Odoo • Image and Text

Did you check your SPG prior to opening your cylinder?

You might ask yourself why on earth is that a thing? Hear me out.

The Submersible Pressure Gauge (SPG in short) is a mechanical device that has a needle attached to some fancy tubbing in order to measure / indicate pressure. How it exactly works is not the point of this article. What however is important to understand, is that is a mechanical device, with moving parts in it. And with everything that moves, eventually - sooner or later - will break.

Proper care of equipment will certainly extend the life of a SPG and I’ve personally seen SPGs that are many years old. Funny enough SPGs that are in console housings seem to last a lot longer in my observations. Possibly this is because the swivel that connects the SPG to the HP hose doesn’t work as much as the console housing protects it from moving. SPGs that can move (spin) likely experience more friction then the once that are stationary in a console housing. I might have been unlucky with the fair amount of SPGs I’ve seen bubbling under water, but it seems plausible. This however does not mean that the mechanical part of the SPG can’t become damaged over time.

As I mentioned earlier, how the SPG works is not super important – Nobody that wants be popular in a Dive Center, is going to quiz you on it - But, perhaps its useful information during a pop quiz... 🙂 who knows!

So in short:  a SPG works (most of the times) using a Bourdon tube, and when pressurized, a flattened tube tends to straighten or regain its circular form in cross-section. The Bourdon tube comes in C, helical, and spiral shapes—although most gauges employ the C shape. The more pressure applied, the stiffer it becomes (or stretches), the more pressure it will read. 

Over time this moving mechanism will retain a little bit of "memory" and therefore the material will get lazy - Meaning the reading of your SPG might be off (read: LESS accurate).

So who cares I hear you say? Well, quite simple: Less accurate SPGs might be an issue while diving and give you a false sense of security, meaning during low gas scenarios you might actually have even less gas with you then you actually think you have!

I am a big advocate of proper gas planning, meaning the 50 bar minimum gas malarky you hear some people say might not be the amount of gas you really need to finish the dive you are actually conducting.  Gas planning however, (as much as it's super important!) is a story for a different time.

What I am trying to advocate here is that looking at your SPG before opening the cylinder is actually not a bad thing. If the SPG reads 0 bar before you open your cylinder, gives pretty good indication the SPG still works as advertised. If it gives a reading more then 0 ( so 10, 20, or even more, like on the photo attached), indicates it’s been used a fair bit of time and you might want to replace it. 

I’ve seen loads of used equipment (rental gear) where the SPG is off, reading “fake” pressures of up to 50 bar! Now understanding that when you start an assent when you have 50 or 60 bar, would mean that with an SPG that shows 40 bar prior to opening your cylinder, you actually only have 10~20 bar left . Not good...!

I have 1586 dives, and experienced out of air gas divers twice, from divers that (luckily) happen to be very close to me just at the end of the safety stop.  Both divers had SPGs what did showed more then 0 bar once we closed the cylinder and purged the pressure. Both divers other then a bit shocked, where fine and learned this lesson the hard way. Personally during all my teaching I mention the above.

Need a new SPG? No problem. We have them available in our shop!

Who am I? I am an SSI Instructor Trainer (Recreation and Technical OC) and Rebreather Diver.

I hope you enjoyed this article and next time you go diving, look at your SPG prior to opening your cylinder and remember to stay safe. 🙂

Want to read more stories? Have a look at: To reel or to spool. What's the difference?