K2-100 altimeter watch
john...@hotmail.com
Beauitful. It even came w/an instructional DVD.
http://www.lacrossetechnology.com/k2-100
Popped the DVD in and it wouldn't play - so that was useless.
Got the manual out and figured out how to set the altimeter/barometric
pressure setting. It even shows the temperature - cool!
So I excitedly set it for my first hike, in which I'd drive from about
2000' to about 7500' and then hike from there up to the summit, which
is just under 12,000'. I turned on the weather channel, got the
barometric pressure, set the watch, and head out. Noticed the
temperature was way off - realized it was effected by my body heat (I
have a hot body) (I was also dismayed to noticed it 'only' goes down to
14 deg. F...usually that's not a problem but most major peaks feature
temps. below that...sure it can measure up to 140 deg F. but I have no
desire to hike in temperatures high enough to make me boil over.
Along the road up the mountain, there were elevation markers, and I
noticed my watch was lagging behind..not a few feet, but more like a
thousand. I re-set the watch at 7650' at the parking lot sign...set
out on the climb, and by the halfway point had said "forget it" and
just turned it back to the regular time-setting. I checked it again
when I returned to the parking lot and somehow my car had elevated
itself nearly 1000' - wow!
So, in short, WTF??
I realize that on the summit of let's say a 12000' peak, it might read
11990, or 12010 - I could live w/that...but if I ever get to climb
Everest and it were to report me at either 27,000' I'd be very upset,
and upset/amazed if it said I'd climbed to 31000'
Get the picture? Wha'ts the point of these things? I was excited about
it - very excited...did I get a lemon watch and dvd or is this typical?
I have a handheld GPS that is accurate to w/in a couple feet as long as
the batteries are fully charged - if they're rundown at all it'll get
frozen..anyway..I'd like to give it another chance but don't know if
i'ts worth it - should I just go back to my 'regular' Pulsar that let's
me guess the temperature, altitude, barometric pressure, but doesn't
frustrate me because it doesn't live up to expectation??
Jim Muller
>So I got this Swiss-technology K2-100 fancy schmantsy altimeter watch.
I wasn't able follow all of your posting, but it seems like there are
a few things you should know about how barometric pressure altimeters
work. They were probably explained on the DVD which you couldn't
view, unfortunately.
First, about that thermometer. It is there because the pressure
sensor needs it to do a temperature compensation. It doesn't have to
read the air temp, just the temp of the watch itself. If you're
wearing the watch on your wrist it will always read warmer than the
air. You can use it as a real thermometer if you take it off your
wrist, but it will probably require 15 to 30 minutes to equalibrate
with the air temperature.
When you set the barometric pressure, be careful. Most of the time
you don't need to do that. If you do set it, you should set it to the
exact pressure where you are. That means you need to be standing
beside a reliable barometer or you need to know the pressure at sea
level while you are at sea level. On some such watches (or so I was
told by someone at Suunto) you can set them so far off that the
display goes crazy. Once it is set properly, which is usually done at
the factory, you shouldn't need to do it again.
The correction you do have to make is for your current elevation.
This is because the barometric pressure can change so much. On
average at sea level the pressure changes about 1in Hg for every
1000ft elevation change. Since the barometric pressure can fluctuate
by 1in Hg due to weather, the elevation it shows will fluctuate too.
So for any hike you need to start out by setting it to a known
starting elevation, usually at the trailhead. If the weather changes
serisously during the hike, that setting will gradually go awry, but
since the weather doesn't usually change very much in a short time you
can live with the error. Some watches have a feature that lets you
"disconnect" the pressure changes from elevation changes. What this
means is that when you engage that mode, the pressure can go up and
down but the watch will ignore it, keeping its elevation value the
same. This is good for, say, when you camp. At the end of a day's
climb you engage this mode. If the pressure changes during the night
you won't have to reset the elevation the next morning to what it had
been when you stopped climbing the day before. Essentially it is a
convenience. It won't prevent error from accumulating due to weather
changes while you hiking.
There is one more source of error, something you can't correct for
easily. That is the air temperature. Cold air is heavier than warm
air. The barometer doesn't know the air temperature, so its
calculations are based on an assumed temp, probably something like
70degF at sea level, and decreasing 3.5degF per 1000ft as you climb.
