Archive for the ‘Pressure regulators and flowmeters’ Category

Pressure regulators and flow meters.

They are either a bull nose fitting or pin index fitting, in the UK at least the large cylinders use a bull nose connection and smaller cylinder pin index.

It simply refers to the means by which the regulator is connected to a cylinder.

Bull nose has a thread on it. For some reason that has never been sorted out and I have never understood, you can connect an oxygen regulator to an air cylinder and an air regulator to an oxygen cylinder, which creates the possibility of thinking you are giving someone oxygen when in fact you are giving them air.

The pin index, on the other hand, is a system that protects against that and other gas errors. On the yolk there are two pins that line up with holes drilled in to the pillar valve on the cylinder. Different gases have different configurations, so air will only fit on an air cylinder, oxygen on an oxygen cylinder, nitrous oxide on a nitrous oxide cylinder and so on.

Below is my photo of an oxygen pin index yolk. You can see the two pins, next to those is the bodok seal that seals the inlet.


Below a bull nose regulator.A basic bull nose pressure regulator made by Ohmeda. (The label with ‘O2’ on it is missing from the end of the bonnet)bull nose O2 regulator

The pressure gauge.

pressure gauge (1)

Inside the back of the pressure gauge showing the bourdon tube, the curved tube that the pressure goes into, as it pressurises it tries to straighten out, in doing so it moves a rack and pinion that is under the brass bit in the middle which moves the pointer.

pressure gauge inside (1)

Check for leaks by turning on the cylinder and pressurising the system (make sure that the flowmeter is turned off), turn the cylinder off.
Watch the needle on the pressure gauge, it should remain where it started, if there is a leak it will gradually drop down towards zero.
Check for the site of the leak with soapy water, it will bubble at the point of the leak.
Also check the pillar valve of the cylinder. If you can’t find the leak unscrew the back plate of the pressure gauge and check inside there.
If it is coming out of the safety blow off valve it is called creep and is caused by a faulty valve seat, being an internal problem if you have no spares you will have to live with it.
If it gets really bad you will know all about it with a harmless bang and you wont be able to use the unit. If you have a small leak and are unable to cure it for what ever reason,
turn the cylinder off when not in use to prevent gas being wasted.
This may sound rather feeble but is probably a reality.

Hopefully most leaks will only be a matter of tightening a joint.
If you hear a popping noise when the flow meter is turned on and the ball bounces up and down in the flow meter it is called motor boating (after the noise of an engine).
This is caused by dirty or worn valve parts inside, again without the spares there is little you can do about it. It is not a problem except in as much as it is a noisy nuisance.
Do not unscrew the flow meter outer cover while there is pressure in the system, there will be 60 p.s.i inside and it will give you a nasty fright.

To test the flow meter.

1. Pressurise the system.

2. Turn on the flow meter and check that you get a good flow to the top of the tube.

3. Turn the flow meter down to say 5 lpm and put your finger tightly over the outlet of the tubing nipple so the gas cannot come out, the ball should drop to the bottom and show no flow, (because your finger has cut it off).

4. If there is still a small flow with your finger over the end, or indeed a flow when the flow meter is turned off. You have a leak. Check for the leak with soapy water.

1. Look at the outer tube for cracks.

2.Check that it is tightly screwed up. put soapy water around the base of the outer tube it will bubble if not screwed up tightly. If it is tightly screwed in place unscrew it. Look very closely at the threads, they sometimes get damaged, check the ‘O’ ring that the tube seats on is in place and not damaged.

3. Check that the tubing nipple is properly screwed into place. brush on soapy water to check (with your finger still over the outlet).

4. Check around the gland nut.

Should the ball still be floating at a couple of lpm  with everything turned off it may be caused by static electricity in the inner tube.

1. Turn off the cylinder and release the system pressure.

2. Unscrew the outer tube.

3. Take out the inner tube taking care not to lose the ball. You can get anti static spray, but simply handling the tube will often discharge the static.

4. Put the inner tube back making sure the numbers are facing the front. Replace the ball.

5. Screw on the outer tube tightly, turn on the cylinder to pressurise the system, turn on the flow meter and leave it running for a few moments. Turn it off and check the ball falls to zero.

Flow meter showing inner and outer tubes and tubing nipple. Made by Therapy equipment Ltd.


Showing inner tube and ball.


Showing filter and two ‘O’ rings, the small one for the inner tube and the larger one for the outer tube.


