Imagine a differential pressure gauge with its H port connected to an air chamber and its
L port to atmosphere. Air chamber pressure is constant at say 500 Pascals and the gauge indicates 500 Pa.

If the tube to the H port was constricted, for example by a variable clamp, what effect would this have on the
gauge reading?

Thinking caps on!

Well, my 25 years old A level Physics knowledge makes me think it’ll go up slightly, as the volume in the entire system has reduced slightly with the constricting of the clamp.

IANAFDE

If the tube to the H port was constricted, for example by a variable clamp, what effect would this have on the
gauge reading?

constricted but open?

All our base are belong to you?

i think you would need to know the flow rate through the clamp

Are you changing the volume of the air chamber significantly?

I’d have to have it explained in words of less than two syllables why the pressure wouldn’t stay roughly the same if you’re not changing the volume…

the pd, would be measured between H at 500 Pa and the pressure just the other side of the clamp.
You could treat this like an orifice late or venturi meter, but work backwards, if you know the flow rate, you can work out the pressure drop across the contraction

There are no dynamics in your question. i.e. you’ve described a static system.

Nothing happens (other than the possibility that your constriction has changed the volume of the overall system by an appreciable amount).

Out of interest is the chamber at 500 Pa (i.e. a fairly good vacuum) or is it 500 Pa above atmospheric? Your questions says one thing and the gauge says another.

There’ll be no flow as such across the constriction – the hp side of the gauge block should be a dead head against the internal diaphragm/piston, otherwise the thing wouldn’t work.

Would you expect a difference if the hp inlet tubing was 10mm rather than 6mm? Assuming the clamp only restricts the hp inlet and doesn’t close it, I reckon you may see tiny fluctuations in pressure as you adjust the clamp, but ultimately it will settle out and there will be zero difference in the displayed pressure differential.

The tube is constricted by say half.

The chamber is at +ve pressure.

I think the airborne ox has it.

Any other thoughts?

If it’s a static system, with no air flow out then both will read the same pressure.

As above, it might flutter when the clamp is tightened/loosened, but it would balance itself again.

If squeezing the tube has only a momentary effect, is there any way I can simulate a pressure drop – i.e. kid the gauge that the pressure in the chamber has decreased?

(I’m worried about how stupid my question could make me appear!)

Without knowing exactly what equipment you have available or what kind of gauge you’re working with, you could:
a) lock in the pressure from the air chamber and then vent from the HP side of the gauge,
b) increase the pressure in the LP side

so there is no flow in the system? then pressure each side of the clamp is equal so pd across….
yeah Ox has it

TFO – b) sounds perfect.

Thank you all.

If squeezing the tube has only a momentary effect, is there any way I can simulate a pressure drop – i.e. kid the gauge that the pressure in the chamber has decreased?

you could flatten the the tube over a length near the inlet, then nip the tube at the chamber side, then unflatten the tube