Barometric pressure difference
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the weather sources give the atmospheric pressure values at sea level, while your watch shows the pressure at your current altitude which should be around 948hPa, assuming the reference 1013 hPa at sea level (since 1hPa ~ 8.78m at low altitudes)
for a sea level air pressure of 1017hPa, you should read 951 hPa
you can check it here https://www.mide.com/air-pressure-at-altitude-calculator
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@Stefano-M64 From Suunto website: “A barometer watch continuously measures absolute air pressure using the built-in pressure sensor. Based on this measurement and reference values, it calculates altitude (altimeter, or alti) or sea level air pressure (barometer, or baro).”
My understanding is that the watch always displays the recalculated sea-level air pressure value. This makes sense—otherwise, the barometric trend would be useless for tracking weather trends during mountain hikes or similar activities. My watch is accurate; it currently shows 1026 hPa, matching the 1026 hPa reported by my weather source (explicitly stated as pressure reduced
to sea level), while I’m at an elevation of 289 m.a.s.l. -
@inkognito a barometer can only measure the absolute air pressure, it can be used to derive the correct altitude or sea level pressure only if properly calibrated (“reference values”), for example by the GPS during an activity.
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@Stefano-M64 I fully agree…
OP’s watch shows incorrect barometer value (that should imho report sea-level pressure, at least mine does and Suunto webpage also indicates this) but correct altitude. So, there is either a problem with the temperature sensor (since temperature correction is also used in the calculation) or something else (like if sensor reports wrong absolute value thats used for calculation)…
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@inkognito currently temperature shows on the watch (on the wrist) is exactly at 30°C and the watch pressure indicate 986hpa (for 973m of altitude which is the correct one).
And my house barometer indicates 1019hpa for 1018hpa by weather sources.
Comparing to the previous information given in my first message, seems to have an offset of -33hpa.I had one Ambit 3 Peak and one 9 Peak Pro in the past and pressure indicated by these watches was always matching the house barometer.
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@Chris_Dx that is your problem. Does the watch altitude change when you shake the watch ? IT can be dirty sensor
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@Dimitrios-Kanellopoulos No altitude is stable and sensor is clean.
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@Chris_Dx what is your town / location ? and what reading does the watch have atm ?
PS. no need for personal data, just a coarse location somwhere (town/ nearby town etc)/. So I can lookup the sea level pressure there
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@Chris_Dx said in Barometric pressure difference:
pressure indicate 986hpa
btw pressure indicate 986hpa looks bad for 973m of altitude
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Also note:
Relative pressure is the atmospheric pressure corrected to sea-level conditions.
To compare pressure conditions from one location to another, meteorologists correct the measured pressure (referred to as absolute pressure) to sea-level conditions. Because the air pressure decreases as you rise in altitude, the sea-level corrected pressure (the pressure your location would be at if located at sea-level) is higher than your measured pressure if you live above sea-level and lower than your measured pressure if you live below sea-level.
Relative pressure is larger than absolute pressure unless you live at or below sea-level.
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@Dimitrios-Kanellopoulos currently I am in Montrottier (France) altitude done by IGN map for my real position is 740m, altitude done by the watch is 756m but nothing abnormal. Temperature done by the watch positioned on my wrist is 30°C. Pressure done by the watch is 986hpa. And météo source give 1016hpa at 11am 1014hpa at 2pm.
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@Chris_Dx doesn’t sound correct friend. Has it always been like this?
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I will say that my race has the same issue. Only caveat, the pressure is spot on when the sensor/watch is wet. I thought this might be a sign of a dirty sensor but I have cleaned the sensor many many times to no avail. The sensor is consistently 0.3-0.4 inhg (sorry, imperial guy here) under the weather service measurements. Altitude always seems to accurately adjust to what I’m doing.
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@Dimitrios-Kanellopoulos I don’t remember since when it’s been like this, because it’s not the main thing I’m looking at. But since the ascents and descents are not accurate, and this information is very important for ultra-trail running, I’m trying to figure out what’s wrong. The ascents and descents were very accurate when I bought the watch, with the initial firmware installed. After the first firmware update, the ascents and descents became systematically overestimated, despite all my attempts to reduce external influences (wind, clothing, stay fix during 10 minutes before moving, etc.). I don’t remember what the initial and first update firmware versions were.
So, this weekend, I tried to do a more “aggressive” cleaning than usual by leaving the watch submerged for 10 minutes in soapy water, and then scrubbing the sensor with an old soft toothbrush, and finally rinsing with clean water. The pressure indication had the offset observed previously, reduced. Currently, for the altitude of 569m, temperature with watch on wrist of 30°C, the pressure indicated is at 1015hp or an offset of -5hpa with the barometer at home and the weather source. This is better than the previous -33hpa. By trying to leave the watch on the table rather than on the wrist, to remove the potential influence of body temperature, I get 22°C, 569m and still 1015hpa. I am not sure I can get more precise.
What was also remarkable was that the altitude during the immersion fell to around 484m, whereas before, the slightest shower would reduce the pressure to around 234m.
And finally, ascent and descent are always overestimated. -
@Chris_Dx sounds like a damanged / something else wrong with the sensor. I would send it for repairs with the note that pressure is not correct.
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If by cleaning the sensor the pressure difference improved from -33hPa to just -5hPa, it seems the problem was the dirt accumulated over the sensor, no? Moreover, a few hPa of difference isn’t a big problem I think - if it’s a constant deviation - since all the correlated values are derived by pressure differences, so the absolute value isn’t a big problem. My Race has a couple hPa of deviation from our very precise barometer we have in the lab.
The problem of overestimated total ascents and descents is an issue frequently reported, likely not related to defective sensors. -
Hello, finally after a few days of monitoring after the intensive cleaning of the sensor, it would seem that the values indicated by the latter have become compliant again.
I checked several times the difference between the watch and the barometer at home which serves as a reference, and this difference oscillates between -2hpa and +1hpa which is quite reasonable in terms of precision.
It was therefore a problem of cleanliness of the sensor.
It is therefore necessary not to hesitate to clean it intensely with soapy water, rub it with a soft bristle brush then rinse with clean water.
Last information, when the watch is immersed or wet during a shower, the altitude no longer varies as it was the case before cleaning. This can therefore be a good indicator of the need for cleaning.
I think the subject can be closed, knowing that the problem is resolved. Thank you all for your comments and contributions to this topic. -
@Chris_Dx I followed your procedure of cleaning with a soft brush after immersion and it worked for me as well. My barometer is accurate for the first time in 6+ months! It also does not drastically change altitude when wet. Thanks for being the one to try it first, I was a little hesitant to clean the sensor with a brush as I was not aware of how fragile the sensor was.