Most regulators are set to 60 p.s.i.
The pressure regulators under anaesthetic machines are quite often the same type and suffer the same problems.
They are set to 50 to 55 p.s.i. This is because the wall supply is normally set to 60 p.s.i and it prevents cylinders draining out (if the anaesthetic machines is plugged into the wall supply) and back feeding the hospital oxygen pipeline system.

Some faults sometimes found.

1. Unable to get the full flow.
a. Check and clean the needle valve.

b. Outlet partially blocked. un-block.

c. Low cylinder pressure. fit the unit to a full cylinder.

d. Regulator pressure adjusted very low. adjust to the correct pressure.

2. A small flow showing when the needle valve is turned off.
a. There may be a crack in the flow tube outer cover.

b. The outer cover may not be screwed on properly.

3. Ball bounces to a popping noise.
a. This is called motor boating and is caused by a dirty valve seat.
(see section on Anaesthetic machines page for repair details).

Maintenance of the regulator.
This refers to the British Oxygen regulators, many other makes will operate in the same way, but there are variations that you will come across.
For more detailed instructions on regulator maintenance see the section on pressure regulators.
They all do the same thing, and that is to reduce the pressure from what is in the cylinder down to a safer level.
Some of the older regulators reduced the pressure down to 12 psi, this is fine for some uses, but is not enough pressure to run many ventilators that are pneumatically driven so now the pressure is reduced to about 60 psi.
This can be done by two sorts of regulator, the 2-stage regulator and the single stage regulator.
The single stage regulator is like the sort used to give oxygen to a patient, and the type that you will see under anaesthetic machines, the pressure is reduced from the cylinder pressure, in one stage to the final output pressure, about 60 psi.
The two-stage regulator also reduces the cylinder pressure, but it will reduce it to about 150 or 200 psi. the operator can then reduce it to what pressure they need between there and 0, this is done by adjusting a large knob on the front of the reducing valve.
The single stage ones do not have this knob, to adjust their pressure you need an Allen key to adjust the screw in the nose of the bonnet.
The two-stage regulator is often used to power drills and other orthopaedic instruments in the theatre where you need a higher pressure and perhaps an easily adjustable one.
Check pressure regulators and flowmeters as follows
1.Inspect the exterior for damage.
10. Change ‘O’ ring or bodok seal if necessary.
11. Turn on the cylinder and make sure the gauge rises to show the expected contents.
12. Turn off the cylinder check for leaks, watch pointer for two minutes, if the pointer falls more than 1.5 mm there is a leak.
13. Slowly open control knob and make sure the needle falls smoothly.
14. Make sure the needle hits the stop before the gas runs out and the ball drops.
15. If you have a 0 ~ 100 psi (or close) pressure gauge, connect it to the output nipple, slowly turn on and check that the pressure rises to between 50 & 60 psi.
If it is more or less adjust with the screw on the bonnet. (see blue arrow in the previous picture).
If adjusting down you will have to let some pressure out of the system to see any resulting change.
16. To check the flow tube and seals, turn on a low flow and seal the tubing nipple with your finger. The ball should drop to zero, if it does not there is a leak in the flow tube, flow tube seals or where the tubing nipple screws in.
17. Check that the gas markings on the inner tube, the pressure gauge and regulator is correctly marked for the gas.
18. Turn the flowmeter on full and check that you have a good flush flow, it should be around 70 lpm but in any event a good rush of gas.
19. Check that the gas markings on the inner tube, the pressure gauge and regulator is correctly marked for the gas expected.
For directly dialled flowmeters only
20. With a calibrated flowmeter, check the accuracy of the of the output flow.
If the output is in error by more or less that 10%, return to the workshop for repair.
For regulators with a Schrader outlet (for example for use with air drills), check the static and dynamic pressures. At a 60 psi static pressure check that the swing is less than 5psi with a flow.
Wipe the unit clean with a damp cloth. Fix a next service date label.THE UN-REGULATED FLOWMETER.

I have seen around flowmeters that have no pressure reducing part at all, the unit is screwed on to the top of the cylinder and the flow regulated by a knob on the side.
This means that a patient or machine could receive the full cylinder pressure off 2000 psi.
In practice the pipe would blow off quite quickly, but I should imagine that the patient could get this pressure for a very short time before the pipe came off.
If you come across this type you should make efforts to warn people of the dangers involved, and try to get it replaced with the proper type.

A pressure regulator with a schrader outlet, a flow meter can be plugged in or for short term use a ventilator could be plugged in and run from a cylinder for say a ventilated patient transfer from theatre to the ward. This one is made by Therapy equipment Ltd a good and very helpful company.


Drawing of the parts of an old BOC flowmeter these had a green anodised body, modern ones will have similar parts.

BOC flowmeter

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