/ Suunto Altimeter- help pls

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Blizzard - on 26 Feb 2013
How do u calibrate the damn thing? Have never been able to figure out how to do it accurately? When I have set it to barometric pressure it never gives me an accurate reading.
In reply to Blizzard: I got given one of their watches with an altimeter on it and although I rarely use it as the battery seems only to last a few months, setting the altitude was really simple following the instructions. I guess the buttons you push change with the model, but does it not work when you do what the instructions say? If so, then it must be a fault I guess.
SteveCarter - on 26 Feb 2013
In reply to Blizzard:
There is a reset sequence, details are in handbook or easily found on the internet, known reference altitude is needed... but have needed to reset mine every time I've used it... reached the conclusion it's worse than useless and now it's just an expensive ornament.
daWalt on 26 Feb 2013
In reply to scarter:
... but have needed to reset mine every time I've used it...

yes, that's necessary.
Did you expect anything other?

David Cowley - on 26 Feb 2013
In reply to Blizzard: agreed, you need to reset it everytime before you use it cause I think the altimeter works off air pressure(don't quote me), but this is easily done at the start of a day (takes seconds) with the use if the contours on your map. Once this is done mine seems to work fine.
SteveCarter - on 26 Feb 2013
In reply to daWalt:
Yeah knew it needed ressetting with known altitude, which was usually done when summit reached... but 30mins later return to the same summit and it would'nt be the same altitude...?
Blizzard - on 26 Feb 2013
In reply to scarter:

Ok, but what do u press to reset the damn thing? I dont have the instruction booklet. Can anyone advise pls?
SteveCarter - on 26 Feb 2013
In reply to Blizzard: Google, it's on youtube
pog100 - on 26 Feb 2013
In reply to Blizzard:

you can find all the manuals here, it is called the internet

http://www.suunto.co.uk/us/en/support/download-center-user-guides

you need to reset at regular intervals during the day, unless the pressure is really stable that day. As stated above they work by the change in air pressure as you climb, but air pressure also changes with the weather.


In reply to pog100:

> you need to reset at regular intervals during the day, unless the pressure is really stable that day.

That's what I found too - I used it 'properly' the first time on a day when I climbed Sergeant's Gully and then Parsley Fern, then onto the top of Snowdow. I was actually map reading perhaps more carefully than i would otherwise to find accurate spot heights so I could recalibrate the altimetre. By the end of the day I was sort of left asking, what's the point? The altimetre was making me 'navigate' more than I otherwise would have needed to, rather than helping.
Martin W on 26 Feb 2013
In reply to Blizzard:
> How do u calibrate the damn thing?

It might help if you could clarify which "damn thing" you are referring to! As TobyA says, Suunto make a number of different models of barometric altimeter and the exact sequence of button presses varies between models.

The basic principle is straightforward, though: when you are at a known altitude eg a spot height on the map, you adjust the altitude that is being displayed until it matches the altitude you know you are at. How you do that will be documented in the manual for you particular device. It's usually a fairly straightforward process requiring just a few button presses, since you may want to do it a number of times during a single day's outing.

> When I have set it to barometric pressure it never gives me an accurate reading.

How do you know - ie where are you getting the other barometric pressure reading from to compare against the altimeter?

In reply to scarter:

> have needed to reset mine every time I've used it

You always have to do that. It's inherent in the way the device works. Even commercial aircraft will re-calibrate their barometric altimeters before and during flight (you can find accounts online of instances where this wasn't done, with predictably unfortunate results). Of course, many commercial aircraft these days use additional altitude instrumentation as well as their barometric altimeter.

One reason for having to re-calibrate the device is that sea-level barometric pressure changes day by day and even hour by hour, and that will affect the height the device thinks it's at.

What's less obvious is that the formula the device uses to calculate altitude from barometric pressure is affected by the ambient air temperature. There's usually a complicated section in the manual which explains how to correct for this, but it's easier just to re-calibrate the altimeter every so often. Assuming that you're really interested in <10m accuracy, of course - if you just want an idea of overall height gained & lost during the day then it may not matter so much. (You'll usually find that the two figures are a few metres out when you get back to where you started from!)

