Is temperature correction a really good idea in orifice FB ?

[Lapin]

Hello
First post on the forum for me. Will show my bench soon

I'm asking myself about the real effect of temperature on measurements, with an orifice-type flowbench.

My reasoning :
The actual flow corresponding to a pressure loss depends on air density
But this is true as for the measuring orifice as for the head port.
Density is a global multiplicator in all point loss and in-line loss formulas.

Thus, the ratio of pressure loss of head and orifice remains the same whatever temperature is.
It shouldn't have any effect on the readings during a test
Am I right ?

Talking about reality now : to guys using an orifice FB with water manometers : did you see a difference on readings while testing the same part at different temperatures or barometric pressure ?

Thanks
Olivier

[Tony]

Yes Oliver you are quite right.
And welcome to the Forum.

We need an accurate and repeatable method of measuring "flow potential" so we can derive a flow figure that is meaningful.
The only way that is possible is to take temperature and pressure variations right out of the picture in a way that is self correcting.

A really crude example of this same principle would be, measuring a steel engine block with a steel ruler. We know that If both block and ruler expand with increasing temperature by the same amount, we can just take the measurement and ignore ambient temperature.

By passing the exact same air through a calibrated orifice, and the piece under test, we can compare the pressure drops across each, and that effectively removes air density and temperature from the measurement.

That is half of it.
The other half is making the measurement against a given test pressure across the test piece, which we need to adjust before taking a flow reading.

If these two requirements are met, the ratio of pressure drops across orifice and test piece will remain constant, and we can then compute a flow figure based on our known calibrated orifice.

did you see a difference on readings while testing the same part at different temperatures or barometric pressure ?

Denser air would create larger pressure drops across both, but the ratio will remain the same.
When you then correct the test pressure back to where it should be, that will also bring the orifice pressure back to where it should be, and there will be no difference in the final measured pressures with changes in ambient air density.

Our expensive steel ruler we used to measure our engine block will only be strictly correct at one exact specific temperature. But we can measure our engine block at any temperature and assume that is what it would be at the same temperature our ruler was calibrated at.

Likewise, our calibration orifice in our flow bench will only be strictly correct at standard sea level pressure and temperature. But we can use it anywhere and then assume our final flow measurement is also corrected back to standard seal level temperature and pressure (whatever that is).