FAQ summary of common temperature sensors
1. What is the difference between mineral insulation (MI) and prefabricated sheath?
The MI is flexible and the manufactured jacket is rigid.
2. How accurate is the temperature measured with a standard sensor?
Based on the published international tolerance, the common thermocouple is usually ± 2.5 ° C, and the PRT is ± 0.5 ° C. Higher precision sensors can be provided according to the order, such as ± 0.5 ° C for T-type thermocouple and ± 0.2 ° C for PRT. All of these values depend on temperature. Tight tolerance 4-wire PRT will provide the best absolute accuracy and stability.
3. How to choose between thermocouple and PRT?
It is mainly based on the required accuracy, probe size, response speed and process temperature.
4. My thermocouple is far away from my controller. Is this a problem?
It may be; Try to ensure that the maximum sensor loop resistance of the thermocouple and 4-wire PRT is 100 ohms. More than 100 ohms may cause measurement errors. Note that by using a 4-20mA transmitter near the sensor, the cable line can be longer and only cheaper copper wire is required. However, the instrument must be suitable for 4-20mA inputs.
5. What is the difference between RTD temperature sensor and PRT sensor?
No, RTD temperature sensor represents resistance thermometer detector (sensing element), PRT represents platinum resistance thermometer (whole assembly), that is, PRT uses RTD!
6. What is the Pt100 temperature sensor?
Industry standard platinum RTD temperature sensor conforming to IEC751 standard, with a value of 100 Ohms @ 0 ° C; This is used in most PRT components in most countries.
7. Should I choose K-type or N-type thermocouple?
Generally, type N is more stable than type K and usually lasts longer; According to the choice of sheath material, N is a better choice for high-temperature work.
8. Is the type of steel I specify for the thermocouple jacket important?
Usually not, sometimes. In some cases, reliability depends on the ideal choice of materials.
9. Are there other types of temperature sensors besides thermocouple and PRT?
There are several, but these two groups are the most common. Alternatives include thermistors, infrared (non-contact), traditional thermometers (stem and dial type), and many other thermometers.
10. Why are so many different types of thermocouples used?
They have been developed for many years and can adapt to different applications worldwide according to the temperature range.
11. What is a duplex sensor?
One with two independent sensors in one housing
12. Why use thermal sleeve?
Protect the sensor from process media and facilitate replacement when necessary.
13. I have used many thermocouples in tests and experiments. Can I make thermocouple connectors myself?
Yes, using a bench welder and thin thermocouple wires - making sheathless thermocouples is simple and cheap.
14. Why should the actual thermocouple connector be used instead of the ordinary electrical connector?
High quality thermocouple connectors use thermocouple alloy, polarized connection and color coded body to ensure perfect interconnection.
15. Why are 2,3 or 4-wire PRT probes provided?
Because all three met. The two-wire system should be avoided and the three wire system should be widely used. The four wire system can provide the best accuracy. Your instrument will be configured as 2, 3 or 4 wires.
16. For thermocouple cables and connectors, why are there two colors for the same calibration?
Since December 1998, the international color code of IEC 60584-3 shall be followed.
17. I need to measure the rapidly changing temperature; What type of sensor should I use?
Fast response (low thermal mass) thermocouples.
18. What is the minimum immersion depth of the PRT probe?
Generally more than 150mm; Increase the immersion depth until the reading does not change.
19. How to accurately measure the surface temperature?
Use a sensor specifically designed for this purpose by Villian or use an infrared (non-contact) sensor.
20. What is the actual difference between wound wire and film RTD?
The winding line provides higher accuracy and stability, but is vulnerable to impact; The film type has impact resistance and faster thermal response.
21. What do the thermocouple terms "cold junction compensation" and "linearization" mean?
For complete instructions, refer to this laboratory facility temperature manual. For most types of measuring instruments, these functions are applied automatically and do not need to be considered by users.
22. There are several different types of extension cable structures; Is choice important?
yes; Some are waterproof, some have higher mechanical strength, and some are suitable for high or low temperatures.
23. Is the sensor with calibration certificate more accurate than the sensor without calibration?
may not. However, the errors and uncertainties compared with the reference sensor have been published, and the corrected values can be used to obtain better measurement accuracy.
24. How long can my sensor last in this process?
Unknown but predictable in some cases; This will depend on the sensor type, construction, operating conditions and handling.
25. I need to use "fail safe" alerts in my process. Can my controller and alarm share the same thermocouple?
Using duplex sensors, one is connected to the controller and the other to the alarm. Your controller may contain alerts, in which case you need to rely on control sensors.
26. Which thermocouple type is required for my application?
This depends on several factors, including the nature of the process, the heating medium and the temperature.
27. What is the longest thermocouple I can have without losing accuracy?
Try to ensure that the maximum sensor loop resistance of the thermocouple and 4-wire PRT is 100 ohms. More than 100 ohms may cause measurement errors. Note that by using a 4-20mA transmitter near the sensor, the cable line can be longer and only cheaper copper wire is required. However, the instrument must be suitable for 4-20mA inputs.
