Working Pressure & Cylinder Size

Discussion of the macOS applications DiveLogDT and Dive Log Manager
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Joined: Fri Apr 08, 2016 6:57 pm

Working Pressure & Cylinder Size

Post by steigw » Fri Apr 08, 2016 7:07 pm


I'm transitioning from a paper dive log and am trying to understand the working pressure and cylinder size section of the Dive Log Manager. I had tracked just the cylinder size for the paper log. So as I enter this historical info, I'm choosing tank type (i.e. steel) and Size (100). but then I see the working pressure is empty. So I looked it up online and see people saying 3,442 is the correct figure. But when I put that into the working pressure field, the program will change the cylinder size to another figure, like 115 for the size.

I edited it back, but does the working pressure even matter? I have the actual pressure for the tank, both starting & ending. If working pressure is truly 3,442 for all steel 100 tanks, why wouldn't it be hardcoded in, so you just choose the tank type / size and working pressure is automatic.

I appreciate peoples thoughts on how to properly treat this.


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Joined: Wed Mar 31, 2010 4:35 pm

Re: Working Pressure & Cylinder Size

Post by support » Sat Apr 09, 2016 8:06 am


The tank's working pressure is certainly a confusing concept. To start with, it is only relevant when you are working with tank sizes in US/Imperial measurements. If you are running Dive Log Manager/Dive Log DT or Dive Log on iOS in metric then the working pressure concept is not important.

The background here is that US and Canadian tank capacities are measured based on the amount of gas they can hold at their working pressure. For example, an 80 Cu Ft tank can hold a volume of gas that would fill an 80 Cu Ft space at ambient pressure (sea level). So you can imagine starting with an 80 Cu Ft room and removing all the air in the room and compressing it to the the working pressure of the afore mentioned 80 Cu Ft tank and sticking it in that tank. Of course, if you fill the same tank to something less than the working pressure it will be holding less than 80 Cu Ft of air (measured at ambient pressure). By contrast, European (metric) tanks measure the internal volume of the tank itself (so that same 80 Cu Ft tank would be approximately 11 liters if it were marked in metric). In this case, the tank can hold 11 liters of air at ambient pressure (of course when you compress the air it can hold much more (80 Cu Ft is approximately 2265 liters). The bottom line is that to interpret the amount of air in an US/Canadian (Imperial) tank you need to know both the capacity and the working pressure that that capacity is measured at.

Dive Log Manager/Dive Log DT/Dive Log store all their data internally in a consistent measurement system (as it turns out it is metric, but that is not really important) and the applications do the appropriate conversions at display time depending on the units that you have chosen to use (metric or imperial). So we need to know both the tank size and the working pressure to correctly work with the volume of gas in the tank when you are using the US/Canadian (Imperial) settings. However, if you do not enter a working pressure for a tank, we use an approximation for working pressure that is pretty close for most common tanks. When you enter a working pressure, we can then make the appropriate adjustments.

In your case, what you are seeing is that you entered a tank size without a working pressure (so we used this approximation) and then when you added the real working pressure, the tank capacity was redisplayed using the new working pressure. Since 3442 is slightly larger than our approximation, the tank capacity was restated as 115 Cu Ft. At this point, all you need to do is change the tank capacity back to 100 Cu Ft and all will be well. In the future, if you first enter the working pressure then the tank capacity you will never see this "re-interpretation" of the tank capacity.

The working pressure for your tank is stamped on the neck of the tank. Unfortunately, there is no hard and fast rule for what the working pressure of a tank will be (each manufacturer makes different choices). For example, many High Pressure Steel Tanks have a 3500 PSI working pressure but some also have a 3442 PSI working pressure. You can also have a 100 Cu Ft tank that is not a "high pressure" tank and it might have a working pressure of 3000 PSI. There are also many steel tanks that have a working pressure of 2250 PSI. Aluminum tanks often have a working pressure of 3000 PSI, but there are 3500 PSI aluminum tanks as well. Just to add confusion to the whole mess, Steel tanks often have a 10% overfill rating during their first hydro period (this is indicated with a "+" next to the working pressure). This does not change the working pressure, but it does allow tanks in this first hydro period to be over filled by 10%. So a 72 cu ft 2250 PS working pressure tank filled to a 10% overfill of 2500 PSI would actually be holding about 77 cu ft of air (you would still enter the working pressure as 2250 in this case). So there is no simple way to "guess" the right working pressure for a tank. We use the approximation so that if you chose not to enter a working pressure and just use the app consistently in Imperial mode then everything will show the right values. Since all the calculations use this approximation they will all be "right" with the exception of any conversion between Imperial and Metric display of tank sizes.

I hope that helps explain the situation. The simple answer is to always enter the working pressure for your tank and then the capacity. Alternatively, you can just leave the working pressure out and let the application use the defaults and the only downside will be any conversion to metric if you ever want to see that.



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