A GPS isn't affected by changes in sea-level atmospheric pressure the way that a barometric altimeter is, and non-mapping GPS units are cheaper then barometric altimeters these days so that may be a better option if the inherent limitations of a barometric altimeter worry you. Don't make the mistake of thinking that a GPS is necessarily more accurate than a barometric altimeter, though. Have a read of
http://www.dcrainmaker.com/2010/05/understanding-sport-device-gps.html
and
http://weather.gladstonefamily.net/gps_elevation.html
to get some idea of how inaccuracies can occur in GPS altitude readings.
SteveCarter - on 26 Feb 2013
In reply to pog100:
Bought Suunto Vector as navigational aid, but even after reset at known altitude it would be inaccurate again within the hour... so when you're on your way down from a hill, in thick cloud(even with reset at summit) I could never trust reading given... and when you can't see any features & don't know altitude how can you reset...?
dpm23 - on 26 Feb 2013
In reply to Martin W: My Ambit's altimeter was all over the place until I decided to read the damn manual! I had it set to altimeter so it assumed I was moving and gaining/losing height when in fact I wasn't. Result was that it interpreted changes in air pressure as changes in altitude and thus appeared to be completely inaccurate. I should have set it to barometric so it knew it was air presssure that was changing(or automatic and hope the watch can work it out).

Like all tools, got to find out how it works and its limitations. GPS is good though, I can now find my car in the Tesco carpark. Will have to read those links.
andic - on 26 Feb 2013
In reply to scarter:

They are not particularly good for use in the UK our weather is too damp and changeable, I have a Core and it is much more stable/reliable on high pressure days than cloudy/rainy/foggy/crappy British weather days.
andic - on 26 Feb 2013
In reply to andic:

which may defeat the object?
daWalt on 26 Feb 2013
In reply to andic:
...it is much more stable/reliable on high pressure days than cloudy/rainy/foggy/crappy British weather days.

yea, unfortunately these things can't tell the difference between change in air pressure due to altitude or climatic conditions.
I'v got the vector, and on a nice day it's fine.
I guess you just have to be careful in changeable weather.
daWalt on 26 Feb 2013
In reply to Martin W:
> barometric pressure is affected by the ambient air temperature.

Good post and a good point, thanks.
worth baring in mind that wearing the thing on your wrist could affect the accuracy.

sounds more useless the more you think about it.
jezb1 - on 26 Feb 2013
In reply to Blizzard: Was using my Suunto a lot last week with work and it proved super accurate - due to our nice stable high pressure.

When the weather is our usual mixed bag, yes they need setting more often, but personally I find them really useful combined with map & compass.
Martin W on 26 Feb 2013
In reply to daWalt:

> worth baring in mind that wearing the thing on your wrist could affect the accuracy.

Nope - at least not on Suuntos. The air pressure sensor is temperature-compensated so the barometric pressure reading is always accurate.

It's the conversion from barometric pressure to altitude that can be affected by ambient temperature. I'm not aware of any barometric altimeter designed for recreational/sport use that attempts to correct for that. As you've realised, the key difficulty is getting a reliable ambient air temperature reading. The upshot is that whether you wear it on your wrist or hang it off the back of your pack, it will report the same altitude - it just won't necessarily be exactly the true altitude.

That's why the manual describes how to make the correction for ambient air temperature yourself if you really need to. Even then you need to have a way to measure the current ambient air temperature, and a record of what it was when you last calibrated the device.

> sounds more useless the more you think about it.

Depends exactly how accurate you need the thing to be, really. I would be reluctant to rely on an altitude reading - barometric or GPS - for accurate navigation in poor visibility (recognising, of course, that if you've got GPS, you should have a positional fix anyway). And do you really need an altimeter to tell you when you've got to the top of something pointy, or back to the bottom of the valley?

An altimeter watch is generally easier to keep about your person all the time - ie you just stick on your wrist and there it is - and the battery should last way longer than a GPS. Nonetheless, I think that generally speaking most people looking for a navigational aid would be better off with a low-end GPS than a barometric altimeter these days. That said, even with GPS you need to understand and work within its limitations if you plan to rely on it to keep you safe.

