Operating Depth of SPG
- Thread starter hongrace
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We did some calculations on a US Divers brass & glass SPG in the mid 1970s and concluded that about 300' had a 4:1 safety margin. Keep in mind that you need a higher margin on a housing that will likely experience minor impact under pressure. Our analysis was crude compared to computer based FEA (Finite Element Analysis) that is common today. I have seen them used on early diving bells rated at 600' to monitor onboard emergency gas, but were rarely used deeper than 450'.
If all materials thickness were the same on the smaller diameter gauges, they would likely have greater depth ratings. I have no idea if they do or if the modern metal housed SPGs have the same specifications.
In the end, all this is background noise. You might check with Rebreather manufacturers who are more likely to have tested their gauges and see which models they use. Dont assume all models on the market have the same depth rating. Multiple catastrophic SPG failures in deep water could escalate into an extremely dangerous problem. I commend you for thinking to research it.
If all materials thickness were the same on the smaller diameter gauges, they would likely have greater depth ratings. I have no idea if they do or if the modern metal housed SPGs have the same specifications.
In the end, all this is background noise. You might check with Rebreather manufacturers who are more likely to have tested their gauges and see which models they use. Dont assume all models on the market have the same depth rating. Multiple catastrophic SPG failures in deep water could escalate into an extremely dangerous problem. I commend you for thinking to research it.
ianr33
Contributor
Dont know what the depth rating is but I'm sure its many hundreds of feet.Never heard of one imploding.
Even if the glass did shatter I dont think it would matter as it would still read pressure.
Even if the glass did shatter I dont think it would matter as it would still read pressure.
It depends on how the failure occurs. I have seen one SPG that failed on a bell that started as deformation of the case, but appeared to hold until a noise was heard outside the bell. There weren't enough pieces left to be certain that the glass failed on implosion, or the deformation bent the bourdon tube enough to leak and internal pressure caused the glass to explode.
There was a small rubber blow-out plug in the SPG, but the case was too trashed to know if it worked or not. Some speculated that the deformation of the case caused the blow-out plug to be blocked and others saw it as a pure implosion. In any case, everything was gone except the deformed brass housing and some glass chards under the retaining ring. Depth was about 460' as I recall.
This incident forced us to build some very expensive external pressure gauges, which is probably the only reason I remember so much about it. Photo attached.
The question posed by hongrace didnt indicate what depth he/she is concerned about. Personally, I would not be too concerned unless I was looking at something beyond the 250' ballpark. There are a lot of technical divers on this board and you never know who on the Internet will read this thread.
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It was Plexiglas, machined with a gland style static O-ring and rated for 2,000Ft/610 Meters. I think it was 1" and the gauge was a standard 2½" rear port. The fitting at the top is a relief valve. It was less expensive to make one super deep model than to design and certify several units just to save some material costs.
Well, I know that the standard 2.5" SPGs have been routinely carried on dives from 3 to 400 feet.
It would sure be nice if the manufacturers would just put the depth ratings on the face of the gauge like watches. Implosions can be pretty dangerous and there is no way to know if success with one gauge holds true with every maker and model.
There was a story floating around in the days of custom Plexiglas camera housings that a guy lost some fingers when one imploded. I can't be sure it really happened but sure find it believable.
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