Pressure rises after you stop breathing on reg.

[Manatee Diver]

I was looking at the pressure graphs on a recent dive, and I noticed that the pressure reported by the transmitter rises about 50-60psi as I am breathing on the other reg.

I can't figure out why, the temperature inside the tank shouldn't be rising, as the air temperature was higher than water temperature.

 

[DiveClimbRide]

Are you sure the air temp in the tanks was higher than the water temp? Wouldn't there be a cooling effect when you breathe on the tank due to the reducing pressure in it? Then when you stop breathing the tank warms back up to ambient pressure resulting in the slight pressure increase. Could be wrong but that's my theory.

 

[broncobowsher]

Did you have the valve opened all the way?
Not using an old J-valve?

 

[Scraps]

A change of that small an amount in the display reading may not reflect an increase in pressure.

I was told to ignore anything in the tens and units places on pressure gauges and computer displays because gauges can meet standards being off by five or ten percent.

This note from the SPG page at Dive Gear Express describes the EU standard:

SPG manufacturers all seem to reference the EN250 SCUBA equipment standards from the European Union for their accuracy specification of these devices. The EN250:2014 standard states "the accuracy of any indicator tested at the following fixed decreasing pressures shall be: ±15 bar at 300 bar; ±10 bar at 200 bar; ±10 bar at 100 bar; ±5 bar at 50 bar." The imperial units equivalent for this standard would be approximately: ±218 psi at 4351 psi; ±145 psi at 2901 psi; ±145 psi at 1450 psi; ±73 psi at 725 psi. In terms of a percentage, from about ±5% in the middle range to ±10% in the low range of the scale.​

Also, if you switched regs because the tank associated with the first reg was getting low, you may be dealing with the fact that gauges can be less accurate that the low end of the pressure ranges they measure. Again, the pressure may not be increasing. Another note from the same web page:

The nature of all mechanical pressure gauges is they are most accurate in the middle of the scale and least accurate at the ends of the scale. Even when perfectly calibrated in the mid-range, in our experience all mechanical SCUBA SPGs have extremely poor accuracy at the very low end of the scale; possibly indicating significantly more (or less) breathing gas than what is really in the tank. That is why divemasters are often heard to say "Be back on the boat with 500 psi." Just to be clear, this is not a reflection on the quality of the gauge but rather a limitation of the mechanical Bourdon tube technology at the very low end of the gauge range.
​

I do not know if either of these issues apply to AI transmitters, but "significant digit over-run" may be an issue.

 

[clownfishsydney]

Broncobowsher has the correct answer, the valve is not fully open or there is a problem with it so that it is not fully open although the handle might be open. The correct symptom is not that the pressure rises after you stop breathing on the regulator, more that it drops when you breath. Have seen this many times. At depth and when a tank is fairly well used, a person will think they have no air left.

 

[DiveClimbRide]

A change of that small an amount in the display reading may not reflect an increase in pressure.

I was told to ignore anything in the tens and units places on pressure gauges and computer displays because gauges can meet standards being off by five or ten percent.

This note from the SPG page at Dive Gear Express describes the EU standard:

SPG manufacturers all seem to reference the EN250 SCUBA equipment standards from the European Union for their accuracy specification of these devices. The EN250:2014 standard states "the accuracy of any indicator tested at the following fixed decreasing pressures shall be: ±15 bar at 300 bar; ±10 bar at 200 bar; ±10 bar at 100 bar; ±5 bar at 50 bar." The imperial units equivalent for this standard would be approximately: ±218 psi at 4351 psi; ±145 psi at 2901 psi; ±145 psi at 1450 psi; ±73 psi at 725 psi. In terms of a percentage, from about ±5% in the middle range to ±10% in the low range of the scale.​

Also, if you switched regs because the tank associated with the first reg was getting low, you may be dealing with the fact that gauges can be less accurate that the low end of the pressure ranges they measure. Again, the pressure may not be increasing. Another note from the same web page:

The nature of all mechanical pressure gauges is they are most accurate in the middle of the scale and least accurate at the ends of the scale. Even when perfectly calibrated in the mid-range, in our experience all mechanical SCUBA SPGs have extremely poor accuracy at the very low end of the scale; possibly indicating significantly more (or less) breathing gas than what is really in the tank. That is why divemasters are often heard to say "Be back on the boat with 500 psi." Just to be clear, this is not a reflection on the quality of the gauge but rather a limitation of the mechanical Bourdon tube technology at the very low end of the gauge range.
​

I do not know if either of these issues apply to AI transmitters, but "significant digit over-run" may be an issue.

They might be off by 10bar in accuracy but their precision is far higher. Ie if it shows it rising from 150bar to 155bar then there was a 5 bar increase. But that increase could be from 140-145 or from 160-165 but either way there is still a 5 bar increase.

Also I belive @Manatee Diver is using sidemount so he would not necessarily be switching tanks at low pressures.

 

[DiveClimbRide]

Broncobowsher has the correct answer, the valve is not fully open or there is a problem with it so that it is not fully open although the handle might be open. The correct symptom is not that the pressure rises after you stop breathing on the regulator, more that it drops when you breath. Have seen this many times. At depth and when a tank is fairly well used, a person will think they have no air left.

If the valve wasn't fully open there would be pressure swings of a lot more than 50-60 psi

 

[Manatee Diver]

Are you sure the air temp in the tanks was higher than the water temp? Wouldn't there be a cooling effect when you breathe on the tank due to the reducing pressure in it? Then when you stop breathing the tank warms back up to ambient pressure resulting in the slight pressure increase. Could be wrong but that's my theory.

That actually makes sense, and the small increase would be appropriate for the few hundred PSI I was breathing.

Did you have the valve opened all the way?
Not using an old J-valve?

Broncobowsher has the correct answer, the valve is not fully open or there is a problem with it so that it is not fully open although the handle might be open. The correct symptom is not that the pressure rises after you stop breathing on the regulator, more that it drops when you breath. Have seen this many times. At depth and when a tank is fairly well used, a person will think they have no air left.

Valve handles are completely open and checked as part of our pre-dive checks (cave diving sidemount). And there is no quick drop associated with a partially opened valve.

@Scraps I was told that AI transmitters are much more accurate (though one shouldn't plan for it). And it is happening all across the pressure range from 3,000 to 1,000. It probably happens below that but we try to keep our reserves high.

 

[lexvil]

I am guessing and without more info on the equipment used it’s just that a guess. Is this in side mount? What transmitters, tanks, valves, regulators? Off hand it sounds more like a small restriction on the HP side.

 

[Manatee Diver]

I am guessing and without more info on the equipment used it’s just that a guess. Is this in side mount? What transmitters, tanks, valves, regulators? Off hand it sounds more like a small restriction on the HP side.

Tanks: Faber LP85s, and HP100s. And Luxfer AL80s.

Valves: Blue Steel and Thermo.

Regulators: Deep Six Signature

Transmitters: Shearwater branded PPS.