If the air is warmer than that, you have to climb further to generate
the same change in pressure compared to a colder day. So if the temp
is higher than what it assumes, the watch will read less change than
you actually climbed. If the temp is colder, the watch will read a
bigger change. You can compute an approximate correction for that
error if you're good with figures or if you carry a calculator and if
you know the air temperature reasonably well. I confess I don't
recall the exact figures because I haven't done it for a year or two
but I can post it tomorrow if you want to know. The thing is though,
that error usually isn't so big as to be a problem. On a hot summer
hike or a cold winter ski day it will matter, but even on a 5000ft
climb it might be only a few hundred feet maximum. Since the
barometric pressure itself can change so much anyway due to weather,
you can't count on the elevation reading being but so accurate anyway.
So you can do that temp correction if you want to, but it is usually
sufficient just to add or subtract a little bit and not worry about it
any more.
Hope this help clear up some confusion.
Jrobb
Re-read your post and therein lies your answer...altimiters that work
off barometric pressure are not as accurate as you GPS device's
altimiter because the GPS can triangulate your position on longitude,
latitude and elevation via satelite (actually by three satelites or
more depending on where in the world you are). The watch only measures
"altitude" via barometric pressure which is only accurate if the entire
air column ablove your setpoint does not have a change in pressure
other than that experienced with a change in elevation. This is also
dependent on the unit being able to calculate the temperature change
along with the increase in altitude and factor that in to give the
current altitude. These conditions can give the false readings of the
1000ft discrepancy like you experienced. The sea level pressure is
14.7psi, and 50% of the earth's atmosphere's mass is located only 18000
ft. from the earth's surface. That means between sea level and 18000
ft. there is a difference of 7.35psi. This change is not linear so
it's hard to speculate the exact change in psi for a given altitude,
but we're talkin close to .408 psi per 1000 ft. You can see why the
barometric altimiter is not highly accurate. They are used on aircraft
as well, however the calculations are adjusted for barometric pressure
and temperature. They also employ radio and GPS altimiters as
backup/redundancies.
J
Jim Muller
I just check how to do the temp correction. It is only approximate of
course but it's good enough. You need ot know what temp vs elevation
curve it assumes. On my Suunto Vector it is 69degF at sea level,
decreasing 3.5deg per 1000ft. For every degree the air is different
from that, the elevation will be off .22%. Do do it properly you need
to know the temp at your starting point is and what it "should be"
according to that assumed curve. The Suunto user manual describes it
by averaging the error at the beginning and end of the hike, but
you're only doing an approximation anyway.
Here is an example. Suppose you start at 1000ft and the temp is
75degF. The assumed curve says it should be 69 - 3.5, or 65.5. You
actual temp is about 10deg higher, so you'll read a change 2.2% too
low. So if you climb to 8000 ft, that's a change of 7000ft. 2% of
that is 140ft. So your reading will be high by 140ft.
Is that a lot? Over a 7000ft climb that's pretty good! The weather
could change by 100ft (.1inHg) on a typical day. You will never get a
reading accurate to within 10ft over such a long trip. I've stiood at
the top of mountains and watched my reading go up and down 30ft just
due to the wind gusts. Do you need better accuracy? Not really. The
main value of an altimeter is as a guage for how well you are
progressing up or down a trail, and approximately where you are on it.
Let's see, Lafayette Place is 1800ft, Franconia Ridge peaks out at
5200 (IIRC), so how far up have we gotten?
Jrobb
could change by 100ft (.1inHg) on a typical day. You will never get a
reading accurate to within 10ft over such a long trip. I've stiood at
the top of mountains and watched my reading go up and down 30ft just
due to the wind gusts. Do you need better accuracy? Not really. The
main value of an altimeter is as a guage for how well you are
progressing up or down a trail, and approximately where you are on it.
Let's see, Lafayette Place is 1800ft, Franconia Ridge peaks out at
5200 (IIRC), so how far up have we gotten? "
The problem with using these is some people will not just use them as
cool gizmos, and will try to use them to find their actual place on a
map (say a topo map) and be off 100ft or even 1000ft. That is where
the barometric altimiter's reliability falls short ( no pun intended).
The result could be life threatening...
J
Jim Muller
wrote:
>On my Suunto Vector it is 69degF at sea level,
>decreasing 3.5deg per 1000ft.
