Q: 5. What are the common terms used on thermowell datasheets and what do they mean?

Thermowell dimensions / scale drawing: EL (sometimes L) = Installation length (of the thermowell) is how far the well will extend into the process once threaded into the process connection. BL = Bore length is the length of the hollow inner part of the well from the opening to the tip. This will be slightly shorter than the overall length due to the thickness of the well at the tip. Datasheet / technical details: Design for temperature sensors EL = recommended installation length of the temperature sensor probe. Sensor connection = recommended process connection of the temperature sensor in order to adapt to the internal thread of the thermowell. Process connection = thermowell’s external thread size or sanitary connection for mounting in the process. When determining compatibility between a thermowell and temperature sensor, consider the following sensor specifications. Sensor dimensions and datasheet: EL on the sensor data sheet should match Design for temperature sensors EL on the thermowell data sheet. Process connection on the sensor data sheet should match Sensor connection on the thermowell data sheet.

Question 1

1. What is a NPS thread?

Accepted Answer

An NPS thread is a version of the popular NPT thread but without a taper. The taper of an NPT thread does two things: it makes a mechanical joint between the two components and creates a fluid tight joint without a seal. The NPS thread is a parallel thread similar to the common “G” BSPP threads. Since the internal threads of thermowells are only used to mechanically hold the temperature instrument, a fluid seal is not needed. The major benefit of the NPS thread is that it is dimensionally equivalent to both the ½” NPT and G½” BSPP threads. This means that those thermowells can accept either a ½” NPT or G½” adapter.

Question 2

2. What is the response time of ifm temperature sensors when thermowell mounted?

Accepted Answer

When mounting a temperature probe into a thermowell, the response time is greatly increased.
Typical T09 (90% of temperature change) response time for ifm RTDS and thermowell assembly is:

20 seconds when assembled using thermal grease
40 seconds when assembled dry (no conductive material added)

Question 3

3. What if the probe does not touch the end of the thermowell?

Accepted Answer

When an air gap exists between the probe tip and thermowell, response time is increased. Adding thermal grease will greatly minimize the increased response time. For applications where thermal grease is needed, ifm can supply thermal grease packet part number UT0063.

Question 4

4. What are the differences between a thermowell and protective tube?

Accepted Answer

Ifm offers both thermowells and protective tubes for temperature sensors, but the products are all listed under the term "thermowell". The main difference is that a thermowell housing is constructed of one piece of metal with no welds, while a protective tube is multiple pieces welded together. As a result, protective tubes have a lower pressure rating than thermowells.

For further information, check out the Technology and performance page.

Question 5

5. What are the common terms used on thermowell datasheets and what do they mean?

Accepted Answer

Thermowell dimensions / scale drawing:

EL (sometimes L) = Installation length (of the thermowell) is how far the well will extend into the process once threaded into the process connection.
BL = Bore length is the length of the hollow inner part of the well from the opening to the tip. This will be slightly shorter than the overall length due to the thickness of the well at the tip.

Datasheet / technical details:

Design for temperature sensors EL = recommended installation length of the temperature sensor probe.
Sensor connection = recommended process connection of the temperature sensor in order to adapt to the internal thread of the thermowell.
Process connection = thermowell’s external thread size or sanitary connection for mounting in the process.

When determining compatibility between a thermowell and temperature sensor, consider the following sensor specifications.
Sensor dimensions and datasheet:

EL on the sensor data sheet should match Design for temperature sensors EL on the thermowell data sheet.
Process connection on the sensor data sheet should match Sensor connection on the thermowell data sheet.

Question 6

6. How is the G 1/2 sealing cone process connection considered sanitary with a thread? And why can't a temperature sensor with a G 1/2 sealing cone be used with a thermowell that has a standard G 1/2 sensor connection?

Accepted Answer

The G1/2 sanitary sealing cone is a special profile that uses a standard G1/2 thread. It includes an additional feature of a tapered sealing system that can make a sanitary “metal to metal” or “metal to polymer” seal ensuring a contamination free connection.

Although the thread is a standard G1/2 profile the G1/2 sealing is not compatible with standard G1/2 thermowells. This is due to the longer length of the sealing cone that interferes with standard mechanical thread systems.

As depicted in the image below, the smooth stainless steel sealing cone is excessively long and bottoms out prior to the G 1/2 threads of the temperature sensor reaching the G 1/2 internal threads of the thermowell. Since the sensor and thermowell cannot thread together, the temperature sensor will not be securely held in place mechanically. Additionally, the temperature probe does not reach the bottom of the well, thereby preventing efective heat transfer and leading to inaccurate temperature measurements.

Frequently asked questions about the use of thermowells