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Temperature Measurement in Small Tubes
Measurement of temperature in small diameter tubes


What is the recommended way to measure temperature of water (300 degC at 200 bar pressure) in a small tube of inner diameter 12mm (1/2 inch)?

In my view, an RTD or thermocouple with thermowell would require a reducer/expander of some sort to be welded to the tube. Is there a better way to do this?

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Spray paint the tube black with 'exhaust' spray paint and use IR to measure the skin temperature.

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@David: The tube is to be insulated since the process temp is quite high. Any other great ideas?

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A common technique is to spotweld a sensor tab (RTD or T/C) to the tube surface, insulate and bring the leads out to your terminations.

You need an ungrounded sensor to minimize common mode noise pick up do to ground potentials.

You can also use the IR approach mentioned earlier with fiber optics, as long as the fiber is suitable for the temperatures.

You can also use a tubing tee for a custom designed sensor installation.


We have this application before, and you can have a surface PT100, RTD.

Cheap Flow meter, China flow meter manufacture

I just stumbled across this.

Siemens has a TO500 multipoint temperature measuring system. It has a separate electronic box and a measuring lance with up to 48 temperature measuring points.

The technology is some derivative of fiber optic technology. The temperature range it can measure is stated as 40 to 800 Deg C

It is apparently contact technology because of a deliberately ambiguous statement in the color brochure: 'chemical resistant'.

The only output is Profibus DP.

I haven't a clue how big the lance is (what it can fit into), how the lance seals from the insertion, or what media it is compatible. Maybe Siemens knows.

I guess what you are getting at is the difficulty of finding small fittings rated at 3000 psi (200 bar).

You can certainly strap a sensor to the outside and insulate it.

I don't know why the thermo-well manufacturers don't make wells to fit inside a standard tee fitting. It would seem to be such an easy thing to do. For example, a solid piece of 3/8" Stainless with the ferrule machined onto it could easily have a hole drilled for a 1/8" RTD or thermocouple.