Silly me, I was off by 10deg. Bad brain cell, bad bad bad brain cell.
The Vector assumes 59degF at sea level, not 69.
> So your reading will be high by 140ft.
The temp error will therefore be 20deg, not 10. So that number is off
by a factor of 2. In other words, the error will be 280ft. Still not
bad over such a large climb. The thing is, you will probably have
places during a hike like that where you can check the elevation and
reset the altimeter.
Jim Muller
>The problem with using these is some people will not just use them as
>cool gizmos, and will try to use them to find their actual place on a
>map (say a topo map) and be off 100ft or even 1000ft.
Well now, I'd argue that finding your position along a trail is one of
the good things you can do with it. The question is really just how
accurate it needs to be. Trails aren't necessarily shown on maps more
accurately than that. Having an altimeter is better than not having
one, but if you're in a position where you need more accuracy than
that you're in big trouble. In the "old days" before GPS, few people
carried an altimeter at all, and no one knew their elevation except by
really knowing where they were anyway.
> That is where
>the barometric altimiter's reliability falls short ( no pun intended).
>The result could be life threatening...
Having your GPS batteries go dead falls even shorter. I say again, if
that becomes life threatening they you shouldn't be there.
john...@hotmail.com
this: Say I'm at 4000' (I know because i'm next to a sign) The watch
reads 3560' let's say...and barometric pressure of let's say 28.80. I
understand that altitude is a function of barometric pressure, so that
if it isn't exactly right its' because of the pressure change. So if I
reset the altitude to 4000', when I check on the barometric pressure,
it'll read say 27.50 (i'm not doing exact calculations here, just go
with me!) Now like I said I understand altitude is a function of bar.
pressure, however it's not possible for me to change the air pressure
around me by pressing a few buttons on a watch!! And that appears to be
the result? Don't like the reading? Change the weather!!
Jim Muller
>OK I think I'm understanding...the one thing that I don't understand is
>this:.. So if I reset the altitude to 4000', when I check on the barometric
>pressure, it'll read say 27.50 ... it's not possible for me to change the air pressure
>around me by pressing a few buttons on a watch!!
Sorry I didn't respond right away. Been x-c skiing for a few days.
The White Mountains got an unexpected dumping of snow a few days ago.
Well, you are absolutely right that you can't change the weather by
punching buttons on a watch. (Hey Bubba, watch this! I'm gonna' make
it rain! :-)
I think the point of confusion here is that your watch most certainly
has two different adjustments, possibly three though only two are
independently adjustable. Let me explain. (All of this discussion
will assume that the temperature vs. elevation matches some curve
which the watch expects. In reality the temperature could be
anything, of course, but the watch doesn't know what it really is so
it has to assume something. As I posted previously, my Vector assumes
59degF at sea level, decreasing 3.5deg per 1000ft. You can always
correct for it if you want to go through the trouble.) Note, I tend
to use the terms elevation and altitude interchangeably, Hope that
doesn't become confusing.
The pressure sensor itself reads just the pressure, period. It is
usually calibrated at the factory, though you probably have the
ability to adjust it yourself. (As I'll explain in a minute, I got
the impression from your first posting that you may have been tweaking
this instead of the altimeter adjustment.) When you make the watch
display barometric pressure, it simply shows the value of that sensor.
This is the pressure *where you are standing* regardless of your
elevation. If you change your elevation or if the weather changes,
this value will change. If you change the pressure calibration
adjustment, it will (obviously) change the pressure display, but also
change the elevation display too.
Now, one complicating factor is that weather reports typically give
the barometric pressure corrected to sea level. If they didn't you
would have no way to know whether a value of 26.50" was high or low.
(For Mt Washington that would be very high, for Cape Cod that would be
low.) We'll get back to this point in a minute, but for now just be
aware that if you try to calibrate your barometer you need to know
what you are calibrating to.
Your watch should have a separate adjustment for altitude only. The
altimeter display uses the difference in pressure between what it
measures at any moment and what it was when you last set the altitude.
