Altitude Issues

rolfsg said:

Maybe one of the dive pros here is ready to test this on a MK1 with a wrong altitude setting and compare is with a reference dive computer. If this should have an impact I think in minimum Garmin should let out a warning that altitude of the MK1 can be wrong and needs to be verified before each dive. If the Altitude does not have an impact on the diving calculation, I am relieved, but still have this known issue for mountain sports. This is still frustrating, but at least not dangerous to my health.

Just a quick post to say that I set the altitude on my Mk1 to 500m (well it was 501m as I entered the water) before my dive today.  I could discern no difference in the displayed depth between the Mk1 and my Perdix.  The displayed NDL was also very close between both devices - about as close as it usually is.

Once I download the data from the devices I'll pop up some graphs.

Garmin-Nate said:

It is going to adjust the NDL based solely on the reading from the barometric pressure sensor on the watch.

Which is what I suggested from my first post (and later posts) in this thread. Thank you for confirming!  It is also why I had no qualms about changing the altitude on my Mk1 before a dive.

Thank you clarifying this Nate, it provides peace of mind to know the altitude issue does not impact the diving algorithm and there is still hope this issue will be solved by Garmin.

Thank you atj777 for testing it, always great to have these factors validated. By the way, I am humbled you are comparing us with James Bond. According to statistics we are 250'000 certified divers in Switzerland, 50'000 of us are diving the Swiss lakes on a regular base, 25'000 even every week what includes winter. This is Switzerland only, not including divers in the Austrian, French and Italian alpine region. So James Bond is definitly not alone :-) Thanks to the elaborate algorithm Albert Bühlmann developed when diving lake Zurich, diving here is very safe these days. This was not the case in the early days of diving in the alpine region when all tables have been developed for sea level.

Now I really am confused - can some explain in simple terms please?  I was led to believe that the altitude is determined in the following way; Initial GPS fix, updated by barometric pressures sensor changes, OR initial Manual fix updated by barometric pressure changes, OR initial reading 'auto-calibrated' by GPS use when tracking an activity AND barometric pressure changes.

To my initial issue - if the barometric pressure sensor (ie altitude reading) cannot be trusted, does that not impact on the NDL calculations?  Yes, I understand there is a separate (?) depth sensor - pressure sensor, however, if the MK1 was showing -47m BEFORE you descend on a dive and you hadn't noticed, you would presumably have all sorts of issues trusting the NDL calculations?

If, as I do, you have to climb up a pretty rugged and high mountain to dive in a quarry and the barometric data is skewed - what then?

What am I missing?

Goose said:

To my initial issue - if the barometric pressure sensor (ie altitude reading) cannot be trusted, does that not impact on the NDL calculations?  Yes, I understand there is a separate (?) depth sensor - pressure sensor, however, if the MK1 was showing -47m BEFORE you descend on a dive and you hadn't noticed, you would presumably have all sorts of issues trusting the NDL calculations?

What makes you think the barometric pressure sensor can't be trusted?  All it does is measure pressure and there's no reason to believe it can't do that. 

The difficulty, and I mentioned this after your initial post, is that air pressure doesn't equal altitude.  Yes, air pressure decreases as your altitude increases (because there is less air above you). But air pressure also changes with weather.  

For example, 1 atmosphere is approximately 1.013 bar (1,013 millibar) - in theory.  However, the actual air pressure at sewa level can range from below 930mbar (tropical cyclone/hurricane/typhoon)  to above 1,050mbar. So, you can't just take the pressure and determine the altitude.

This link that I posted earlier goes into a lot of detail how altitude can be calculated:
https://www.outdoorgearlab.com/topics/camping-and-hiking/best-altimeter-watch/buying-advice

When it comes to determining NDL, tissue loading, deco commitments, etc. all that matters is pressure. Estimated altitude is irrelevant (and to some extent even depth is irrelevant).  It is the increase in pressure that increases the partial pressure of Nitrogen and so influences the rate at which it dissolves in blood and tissues.  Similarly, decreases in pressure lower the PPN2 and influences the rate at which nitrogen leaves blood and tissues.