And like any device, both are subject to user error. I once nearly missed a "pimple on a flat plateau" Munro because my GPS was set to the wrong map datum. I was being a bit lazy with my nav and it was a misty day; it was only when the mist cleared for a few seconds that I spotted a higher point a few hundred metres away! Equally, I spooked myself quite badly one time on a misty day in Coire Leis, when the altimeter reading kept going down while I was definitely climbing upwards! It was only later that I realised that I'd managed to switch it in to barometer mode early on in the ascent. By chance the pressure and altitude readings were very similar to begin with so I hadn't spotted the change.
Petarghh - on 26 Feb 2013
In reply to Blizzard: Depends on the model, its as simple as pressing and holding a couple of buttons then setting the height.

whenever I use mine properly (set it up to the correct pressure and altitude) it proves to be very accurate.

also the best feature is the height change mode, where even if it isnt calibrated you just zero it to a point (like the foot of a multi pitch rock climb) then it calculates the amount ascended/descended from that pooint. You can get a good idea of the pitch length and how far you are along the topo. Extremely useful on long alpine rock routes with multiple in situ belay stations !

Pete.
ads.ukclimbing.com
daWalt on 26 Feb 2013
In reply to Martin W:
> air pressure sensor is temperature-compensated so the barometric pressure reading is always accurate.

Ah, thanks.
I'd misunderstood that point.
Martin W on 26 Feb 2013
In reply to daWalt: You read/quoted my earlier post rather too selectively!

I should more accurately have said that the formula the device uses to calculate altitude from changes in barometric pressure is affected by the ambient air temperature. An altimeter watch doesn't convert directly from a given barometric pressure to an altitude. Rather, given the barometric pressure recorded at a known reference altitude (ie when the user calibrates the device) it uses the difference between the current barometric pressure and that recorded at the reference altitude to calculate the change in altitude from the reference. That calculation is more accurate if you include a correction for the ambient air temperature at each measurement point - which the device itself can't do.
drmarten on 26 Feb 2013
Air pressure typically can drop about 2mb/hPa an hour with approaching bad weather in the UK. Normally there is no change or 1mb/hPa per hour change. A mb/hPa is equivalent to approximately 10m vertical distance at the altitudes we are using (ie not heading to orbit!).
If an altimeter is displaying results that don't really match those figures then it's either broken or not worth bothering with anyway.
Steve Perry - on 26 Feb 2013
In reply to Blizzard: I've always found it really good (Vector) as long as it's reset each time you go out. Off the top of my head, get it to the altimeter and hold the top left button for a few secs and the height will flash, then adjust up or down with the bottom left or right buttons. Once at your height press the top right button.
victim of mathematics - on 26 Feb 2013
I was always under the impression that having set it at the start of the day, if I get to a point where I know my height (eg the top of a mountain), then if the altimeter tells me I'm higher than that, it means the air pressure is dropping (and vice versa). Which is quite useful to know. Are you telling me that I'm wrong, and it's actually just the ambient temperature which has changed? Or both?
RockShock on 26 Feb 2013
In reply to victim of mathematics:
> I was always under the impression that having set it at the start of the day, if I get to a point where I know my height (eg the top of a mountain), then if the altimeter tells me I'm higher than that, it means the air pressure is dropping (and vice versa). Which is quite useful to know. Are you telling me that I'm wrong, and it's actually just the ambient temperature which has changed? Or both?

I guess, looking at http://www.skybrary.aero/index.php/Altimeter_Temperature_Error_Correction that temperature may have an impact on the reading (ie: with colder temps, the height displayed will be lower than real). Given the change rate (4% per 10C) I guess it can affect the observations you made (regarding the overall pressure rise/drop).

If you get to the peak,
Martin W on 26 Feb 2013
In reply to victim of mathematics: It could easily be a combination of both a change in the sea level atmospheric pressure and the device's inability to adjust for ambient air temperature. Basically, these are both factors which can affect the accuracy of the altitude readings.

Section 6.6 "Effect of Air Temperature on Altitude Measurement" on pages 42-44 in the Vector user manual explains how to calculate the correction. AFAIAA the Core has the same limitation - there's no conceivable way that it wouldn't because (again) the device can't reliably know the ambient air temperature - but for some reason they seem to have chosen not to document it.
dregsy - on 02 Mar 2013
In reply to Blizzard: What sort of differences are we talking about? I got a core for Christmas and used it on a walk with the kids in the Lakes. Despite it reading a few metres out all day it was consistent, As a sceptic I was impressed.
xplorer on 03 Mar 2013
In reply to Blizzard:

I've got a vector and its spot on, I set at known height way points using a map and again I set again if the weather changes.

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