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Altimeter 'spikes' on Vista HCx


AlunS
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I was being a bit puzzled by sudden spikes or glitches in the altitude in my track logs. Suddenly the reading would suddenly plunge 20 metres or so and then pop back up again, sometimes jumping the same amount into the air.

 

Anyway, to cut a long story short, I discovered that it was due to me taking the GPS out of my holster, to check on one thing or another, or to use it to navigate to a waypoint, and holding it with my left hand index finger nestled in the convenient groove in the back of the unit ... yes, where the little hole is for the altimeter sensor!!!!

 

At least I know know, but it does make the thing a lot more difficult (for me at least) to hold properly ... that groove on the back looks like it was designed from the outset for a finger to nestle in it, so why put the sensor hole there where it's just asking to be covered up?

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To be fair to garmin it is incredibly difficult to hold the unit comfortably while blocking the barometer hole. But you seem to have found a way.

Got out of bed the wrong side this morning did we? :D

 

I'd disagree 100% with your premise that it's 'incredibly difficult' .. As I said, I hold the unit in my left hand, despite being right handed as it makes operation of the click-stick easier, and means my hand doesn't obscure the screen. In this position my index finger falls naturally into that groove, especially if I have to use the click stick when the support given by my finger from behind makes that easier too.

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Downwards, often, as the pressure then usually increases inside. But I've also seen that wringling the case, as you sometimes have to do on an old Vista, to make the screen work, can lead to fluctuations in reported elevation, in spite of the hole not being covered. It probably causes mechanical strain on the air pressure sensor assembly.

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Downwards, often, as the pressure then usually increases inside. But I've also seen that wringling the case, as you sometimes have to do on an old Vista, to make the screen work, can lead to fluctuations in reported elevation, in spite of the hole not being covered. It probably causes mechanical strain on the air pressure sensor assembly.

Hmmm, maybe you're right .. I was squeezing reasonably hard to make sure I covered the hole completely, and subsequent tests show that if I just cover the hole lightly it doesn't happen, but if I squeeze the case as well a bit it does.

 

My latest theory then is that what's really causing the problem is me removing it from the case I have. I have this case ( https://buy.garmin.com/shop/shop.do?pID=814 ) that I also had for my old etrex Venture, but the newer models appear to be slightly bigger and are quite a tight fit in these cases. I have the case more or less permanently attached to the shoulder strap of my rucksack, meaning I have to take it out of the case to use it, and it's necessary to grip the thing quite firmly when removing it from the case. I did a few dummy runs with it in the case on my rucksack, and it appears I'm gripping it with my thumb in the groove over the hole which is a pretty natural place to grip it if you're trying to remove it from a tight case.

 

I think I may have to invest in another kind of attachment, either the button clip or the stretch holster. Has anyone had any experience, good or bad, with either of these?

 

https://buy.garmin.com/shop/shop.do?pID=1035

 

https://buy.garmin.com/shop/shop.do?pID=1034

Edited by AlunS
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Squeezing the unit may lead to the same result.

In a previous thread along these lines, someone said that rapid deceleration of a car could cause similar spikes, due to the "piling up" of the air in the front of the passenger compartment; that's something I've been meaning to check out, but haven't done so yet. My own observation (also not thouroughly checked out yet) is that when I drive my car over a pass the GPSr seems to underestimate the altitude. One day while doing this I decided that because the car was closed up the pressure inside might not be equalizing quickly enough during the rapid ascent; I opened the window and thought I observed a spike in the elevation profile. This is again something I want to investigate further, but haven't come to any firm conclusions.

Edited by Hertzog
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Squeezing the unit may lead to the same result.

In a previous thread along these lines, someone said that rapid deceleration of a car could cause similar spikes, due to the "piling up" of the air in the front of the passenger compartment; that's something I've been meaning to check out, but haven't done so yet. My own observation (also not thouroughly checked out yet) is that when I drive my car over a pass the GPSr seems to underestimate the altitude. One day while doing this I decided that because the car was closed up the pressure inside might not be equalizing quickly enough during the rapid ascent; I opened the window and thought I observed a spike in the elevation profile. This is again something I want to investigate further, but haven't come to any firm conclusions.

1. I have a Vista (B&W) - if I cover the vent hole with my index finger and squeeze the top of the unit with my thumb gently (by pinching lightly between thumb and index finger), I can easily generate a downward elevation spike of about 20 to 30 metres. (Squeezing the unit increases the air pressure on the sensor, therefore it assumes a sudden drop of elevation.) Squeezing tightly (but not enough to make me think I might break the unit!) can generate an apparent elevation drop of 100 metres or more. These elevation "spikes" can be maintained for as long as I continue to pinch the unit while blocking the vent. Pinching the unit, but being careful to NOT block the vent, shows elevation changes of only a couple of metres, and these elevation changes quickly self-correct in a couple of seconds, as the changing air volume inside the unit re-balances with outside pressure through the vent.

 

2. Driving in my car (Mazda 6 wagon) with all windows closed at around 100 km/hour on a flat road shows a steady elevation. If I stop and get out, I will see an apparent elevation drop of around 10 to 15 metres. If I stay at around 100 km/hr and open the driver's window, I will see a similar apparent elevation drop. That is, when driving at speed with the windows up, the internal air pressure is slightly lower than ambient. This is to be expected. Modern car designs are generally "slippery", and have a zone of moderately high pressure at the base of the windscreen (where the air intake vents are), and a zone of low air pressure along the sides and at the back (where the cabin exhaust vents are). This is all basic aerodynamics, used by car designers to get positive ventilation of the cabin even with the windows closed (unless you close all the vents, or operate on "recirculate" mode). The internal cabin pressure should be negative (less than ambient) to ensure air is positively pulled out of the cabin through the exhaust vents - that is, the effect of the negative pressure at the exhaust vents should be stronger than the effect of the positive pressure at the inlet vents. This provides positive free ventilation of the cabin, and is better than having positive pressure in the cabin, which will not recirculate air through the cabin as effectively. (Note - actual air pressure inside a vehicle cabin will depend on the vehicle body shape, location and size of vents, windows and other leaks, vehicle speed, etc, etc.)

 

Hope this helps!

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If you want to see altimeter spikes and drops of 300-400 feet in seconds (accompanied by ear pops), take the unit on a New Jersey transit train ride into New York City. You get the weirdest, rapid pressure fluctuations as soon as the train enters the tunnel. I've seen mine plunge to -248 feet and back to +300 feet in seconds.

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