International standard for thermowell


Thread Starter


Hi all,

I am working on temperature transmitters specifications for a project in Energy.

According to my process conditions, I choose RTD technology.
We are going to install threaded thermowell.

I used to work with:
- IEC 60751 (Industrial platinum resistance thermometers and platinum temperature sensors)
- IEC 61152 (dimensions of metal sheathed thermometer elements)
- and sometimes ASME PTC 19 3 (tmermowell) according to the flow
- our general / master specification.

Do you know an IEC or ISO standard (not an API, ASME etc.) for "standard thermowell insertion length" (as API 551) or unsupported length (as ASME PTC 19.3) ? I am not looking for the rule to specify this length but I am looking for a standard that specify some standard length according to the size of pipes (inch or DN) . Thank you for your comments,

19.3 has been superseded by 19.3 TW which is considerably more restrictive on allowable lengths.

It is completely contrary to traditional practice that emphasizes placing the thermowell in the central one-third of the pipe. It does this to avoid thermowell failure. Most of the well suppliers have codes to sort out the safe designs.

regarding measurement errors:

the dynamic error depends mostly on the mass of the tip section and the fluid velocity,

the static errors are a function of the pipe wall deficit (i.e. process temperation and pipe wall temperature). If you insulate the pipe, including the thermowell connection the static error is drastically reduced.

placement errors, involve measuring the temperature where the process profile is non-uniform. Herein lies the usual conundrum: if you have a process that is not at uniform temperature, then you and your process engineer have to define just what do you mean by temperature.

The traditional thermowell "rules" tend to be risky in terms of failure (fire, equipment damage, personnel hazards).
OK received, thank you for these comments.

So, no particular reference for some standard length of thermowell from IEC or ISO?


Mitch Johnson

Hi Bruno,

Probably the closest thing to what you are looking for is in the Process Industry Practices (PIP) standards, particularly PIP PCFTE100. That standard has suggestions such as: 4.5" U dimension for a threaded well installed at a perpendicular orientation into a 6" pipe.

That said, the point made by the previous person posting (try saying that 3 times fast) still has relevance. The recommendations by PIP PCFTE100 all contain a note requiring that the well meet criteria for construction in another PIP code, namely PIP PCETE001. And guess what that code requires?.... application of 19.3 (now superseded by 19.3TW-2010 since the last publication of the PIP standards was in July 2008).

So, if you are looking to get a good starting point for immersion lengths, the PIP standards are a great place to start. However, these proposed lengths should be evaluated against the 19.3TW standard to ensure that the design is mechanically sound. Get a copy of 19.3TW, check it out or use a free software like SwiftyCalc ( that applies 19.3TW to assess whether or not your proposed insertion length is mechanically sound.

Best regards,

[email protected]
Apart from Line Size, Thermowell lengths depends on fluid, velocity, point of installation, etc. For good measurement it is always recommended to calculate length as per the standards. Some times thumb rules are used but they will be for similar trains or units

Your key question is how to select thermowell length according to pipe size.

Unfortunately that is the traditional thermowell sizing method, and it often results in thermowell and sensor failures since the fluid velocity is largely independent of pipe size.

An increasing number of thermowell failures occurred since 1990 as a result of improved piping alloys that permitted increased fluid velocity and the financial pressure to use the most economical pipe size.

Use of traditional length selection methods, increase the risk of well failure. So if you use such methods, be certain that it is only in low velocity, low pressure, low risk services.

If you are dealing with off-shore platforms, LNG unloading, Medium pressure, High Pressure, and supercritical steam, lethal service, or steam turbine services, you would be wise to use the 19.3TW standard as a starting point. There have been too many fires and damaged rotating equipment to do otherwise.