure temperatures higher than any other device and can be measured without direct contact with the measuring surface. However, infrared sensors are often not accurate enough and are sensitive to surface emissivity (more precisely, surface emissivity). Infrared sensors can use fiber optic cables to measure surfaces that are not within direct line of sight.

Q: What are the two commonly overlooked considerations when choosing infrared temperature measurement equipment?
Answer: The measurement surface must be completely within the field of view and surface emissivity must be considered.

Q: What is a good way to solve the problem of electrical noise?
Answer: 1) UseLow noise shielded wireConnectors and probes. 2) Use instruments and connectors that can suppress EMI and RF radiation. 3) Consider using analog signal transmitters, especially current transmitters. 4) Assess the possibility of using digital signals.

Q: Can I still measure temperature if the component is moving?
Answer: Sure. useInfrared equipmentOr direct contact sensors with slip ring components.

Q: Can the two-color infrared system be used to measure low emissivity surfaces?
Answer: It can only be used in high temperature environments, that is, environments above 700 ° C (1300 ° F).

Question: IfInfrared pyrometerWhat kind of error will occur if the diameter of the light spot is greater than the target diameter?
Answer: This will be uncertain. This value will be a weighted average and may not be reproducible.