Pressure gauge

[Spiderman]

I would really like to measure the pressure in my Cressi SL100. I've found a Cressi gauge for sale that costs nearly 3/4 of what I paid for the gun

Is there any alternative out there? I was wondering whether I could somehow adapt a pressure gauge from one of my welding regulators?

 

[tromic]

You do not need a gauge. You can use a home scale.

One should slowly push the shaft into the gun (5 - 10 cm) and read F1. After that read F2 on the scale while slowly unloading the shaft.

 

[popgun pete]

Any inflation pressure gauge lets some air escape as the internal pressure is measured, so you want this volume of exchanged air to be kept very small. Pneumatic speargun pressure gauges are designed to keep this volume to a minimum. Thus when the gauge is removed from the gun only a small amount of air is lost.

 

[popgun pete]

One way to measure pneumatic speargun pressure while pumping is to build an in-line pressure gauge attachment with a tee-tube connector mounting the dial type pressure gauge off to one side. The in-line attachment's connection to the hand pump mimics the back end of the gun and hence has its own inlet valve and possesses the same "dead space" as seen by the pump. So you pump up the attachment and as soon as the pressure inside it is greater than that inside the gun then the gun's inlet valve opens and the pressure equalizes between them. This only works for increasing air pressure, not when letting air out. The advantage is once you reach the desired pressure level in the gun the pressure gauge can be disconnected without losing any air pressure in the gun. Obviously air will be lost from the attachment as its own inlet valve will have to be depressed to release air pressure inside it for it to be removed from the gun, but provided the internal volume of the attachment is not too large this will not involve too much extra pumping effort.

 

[spoolin01]

Is the need for a valve in the T-extension because of the dead space in the T and the small pump volume,so you don't reduce your pump stroke compression ratio to zilch? I'm working on a means of getting up to 400 psi or so variable intermediate pressure from my scuba tank and had just planned to plumb it directly to the gun port adapters, with a pressure gauge in line. I saw your description and wondered about the purpose of that second valve in the T, if I understood your description correctly.

Tromic, I love the elegance of your simple method. I'd have spent hours in the garage working out a solution and never have thought of this one!

 

[popgun pete]

Is the need for a valve in the T-extension because of the dead space in the T and the small pump volume,so you don't reduce your pump stroke compression ratio to zilch? I'm working on a means of getting up to 400 psi or so variable intermediate pressure from my scuba tank and had just planned to plumb it directly to the gun port adapters, with a pressure gauge in line. I saw your description and wondered about the purpose of that second valve in the T, if I understood your description correctly.

Yes, without the second inlet valve where the hand pump actually hooks up to the T-connector the "dead space" would effectively become the interior volume of the T-connector tubing, plus the pressure gauge's own sampling volume and the connection volume at the back end of the gun port. That would significantly lower the compression ratio of the pumping set-up as you have indicated.

One way to pressurize a gun from a scuba tank would be to have a smaller intermediate tank which you carefully pressurize first, that tank having its own pressure gauge attachment which allows you to monitor the pressure build-up inside it. A hand control valve fitting between that tank and the gun would stay closed until you had set the pressure in the intermediate tank. That way you control the pressure in the intermediate tank before opening it up to fill the gun, the scuba tank valve always being closed off when doing so. If the intermediate tank and fittings had exactly the same interior volume as the gun then the pressure would halve as the air was shared between them, presuming that the gun had zero pressure when the process started. A repeated step filling process could be used to fill the gun as once the intermediate tank pressure did not drop, as indicated on the pressure gauge when the control valve to the gun was opened, then the gun and intermediate tank would be at the same pressure. If you were filling say a "Black Sea" gun to 1,500 psi or 100 Bar then it would be worth doing it, but personally I think you should stick to using a hand pump, maybe using a small compressor to take the gun up to 200 or 300 psi first. Things can go very wrong in a big way if you fool around with high pressure air, so never connect a gun directly to a scuba tank. That caution is more for the benefit of other readers as I assume that you have an appreciation of these matters.

 

[spoolin01]

definitely good advice regarding caution surrounding high pressure air especially where an appreciable volume is involved, I've seen the pics and heard of the tragic carnage where scuba tanks have blown. A guy in Florida recently lost his life when he dropped a small tank he was carrying out the door, and it took out windows and furnishings and damaged some cars, if I recall.

I remember when I first learned what the actual pressure inside some of these pneumo guns is (up to 30 bar!!), I thought - I never realized I'm carrying a little bomb around with me everywhere. Something I've well understood and regarded when handling scuba tanks. It's worth remembering and respecting during storage and handling of these guns. I've no doubt a nose cap or handle letting go at full pressure could kill or seriously maim. I've posted a related thread about a barrel from a Scubapro Magnum that has got me worried.

For my planned gun pressurizer, I've got a couple of high pressure variable step down regulators to choose from, that are rated for 5000 or so psi, and provide 0-400 or 0-500 psi out. I've got a suitable scuba tank yoke and am just now making sure I've got the rest of the fittings and hoses of pressure-rated quality. I don't have any idea what my typical fill pressure is during hand pumping, I'm guessing it's maybe 15 bar, but I'll find out (for grins I'm going to try the bath scale method). Anyway, I think this should work and can be done safely with care and good fittings.

A typical scuba regulator puts out about 110-150 psi at the secondary reg/BC inflator hoses. That's enough to get started when testing a gun rebuild or dealing with an empty gun, and you can easily step that down again with standard shop regulators. I use that kind of setup for filling my tires and inflatable boat from my tanks, since I don't have an air tool compressor.

 

[popgun pete]

It appears that Omer already make a T-connector unit, I just found this while looking for something else on the Omerdiving web-site. Here is their description of the device.

"The complete pressure gauge instead is made up of a brass male attachment with a high pressure release valve with an attachment point for a hand pump as well as an attachment point for a tank. With the complete Pneumatic Pressure Measuring Kit the gun can be pressurized or pumped up while simultaneously taking a pressure reading."

 

[Ubykh]

which manufacturers pressure gauge fits-suits to Salvimar Vuoto Air? i am waiting information from Salvimar Vuoto owners.How they measure their guns pressure?any webstore shipping this product to europe?