Question 1

How is response time for a temperature instrument specified?

Accepted Answer

“T05” and “T09” refer to the reaction time in seconds for the sensor to measure a change in temperature, typically 0…100 °C. T05 and T09 are the times the measurement take to reach 50% and 90% of the temperature change.

Response time as shown on our datasheets are for instruments directly inserted into the process using water as the medium. If thermowells are used, the response time will be affected. Heat transfer grease can be used inside the thermowell to improve the response time.

Question 2

Why would I use a thermowell in an application?

Accepted Answer

Thermowells provide:

Ease of instrument inspection without opening the process system.
Higher pressure rating than probe alone.
Resistance to high flow, viscosity, and abrasive media.

Matching thermowells can be found on the Accessories tab of the instrument's datasheet.

Question 3

Why do some manufacturers make spring-loaded probes?

Accepted Answer

When the probe of an RTD is spring-loaded, consistent contact between the RTD, probe and process is guaranteed. Because ifm's tip design ensures precise placement of the RTD every time, spring-loading is not necessary for repeatable results.

Question 4

How does thermowell mounting affect response time, both with typical spring-loaded probes and ifm's tip design?

Accepted Answer

As seen in the image below, using thermal grease to improve heat transfer between the RTD and the process improves response time. Note also that testing has shown that ifm's tip design provides faster response time than spring-loaded probes.

Question 5

What is the advantage of an integrated display on the instrument?

Accepted Answer

The value displayed is the actual process temperature and it is unaffected by programming or scaling. It provides a reference value to confirm correct system configuration.

Question 6

ifm’s temperature instruments use RTDs. What is the difference between an RTD and a thermocouple?

Accepted Answer

RTDs (Resistance Temperature Detector) change a very predictable amount of resistance for every degree change in temperature. ifm offers both Platinum (Pt) 100 and Platinum (Pt) 1000 RTDs. Pt100 devices change 0.385 ohms per °C and Pt1000 devices change 3.86 ohms per °C. RTDs are capable of measuring into the 600 – 700 °C range, and they provide an absolute measure of temperature. They are stable over a long period and are generally more accurate than thermocouples. RTDs use standard instrument cabling and do not need exotic alloy thermocouple wire. The overall total cost of ownership for RTDs is less than an installed thermocouple system.

A thermocouple consists of two dissimilar exotic metals joined together. The joined contact point produces a sensitive millivolt signal that varies as a function of temperature. Thermocouples are capable of high temperature measurement (1500+ °C), but only provide a relative temperature difference between the tip and leads. Thermocouple wire is required to connect them, and they can be sensitive to electromagnetic noise.

FAQs - TN temperature sensor