Perdix AI vs SPG pressure difference?

[ickebins]

Hi Shearwater team,

There currently is a thread discussing the pressure difference of Perdix AI and SPGs. It seems that the AI transmitter shows less than the SPG. As I'm a Perdix AI user myself a qualified answer would be much appreciated.

The thread I'm referring to is Actual Experience with Shearwater AI and it contains pic and videos for more details.

TIA

 

[guyharrisonphoto]

I have heard this about other AI computers as well. On my Galileo, the SPG is usally showing 100 or so PSI higher than my computer. I have heard Oceanics are like that, too.

Don't know why.

 

[Newdiv]

I rather it show less than more.

 

[tursiops]

100 psi, either direction, is really nothing, given the accuracy of both devices.

 

[emoreira]

SPGs used in consoles are pressure gauges of relatively low precision, as they have to be hard gauges to withstand hard work in a hard environment. They are built to have the biggest precision in the low range, where it's more important.
SPGs are normally of the Bourdon tube type.
AI pressure gauges are electronic devices working with piezoelectric pressure sensors. A radical different concept of how to measure pressure.
So, going from one end to the other end of the scale, it could be reasonable that compared both measurements they show different readings along the scale in the order of 100 psi.
Manufacturers try to keep the relative error within a certain margin along the scale. 100 psi is almost nothing in the upper end and could be big enough in the lower end.

 

[Scared Silly]

There are to issues to think about with such devices - accuracy and precision.

Accuracy - does the device give the correct value?
Precision - does the device give the same value?

Related:

Another is resolution - what is the smallest increment that can be resolved. People often think digital devices are more precise because they can resolve very small increments but that is a falsehood.


In the case of analog devices the precision is around +/- 25 psi. In the case of digital devices the precision is around +/- 5 psi (1% of full scale). Note that both of these measurements are precision and have nothing to do with the accuracy. It could be that either devices are not accurate.

 

[CarcharodonCarcharias]

I've mentioned in other posts before that my OCi + Tx will usually have 10 bar less than my DR SPG. Given that Perdix and Oceanic use same Tx, I am not surprised.

 

[KeithG]

Have you compared multiple mechanical SPGs? My SPG reads about 200 PSI lower than my divebuddies SPG in the 3000 PSI range. I have no idea what the offset is below 1000 PSI.

 

[Justin4092]

Maybe someone can explain this my perdix ai reads 1000 psi lower then any other spg or dive computer yet as time goes on they end up matching up around the 1500psi range I am new to the perdix so I want to know if it is something I have done or something with the transmitter

 

[Joneill]

Maybe someone can explain this my perdix ai reads 1000 psi lower then any other spg or dive computer yet as time goes on they end up matching up around the 1500psi range I am new to the perdix so I want to know if it is something I have done or something with the transmitter

So, it sounds like your Perdix AI never shows a full 3000 psi tank? Something definitely seems off if that is the case and you show that large of a difference. I only saw about ~100 psi difference between my Perdix AI and my SPG when I had an SPG (Perdix always lower). I added a second transmitter with my Teric and I see no more than ~10 psi difference between Teric and Perdix AI.

There are no user configurable settings that would affect the direct pressure reading, so I’d check with Shearwater - perhaps it is a faulty transmitter calibration?