28. Do you need power when using the transmitter, and how long can the extension cord run when installing the transmitter?
If the measuring instrument does not contain 24Vdc and 20mA power supplies, 24Vdc and 20mA power supplies are required. Long distance copper cable can be used.
29. What precision can the Pt100 detector achieve at a certain temperature; If better grade detectors are used, what impact will this have on accuracy?
Refer to the temperature manual for Pt100 temperature sensor tolerance information.
30. What sensors do I need to work in molten metal or corrosive environments?
There is no simple answer, but special grade stainless steel, 304 stainless steel, Niclobell and ceramics provide alternatives.
31. How much accuracy loss will be caused by using the transmitter online?
This depends on the accuracy of the specified transmitter; There will always be some degradation.
32. Since most instruments only need 2 or 3 wire Pt100s, if I make corrections on the 3 wire system and merge them into a single leg, can I achieve a 4 wire system?
no Cable length and ambient temperature changes come into play.
33. Can I still buy old BS color codes? Why was everything handed over to IEC?
Some companies can provide some products for "old" and outdated BS colors, but the current IEC standards are internationally recognized.
34. What is the difference between prefabricated thermal sleeves and solid drilled thermal sleeves?
The fabricated heat bag uses a welded structure to allow a relatively long immersion length; The thermal sleeve is made of solid materials.
35. If there is thermowell in my process; How long can my temperature sensor use?
An additional 50mm is used to compress the gland (if used) or probe length to fully insert into the well (if the thread is below the head).
36. What is the typical pressure rating of the thermal sleeve/thermal bag? What is the thermal response time of the thermowell?
It is usually dozens of bars and tens of seconds more than the sensor. However, refer to the full supplier specification - values will vary.
37. What is the difference between flat film and wound Pt100 components?
The thin film is deposited on the substrate using platinum; The spiral wound platinum wire in ceramics is used for winding. The winding line provides higher accuracy and stability, but is vulnerable to impact; The film type is impact resistant and
Faster thermal response.
38. If copper can be used at the same point on each leg of the thermocouple, can I use a copper connector on the thermocouple?
Yes, but only if both legs are kept at exactly the same temperature. Not recommended.
39. If I add two cables of the same length to the simplex thermocouple sensor of two instrument units, will I get the same reading as using the duplex sensor?
Yes, provided that the instrument input is a real potentiometer and no current is measured. Not recommended.
40. Why use insulated hot contact sensor on the instrument?
Eliminate the possibility of measurement error caused by grounding circuit, and reduce the risk of voltage rise of electric heater.
41. What is automatic cold junction compensation?
This is a characteristic of most measuring instruments, which allows the thermocouple input to be at a variable temperature rather than stabilized at 0 ° C.
Remember that the sensor type must always be suitable for the measuring instrument and use the correct extension cable. The information provided here is for general guidance only and is not conclusive - it is not intended to be the basis for product installation or decision-making.
42. Why use thermocouples?
benefit
● Temperature range: The most practical temperature range, from low temperature to jet engine exhaust, can use thermocouples. Depending on the wire used, thermocouples can measure temperatures from – 200 ° C to+2500 ° C.
● Robust and durable: thermocouple is a sturdy and durable equipment, free from impact and vibration, and suitable for use in hazardous environments.
● Fast response: Due to the small size and low thermal capacity of the thermocouple, the thermocouple responds quickly to temperature changes, especially when the sensing junction is exposed. They can respond to rapidly changing temperatures in hundreds of milliseconds.
● No self heating: since the thermocouple does not need excitation power supply, it is not easy to self heating and is intrinsically safe.
shortcoming
● Complex signal conditioning: a lot of signal conditioning is required to convert thermocouple voltage into available temperature readings. Traditionally, signal conditioning requires a lot of investment in design time to avoid introducing errors that will reduce accuracy.
● Accuracy: except for the inherent inaccuracy of thermocouples due to their metallurgical characteristics, the accuracy of thermocouple measurement is only as accurate as the measurable reference junction temperature, which is traditionally within the range of 1 ° C to 2 ° C.
● Corrosion sensitivity: because the thermocouple is composed of two different metals, in some environments, corrosion over time may lead to a decline in accuracy. Therefore, they may need protection; And maintenance is essential.
● Sensitivity to noise: noise of stray electric and magnetic fields may be a problem when measuring microvolt level signal changes. Twisted thermocouple pairs can greatly reduce magnetic field pickup. The use of shielded cables or routing in metal conduits and protective devices can reduce electric field pickup. The measuring equipment shall provide signal filtering, whether hardware or software, which has a strong ability to suppress line frequency (50 Hz/60 Hz) and its harmonics.
43. Thermocouple error
These errors are introduced by thermocouples. The voltage generated by the thermocouple is proportional to the temperature difference between the point where the temperature is measured and the point where it is connected to the equipment. Due to impurities in metals, the temperature gradient on the thermocouple line will introduce errors. Compared with most measuring equipment, impurities may be large. Refer to the thermocouple documentation for its impact on the overall measurement accuracy.
