Question Surface air pressure reading deviation +4% from official weather report: an acceptable error?

[MarkA]

All good points, thanks chaps. Interesting to qualify the deviation in depth: so 40 or 50 mbar would be less than 50cm depth deviation, which I suppose is neither here nor there.

This might be simply a case of a putting a number in front a dumb customer (yours truly) and then it's too easy to fixate on comparing that to a number that was sourced entirely differently and that it's not valid to compare to. Although where I am (UAE) has a very large low pressure zone currently, certainly not 1035 mbar, and still very likely lower than 1035 - 20 tolerance = 1015 mbar. Still annoying when it wasn't a cheap bit of kit, and the large deviation is opposite sides of an analogue home barometer. Why would a solid-state piezo sensor be so far out?

I'll try to compare it side by side with a couple of equivalent computers and get a feel for the variance.

Have you tried a barometric pressure app on your cell phone for another data point? I am sat here with it showing 900.6 mbar (altitude of approx 1060 m). My OSTC dive computer shows 4 mbar higher. The OSTC has a trim for atmospheric pressure if needed.

How does shearwater use this atmospheric pressure? Is it used for automatic altitude adjustment, with an increase in pressure increasing the no stop time?

 

[MarkA]

Hi @boqurant

I have a barometer at home. My Teric and the barometer have ways been within just a few mbar over a reasonably wide range (985-1928 mbar)

Have you asked Shearwater your question? Seems that your Tern TX exceeds the value stated in the manual
View attachment 855569

Do you mean 1028 mbar?

 

[boqurant]

Have you tried a barometric pressure app on your cell phone for another data point? I am sat here with it showing 900.6 mbar (altitude of approx 1060 m). My OSTC dive computer shows 4 mbar higher. The OSTC has a trim for atmospheric pressure if needed.

How does shearwater use this atmospheric pressure? Is it used for automatic altitude adjustment, with an increase in pressure increasing the no stop time?

That's a good idea - I didnt even know my iPhone had a barometer!

With all the caveats that I'm accessing this number through an entirely black box process (via an app, which reports a system-level value, which should be something to do with an onboard sensor calibrated to I dont know what, which could be modified by weather reports etc etc) it reports 994 mbar in the same location as my Tern reports 1035 mbar.

 

[inquis]

How does shearwater use this atmospheric pressure? Is it used for automatic altitude adjustment

Yes, decompression calculations (including NDL time) are based on surface pressure.

 

[boqurant]

How does shearwater use this atmospheric pressure? Is it used for automatic altitude adjustment, with an increase in pressure increasing the no stop time?

The manual does state they use this for altitude adjustment, yeah. What slightly bothers me is that the tolerance with depth isn't linear: at the surface their stated tolerance is +/- 20 mbar, but at 14atm is +/- 100 mbar. I have no plans to be anwhere more than 4atm (!) but it looks like the deviation grows with depth, so given that at the surface I'm +40 mbar, by 4atm I could be +80 mbar (maybe?) off which is still only approx 1m, but could give spurious PP02 values etc

 

[MarkA]

That's a good idea - I didnt even know my iPhone had a barometer!

With all the caveats that I'm accessing this number through an entirely black box process (via an app, which reports a system-level value, which should be something to do with an onboard sensor calibrated to I dont know what, which could be modified by weather reports etc etc) it reports 994 mbar in the same location as my Tern reports 1035 mbar.

Yes. Agree with your assessment. Station pressure should show where you are (not tweaked for sea level). At least it is in the same ball park as the weather forecast.

 

[inquis]

it looks like the deviation grows with depth, so by 4atm I could be +80 mbar (maybe?) off which is still only 1m, but could give spurious PP02 values etc

Yes, PO2 might be 1.43 ATM rather than 1.40. Use caution.

 

[scubadada]

Do you mean 1028 mbar?

Yes, fixed

 

[boqurant]

Hi @boqurant

I have a barometer at home. My Teric and the barometer have ways been within just a few mbar over a reasonably wide range (985-1928 mbar)

Have you asked Shearwater your question? Seems that your Tern TX exceeds the value stated in the manual
View attachment 855569

Iiiinterestering, thanks for the screengrab you shared, but I took another look at the manual for the Tern/TX (https://cdn.shopify.com/s/files/1/0838/3732/1530/files/TernTX_Manual_Rev_B.pdf?v=1709841056) and I think what you shared is for the Teric (which has those values)

What I see on the Tern/TX manual is "Meets EN 13319 for depth accuracy", which is a different statement, and potentially a very different degree of tolerance

I tried looking up what DIN EN 13319 actually states / allows for, but despite it being publicly-funded I can't find any PDFs for free, only paywalls... hey ho. PDF Download DIN EN 13319 [EN]: Diving Accessories - Depth Gauges and Combined Depth and Time Measuring Devices - Functional and Safety Requirements, Test Methods

Nonetheless, all quite interesting and a lesson in carefully reading the manual!

FWIW the Peregrine manual (https://cdn.shopify.com/s/files/1/0838/3732/1530/files/Peregrine_Manual-metric_2021.pdf?v=1710186733) is very detailled and gives +/- 20 mbar, but the Peregrine TX manual omits Accuracy entirely

 

[scubadada]

Hi @boqurant

Yes, the screen shot was from the Teric, the computer I have.

Ask Shearwater, they have always been very responsive to my queries.
Info@shearwater.com