Shearwater Perdix II AI & Leonardo SPG PSI difference

[dj90423]

Recently did five, two-a-day dives in Cozumel. I noticed that there was a difference in my tank pressure of at least 100 PSI between my dive computer/AI and my Leonardo SPG. Does my Perdix II need to be calibrated or is there a logical explanation?

 

[tridacna]

Recently did five, two-a-day dives in Cozumel. I noticed that there was a difference in my tank pressure of at least 100 PSI between my dive computer/AI and my Leonardo SPG. Does my Perdix II need to be calibrated or is there a logical explanation?

Go to a diveshop where they have a tank of known pressure. Connect your devices and compare readings.

FYI: It’s a Cressi SPG. Leonardo is the computer.

 

[inquis]

Odds are the SPG is off.

 

[Sascha314]

All measurement instruments have an accuracy, typical SPG have +-10%. There is an assirional chance you read the needle off a bit from the wrong perspective, which adds another uncertainty to the needle.

So 100 psi of for at least a reading off about 800 to 1000 psi is well within this (remember both have uncertainties!)

Another fact could be that an electrical sensor is temperature compensated, but not sure if thats the case here.

 

[OMyMyOHellYes]

Me, personally, I'd worry more about the Cressi SPG than the Swift or MH8 transmitter.

None of these are gonna be dead nuts on, but ...

For reference, I usually wear two transmitters and two computers. Usually they're within a few psi of each other. sometimes I've seen a split of maybe 50. IIRC a couple times maybe 100 (I assume one was on the sunny side and the other was in the shade pre-splash?)

But I think getting wrapped up on that is a preoccupation with inconsequential increments. Especially if comparing steam gauges to digital equipment. See above re: the 20% absolute value psi error range specification (+/-10%). SW Swift is also expected to be CLOSE to the real pressure (again nothing is exact) but claims +/-5% so that would be a 10% range.

But it is not +/-10% across the entire pressure envelope (but I'll use that for simplicity sake).

BUT
When comparing two reference points, one may be at the lower end of the tolerance range and the other could be at the opposite high end of the range, exacerbating the reported splits.

If I got two SPGs and mid-dive I'm at 1750 psi. One SPG could read 1925 psi and the other 1575 - a 350 psi difference and yet both could still be correct within specified tolerances.

From the good folks at DGX: "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." Pressure Gauges (SPG)

Sounds reasonable.

If one shows 400 and the other shows 500 when you surface, do it matter? If it is of concern, follow the one that shows the lowest pressure.