Using that difference it calculates how much your elevation must have
changed since then. Note, it isn't just a matter of multiplying the
pressure change by some constant. Air gets thinner as you go up, so
the calculation has to use an air density that changes as you go up or
down. The only way it can do that is to know the actual pressure
where you are at any given time. If your pressure sensor is
uncalibrated badly enough, the computation from pressure difference to
elevation change will be quite wrong because the air density is not
what it thinks it is. The fact that you showed a 1000ft error
suggested to me that the watch was using a wrong air density. Also
the wording of your posting suggested that you had indeed been
adjusting the pressure calibration, not the altimeter reading, so this
could explain why you had such a large error.
The third adjustment you may have is "sea level pressure". Here's the
deal. Theoretically if you know both the pressure where you are
standing and your elevation, you should be able to compute what the
sea level pressure is (assuming the temp is correct, of course). Your
watch may have a display of sea level pressure, based on what it reads
for pressure and what it thinks your elevation is. This is useful for
two reasons. One is that you may wish to know what the weather is
doing, and a barometric reading is meaningless unless it is corrected
for elevation. The other reason is as an aid to setting the elevation
adjustment. You may find yourself wanting to set the elevation but
you don't have a map or trail sign. However if you have access to a
weather report for your area you can adjust the "sea level pressure"
reading. Your watch may be different from mine, but at least with
mine that adjustment really adjusts the elevation reading. In other
words, it is a different way to set the elevation, not as accurate as
setting the elevation directly at a known location but better than
nothing. Your watch may have this feature, or it may not.
So what does it all mean? You really have two adjustments, one the
pressure sensor which also affects the elevation reading, and the
other the elevation itself. You need to learn which is which, and
adjust only the elevation value. This stuff is probably explained on
the DVD you couldn't see. Maybe the DVD will play on a different
machine. Or you may be able to find the Owner's Manual (or User
Guide, or whatever they call it) on their website. Many companies do
that. In any case, you *really* need to see that DVD or Owner's
Manual.
You also need to recalibrate the pressure sensor (after first learning
which adjustment is which!), assuming you really did get it messed up.
If you don't recalibrate it, it will never be accurate. However all
is not lost. The fastest and most accurate way to calibrate it is to
find a real Hg barometer, perhaps at a local high school. Simply
adjust the watch so that its barometric pressure reads exactly what
the barometer says. (Be suspicious of the calibration of dial
barometers that people sometimes have as decorative weather stations
on their walls! They may be mis-calibrated too.)
Another way to calibrate it is to a weather report, providing you can
correct for your known elevation and providing it refers to the
weather reasonably close to you. You may also have to note whether
the pressure has been changing rapidly over the last few hours.
Websites like Accuweather and Intellicast give readings from the last
24 hours. The elevation correction isn't hard. Near to sea level you
can assume about 1in per 1000ft, so for example if you live 50ft above
sea level your watch should be set to 0.05in less than what a weather
report says. The closer you live to sea level, the smaller the
correction will be, and the less error any temperature effect will
cause too. If you live at 8000ft, you'll have a hard time getting it
accurate this way though.
A third way is if your watch has a "sea level pressure" reading. You
first set your elevation to what you know your house is from looking
at topo maps, then check to see how far off your "sea level pressure"
reading is from the most recent weather report. Once you know how far
off it is and whether it is high or low, go to the barometric pressure
adjustment and change it a little bit. Don't try to do the whole
correction at once because it could overshoot, especially given how it
uses a varying air density. Instead, do the whole process several
times, iterating in to a final correction. Each time, first reset
your elevation to your house's elevation, go back to the sea level
pressure reading and check its error against the weather report again,
then adjust the barometric pressure reading a little more. After a
few times you should see it converge to a final good value. (Again,
this assumes certain temperatures, but if you live close to sea level
that won't matter.)
Unless you live right at sea level, correcting to a weather report by
any method will probably not get it as accurate as direct comparison
to a barometer, but it will certainly get it good enough to make the
altimeter good again. Remember, you never really need to know the
barometric pressure super-duper accurately, and the altimeter is
always subject to errors from temperature and from weather changes.
So though it is nice to have all the adjustments as accurate as
possible, they don't need to be absolutely dead-on accurate.
I just spent three days skiing at Bretton Woods. I like to check my
altimeter while skiing. It read the same this time over those trails
as it did several years ago when I first tried it. Great fun, and
it's nice to know it is still accurate.
john...@hotmail.com
now appears to have a dead battery, so once I replace that, I will see
what happens!
Thanks again,
John