Expert: Paul Soderman - 11/15/2000

Question
Dear Sir,

I am a university engineering student doing my final year thesis.  I am constructing a nozzle flow meter for high speed flow of nitrogen gas at cryogenic temperatures.

Is it possible to measure the static temperature of a gas flowing at high speeds inside a pipe?  Since high speeds are involved the static
and total temperatures can be fairly different.  The only way I know is to take measurements of the total pressure, total temperature and also of the static pressure, and then using gas dynamic equations to obtain the static temperature.

i.e.   Po / P  =  [To / T]^(k / k-1)              ^ - used to represent "power of"

Now the total pressure and total temperature would have to be measured using a pitot tube and thermowell respectively and the static pressure using a wall pressure tap.  I find this whole procedure too tedious.  Is there a way to set up a thermometer that reads the static temperature for high speed gas flow directly.

Thank you for any assistance you can give me.  

Answer
The only way to measure static temperature in a pipe would be to have a thermocouple mounted flush with the pipe wall.  This may be problematic if there is some temperature gradient between the inside and outside of the pipe since you are essentially measuring the temperature of the pipe wall.
If there is a gradient, the outside of the pipe must be well insulated.  Of course, this technique is only valid for steady-state flows after sufficient time for the pipe wall to reach an equilibrium temperature.