As I showed yesterday, my "altitude" was 501m just before I descended for a dive but the NDL displayed on my Mk1 was as close as it usually is to the NDL on my Perdix (same algorithm and gas factors).

It has been explained to me by the Garmin techs that the altitude reading is a calculation made by changes in the barometric pressure.  The only time the altitude is 'fixed' is either with a gps or manual entry, all subsequent readouts are made based on barometric pressure changes.

It follows that if the dive calculations are made using barometric pressure reading at the start of the dive then they are subject to the same errors - bad data - as the altitude reading.  If I start a dive at 1000m altitude and the barometric sensor (altitude) is telling me I'm at 100m then to my mind there is a problem.

Garmin-Nate said:

It is going to adjust the NDL based solely on the reading from the barometric pressure sensor on the watch.

The Garmin tech stated that the ndl calcs, and therefore I would suggest all the dive related calculations use the barometric pressure data along with a seperate pressure sensor for actual depth.

Whilst this may not be a concern for 99% of recreational divers out there it is still an issue : https://en.wikipedia.org/wiki/Altitude_diving

I also agree that if you are altitude diving (the Dead Sea for instance) you would be more careful in your planning, post dive activity and recovery times.

Goose said:

It has been explained to me by the Garmin techs that the altitude reading is a calculation made by changes in the barometric pressure.  The only time the altitude is 'fixed' is either with a gps or manual entry, all subsequent readouts are made based on barometric pressure changes.

It follows that if the dive calculations are made using barometric pressure reading at the start of the dive then they are subject to the same errors - bad data - as the altitude reading.  If I start a dive at 1000m altitude and the barometric sensor (altitude) is telling me I'm at 100m then to my mind there is a problem.

You need to understand, and I don't know how to make it any clearer, barometric pressure IS NOT the same as altitude.  You need to stop thinking of it that way.

While altitude will influence barometric pressure, so does weather, while the weather will have a smaller effect, it will still have an effect and that's what makes determining altitude so difficult.

For example, under "normal" conditions, the barometric pressure at sea level is around 1,013mbar and around 950mbar at 152m.  BUT, and an important but, a severe low pressure system in a storm could drop the pressure at sea level to 950mbar. So, if a pressure sensor reads 950mbar, how do you know if the altitude is 0m or 150m or somewhere in between?  This is why determining altitude is so difficult.  It is not the pressure sensor that is a problem but rather how do you interpret the reading from the sensor?

The sensor reading is not wrong.  It is just measuring the actual pressure.  If the reading is interpreted incorrectly, the displayed altitude may be wrong but the pressure is still the pressure.

So, it is this pressure reading that is used for NDL, etc.  The pressure reading is still the pressure and NDL, etc, is based purely on changes in pressure relative to that initial reading.

In fact, using the pressure reading is actually more accurate that the actual altitude as it takes into account variation in pressure due to weather which has an effect on NDL etc.

Here’s a scenario that might help your understanding.

You are sitting on your back veranda sipping a beer. It’s a beautiful day. The sun is out, it is warm and there is a gentle breeze.

You calibrate the altitude on your watch using GPS.

You finish your beer and fall asleep.

You awake 90 minutes later and the weather has turned. It is now cold and very windy. It has started to rain. A storm has rolled in.

The barometric pressure has dropped by 50mbar since you calibrated the altitude on your watch. A drop of 50mbar is equivalent to an increase in altitude of more than100m!

Your watch detects this change in barometric pressure. What does it do? Does it increase the altitude? If it does it will be incorrect (even though the pressure reading was correct) as your altitude has not changed.

Does it maintain the same altitude? Why would it do that? How does it know your altitude hasn't changed?

This is a relatively simple scenario. What does the watch do if you were out and about when the storm rolled in and your altitude was actually changing? How can the watch maintain accurate altitude?

These are all problems even if the barometric pressure sensor is 100% accurate.