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

[boqurant]

Just noticed that the surface pressure barometric reading on my new as-yet undived TernTX is about 4% higher than the official weather report in my area (it shows 1035 vs 998 reported). Is this within some level of acceptable safe tolerance?

Naturally perhaps shouldn’t expect too much precision, and I guess it's all relative so the depth calcs are probably unaffected, but this still seems a bit sloppy. Does anyone else have the same issue?

 

[Still Kicking]

How do you know the computer isn't right and the weather report isn't wrong?

Worry about stuff that matters.

 

[boqurant]

A man with two watches is never sure...

But in this case I'm just hoping that folk could chime in with an idea of the acceptable tolerance level. I've since heard of that being +/- 20mb, which would put mine out of bounds

 

[OMyMyOHellYes]

Unless you are standing next to the WX station, at the same moment when the report is taken, and it is markedly different, then why worry? Unless using the Tern to forecast WX, why would it matter? (And the Tern's pressure monitoring mechanism is not intended to do that - it's to help determine approximate depth (no instrumentation system within reasonable cost is bang on - in aviation, so long as your altimeter, set to the proper pressure on the ground, at the point where field elevation is measured, is within 75 ft of field altitude, it's considered good enough AIM 7-2-3.))

There are four primary variables that can come into play measuring altimetric error:

1. Instrument error.
2. Position error from aircraft static pressure systems.
3. Nonstandard atmospheric pressure.
4. Nonstandard temperatures.

I would not expect the Tern to take all of that into consideration and compensate to -0- error.

Close enough is good enough.

If the Tern's variance is less than 50 mb off, you're talking about a foot and a half of water depth, plus or minus. You'll find way more variation than that on the depth gauges on your cattle car.

 

[FormerAbDiver]

@boqurant the barometric pressure from where you are standing to across the street could be different. And getting a reading from the weather channel is problematic at best. Where is their barometer located? Has it recently been calibrated? Is the instrument accurate at all? You get where I'm going with this.

If this is really important to you about the only way I know to get an accurate reading is to go to the airport. Find out at precisely what spot on the airport they measured the field elevation, taxi a small plane to that exact spot, set the altimeter to field elevation and then the altimeter will show the actual barometric pressure at that location.

Alternatively, if you are in the United States you can drive to the airport, find the FAA Flight Service Station, ask them where their reporting equipment sensors are located, and if it's on the same building then ask them what the barometric pressure is.

Of course again the reading will only be true at those locations. As soon as you go somewhere else the setting you made is likely inaccurate.

Or you could just set it to 29.95 and live with the difference. The biggest of course would be during a hurricane, but probably aren't diving then anyway.

Good question though.

 

[OMyMyOHellYes]

FORMULA: Multiply millibar deviation by 0.4016 to determine how many inches of water your water depth pressure readings are off (approx).

Note: does not address temperature or salinity factors, but pretty damn close.

And remember, the dive tables and deco algorithms are just best estimation anyway.

 

[scubadada]

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 (985-1028 mbar)

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

 

[greeniguana]

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

I doubt the pressure accuracy value listed in the manual is relative to weather reports....

 

[OMyMyOHellYes]

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

Without a few more data points, (location, temperature, timing, altitude ASL for the reporting station vs. Tern altitude from the TV WX report) I don't think that assertion can be made reliably.

 

[boqurant]

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.