Temperature drop in Pressure measurement impulse lines 2

I, too, heard the rule of thumb 100 Deg (F or C, never clear) drop per foot of impulse tubing myth.

Then I ran into a site where someone had connected a draft range pressure transmitter to only a foot of 1/2" inconel impulse tube in a furnace running at about 1800 Deg F and the transmitter had been in service for some years without a problem. That short one foot length should have conducted enough heat by the rule-of-thumb to bake the transmitter's electronics.

The installation also included a sheet of galvanized steel between the transmitter and the furnace to prevent the furnace wall's radiant heat from damaging the transmitter. The transmitter body and impulse tube compression fitting was cool enough that I could put my hand on the transmitter body and the tube fitting.

So I went looking for an actual reference and I found Kulite's app note with the math and the temperature curves for impulse tubing temperature reduction.

Link

The last two paragraphs sum up the falsehood of 100 deg/ft myth:

"From the curves, for a given length of tubing, better isolation is obtained with steel, rather than other materials, such as brass or copper. Where frequency response is a consideration, use steel tubing, to minimize the length. At temperatures above 1000°F, brass and copper use is limited. Smaller diameter tubing requires shorter lengths, to isolate the transducer, for a given source temperature. A compromise must be made, between tubing diameter and length, and frequency response.

As a rule of thumb, one foot of steel tubing, any diameter, will isolate a transducer from any temperature."

Some caution, using a Ni alloy will give you the shortest, then SS, and then steel.
I worked in a plant where our rule was 12" for Ni and SS, 18" for steel. We usually used 3/8sch40 pipe for these.
IF\f the hot side was over 1200F we required radiation shields also.
At one time the maintenance manager had done tests with temp sensors to verify that this was adequate.
For 350C I would think that a 6" length of 3/8 SS pipe would be fine.

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P.E. Metallurgy

If the impulse line contains condensate, liquid or totally dry gas, 100 degrees C per INCH of tubing is likely accurate, unless you're using heavy wall large diameter tubing.

If the impulse line contains steam or other condensible gases, I've seen an impulse line 10 ft long which free-drained contrary to commonsense and which cooked a device at the far end. If there's no deliberate condensate trap U bend or pigtail, designed to condense liquid and keep it there, don't count on the line losing significant heat if the stream contains or might some day contain condensible vapours.

Many thanks guys, this is very useful.
Special thanks to danw2 for the enlightening link.