Does this sound familiar? You are getting ready for a sidemount dive. You are at the point where it is time to calculate your turnaround pressures. You look at your SPGs to see what your starting pressures are and are surprised to discover the starting pressures for each bottle are as much as ten percent apart — possibly more. What is going on here?
A possible explanation is that, when you filled your cylinders, one was closer to empty and the other closer to full. This means that, if you fill to the same ending pressure, the cylinder that began at a substantially lower pressure will be hotter and, consequently, appear to lose more pressure as it cools. This would also mean that you have poor gas-management skills, and failed to keep your two cylinders close in pressure during the previous dive. Hopefully, this won’t be the case.
The more likely situation is that your two tanks are, in fact, very close in starting pressure. The problem is that your SPGs aren’t accurate and return readings that are substantially different from one another. Confirming this is easy.
- Put one of your sidemount regulators on a full or nearly full cylinder. Note the pressure.
- Now put the other sidemount regulator on the same cylinder and see whether the reading is the same or different.
Don’t be surprised is the two SPGs read as much as 21 bar/300 psi apart (possibly more).
What’s going on here?
Many sidemount divers simply assume their SPGs are reasonably accurate. This is not always the case. Even brand-new, fresh-from-the-box gauges can be off by as much as ten percent or more. It is even possible that your SPGs are off by the same amount, both returning a reading of, say, 248 bar/3,600 psi when your actual pressure in both cylinders is closer to 220 bar/3,300 psi.
Having an inaccurate SPG is also a problem when backmounting, as it affects your ability to accurately gas match with buddies. However, it generally does not affect your ability to adhere to the Rule of Thirds. Two thirds of an inaccurate starting pressure is still a valid turn pressure.
Sidemounters, on the other hand, need to be especially concerned with the accuracy of their SPGs, as it directly affects the ability to keep both cylinders in balance and to make gas switches at the best possible times. So, if you determine that your SPGs give substantially different readings from one another, what can you do?
Strategies for dealing with inaccurate SPGs
Depending on circumstances, there are a number of strategies you can follow if you determine your SPGs are off. These can include:
Ignore the problem: Sticking your head in the sand works about as well for cave and technical divers as it does for ostriches. Which is to say, it usually doesn’t. Still, if the disparity between gauges is substantially less than, say, 10 bar/150 psi, the risk factors associated with ignoring this difference are likely minimal. Past this, however, you may want to take more meaningful action.
Replace one or both SPGs: If you determine that one of your SPGs is substantially off from not only its mate, but other SPGs you compare it to, the best course of action may be to simply replace it with a more accurate one. Of course, the best time to do this is when you first purchase your SPGs. Don’t leave the store without checking the SPGs against one another, as well as any other SPGs that may be available. Make sure you go home with two SPGs that read within at least 7.5 bar/100 psi of each other.
Do the mental gymnastics: Generally speaking, anything that further complicates the process of gas management is something you want to avoid. Still, if you are out in the field when you discover the disparity between gauges, you may not have a choice (other than, of course, simply aborting the dive).
So, how does this work? Let’s use an example. In this instance, we’ll use psi, as the numbers are better for this example.
- To start, be aware that your SPGs are most likely not off by a fixed number of psi, but rather by a percentage of the total scale. In other words, gauges that read 300 psi apart at around 3,000 psi will only read 200 psi apart at 2,000 psi and 100 psi apart at 1,000 psi.
- In this example, we will assume you are using two cylinders that you have confirmed have identical starting pressures by checking them with a single gauge.
- We will also assume that, at the start of the dive, one SPG reads 3,300 psi while the other reads just 3,000 psi. As we know, however, this actually represents the same volume of gas in each.
- You determine your turnaround pressures by applying the Rule of Thirds to each cylinder independently. For the 3,300 psi cylinder it will be 2,200 psi; for the 3,000 psi cylinder it will be 2,000 psi. Again, this actually represents the same amount of gas.
You can probably think of a number of variations on this strategy, but you get the general idea.
Which strategy is best?
This will depend on a variety of factors, but in general:
- If the disparity is minor — say substantially less than 10 bar/150 psi — you may be able to ignore the problem without significant risk.
- If the disparity is significant, but you are not in a place where you can swap out gauges, you can either abort the dive or do the mental gymnastics required to calibrate for the difference in pressure readings.
- By far the best solution is to simply ensure you have two SPGs that produce identical pressure readings or reasonably close to it.
It’s important to remember that, when purchasing sidemount SPGs, you cannot simply assume that, even though they may be new, they will be accurate or provide identical readings for the same cylinder. Test the gauges for accuracy and don’t leave the store until you have two that read reasonably close to one another as well as to other gauges whose accuracy is not suspect.
It is also important to understand that gauge accuracy can vary over time. That’s why, even though you started out with SPGs that read nearly identical, you need to check them periodically against a single cylinder to ensure this degree of accuracy remains.