High pressure / Low pressure DV Test

[derek perry]

Hi
High pressure / Low pressure DV Test
I was taught and have taught the following for 30 years

High pressure test
Turn cylinder on and check contents gauge.
Turn off cylinder watch gauge for drop in pressure. If no needle movement then you have tested for leaks, while the DV is under high pressure. If you have a leak it could be either the Low or High pressure side of the DV

Low pressure test
Purge valve to zero. Breathe in on each 2nd stage and if there is resistance with no leakage, you have tested for leaks while the DV is under low pressure. Any leaks during this test would mainly be on the Low pressure side.


Another experienced instructor was recently on a course where the national instructor taught the following

Turn on cylinder and check pressure.
Turn off cylinder and partly purge DV.
Look at contents gauge
If it doesn't go up or down then it is OK and you have completed both low and high pressure tests.
He was told that if the needle went up there was a high pressure leak.
If it went down there was a low pressure leak

I have argued that the needle going up will only indicate that there is a leak on the cylinder valve into the DV 1st stage and therefore you are not testing the DV at all. The contents can't go higher as it is at the pressure shown on the gauge, where would the additional pressure come from?

A needle going down will indicate a leak in the DV while under pressure and not a low pressure test.

Any thoughts


Derek

[PeteM]

Any thoughts

I agree with you, you can not do a low pressure test with a system pressurised to a high pressure.

Only thing I do different to you is I breath down the system from high to low pressure rather than purge it whilst watching the guage, that way you can also test for a sticking needle

[derek perry]

:=Any thoughts

I agree with you, you can not do a low pressure test with a system pressurised to a high pressure.

Only thing I do different to you is I breath down the system from high to low pressure rather than purge it whilst watching the guage, that way you can also test for a sticking needle

Yes sorry I do same because you can also taste the air at the same time and check the 2nd stage is flowing alright

Derek

[Andy Wade]

:=:=Any thoughts
:=
:=I agree with you, you can not do a low pressure test with a system pressurised to a high pressure.
:=
:=Only thing I do different to you is I breath down the system from high to low pressure rather than purge it whilst watching the guage, that way you can also test for a sticking needle

Yes sorry I do same because you can also taste the air at the same time and check the 2nd stage is flowing alright

I also breathe it to empty and suck hard on the mouthpiece as a test for the exhaust valve leaking.



.

[J Abbott]

I also breathe it to empty and suck hard on the mouthpiece as a test for the exhaust valve leaking.



This also checks the integrity of the diaphragm.

If you suck too hard you can pull the exhaust valve into the 2nd stage body and cause a leak.

John

[Andy Wade]

:=
:=I also breathe it to empty and suck hard on the mouthpiece as a test for the exhaust valve leaking.
:=
:=

This also checks the integrity of the diaphragm.

Yes, that too.

If you suck too hard you can pull the exhaust valve into the 2nd stage body and cause a leak.

Indeed that is possible.
Maybe I should have said 'a little harder'.
Basically, if the air is turned off and you can still breathe off the valve, (even if it's only a tiny amount) then you have a leak somewhere, the exhaust valve seating or a hole in the diaphragm are likely.
The bottom line being that if you use it for a dive, you'll be getting water in your mouth with each breath.





.

[Philip Smith]

Hi
High pressure / Low pressure DV Test
I was taught and have taught the following for 30 years

What I was taught and teach:
Inspect cylinder O-ring (or regulator O-ring, if DIN) and regulator hoses, contents guage and mouthpiece for damage (e.g. blisters on hoses, splits in mouthpiece) before connecting to cylinder.

Inspect sintered gauze inlet to first stage for signs of rust from cylinder interior.

Open cylinder valve briefly to blow out any dust or water from the orifice.

Fit regulator to cylinder; pressurise gently; check pressure.

Breathe deeply from 2nd stage watching contents gauge needle for movement (dropping and rising with each breath) indicating a restriction in the HP air flow (e.g. clogged inlet, cylinder valve not fully open/faulty).

Close cylinder valve, watch for fall in gauge pressure indicating HP or IP leak (usually HP leak at contents gauge swivel or cylinder O-ring).

Breathe pressure down to zero gauge pressure then try to inhale gently (shouldn't be possible). Leak likely to be due to exhaust valve not seating, but could be hole in diaphragm or mouthpiece, or leak in 2nd stage body. Sometimes direct feeds don't seal when not connected.

Turn on HP valve again before use!

If it doesn't go up or down then it is OK and you have completed both low and high pressure tests.
He was told that if the needle went up there was a high pressure leak.
If it went down there was a low pressure leak

The HP could only go up if the cylinder valve was leaking. It could go down due to a leak on the HP or IP side.

Philip Smith