| What is an Altimeter ?Right back to basics, an | | | | knack, but pretty soon you'll have a good idea of |
| altimeter is a device which measures changes in | | | | what's going on around you. When you see that baro |
| height or as the pro's say - altitude. The simplest | | | | at 980 and plummetting, well maybe you'll put the |
| way of measuring altitude is to measure the way the | | | | bike back in the shed and meet your mates down |
| pressure of the air around us changes as we ascend | | | | the pub to talk about how good it was last week. If |
| or descend, the air gets thinner the higher we go up. | | | | you're a canoeist - maybe this weekends going to |
| Unfortunately this change is very slight (unless you | | | | have some big water !!!Yeah, but you said the altitude |
| have a jet !) and you need an extremely accurate | | | | affects the air pressure and the weather affects the |
| instrument to measure the change in air pressure | | | | air pressure - how do I know which one's affecting |
| experienced by walkers, or even downhill ski racers. | | | | the readings ?Well now, an advanced user!!! You're |
| Until recently the only way to do this was with a | | | | right, they do affect each other, and the altitude has |
| very sensitive mechanical device, too big, delicate and | | | | much more effect than the weather, so if you're |
| expensive for the average Joe to carry around. Then | | | | going up and down a bit the weather becomes quite |
| electronic gauges were made for aircraft - but these | | | | hard filter out, but you can do it. The main thing is to |
| were incredibly expensive, very exotic. And then | | | | keep calibrating it, if the weather or height is |
| those electronics whizz kids went and miniaturised | | | | important - keep calibrating. Generally you'll calibrate |
| them, shrunk them until they fit into an ordinary | | | | the watch from a map height, this will ensure the |
| watch !!! Now everyone can have one.But air pressure | | | | altitude is always accurate. But when you calibrate, |
| changes with the weather too doesn't it ?Yes it | | | | stop and think a minute, is the watch reading low, or |
| does, but not enough to make the alti-meter too | | | | high. If it's reading low, it's good news, since you last |
| inaccurate and with a little care you can actually read | | | | calibrated the air pressure has risen - remember - |
| the changes in the air pressure and get an idea of | | | | higher pressure means lower altitude - so the watch |
| the weather to come. In other words you can have | | | | reads low. If the altitude is reading high, the opposite |
| your own weather forecast on your wrist !But what | | | | applies, the pressure has dropped - it could be bad |
| will I do with an Altimeter-watch ?It depends upon | | | | weather ahead. The amount you have to change the |
| what you do with your life, but just imagine how | | | | watch by also tells you something - a metre or two |
| many of todays action sports involve going up... and | | | | means the weather is really pretty stable, 10 metres |
| then coming back down. Imagine knowing how far | | | | and you'd better keep an eye out. For a rough guide |
| you've climbed from the bottom of the hill, your total | | | | the pressure changes about 1 mbar for every 8m of |
| height gain for the day on your mountain bike, how | | | | height, thus at if you are at 800m and the air |
| far you've ski'd down a hill - and how long you took. | | | | pressure is 900mbar, at sea level it is 900+800 |
| You can go back next week and try do more, faster | | | | 8=1000mbar. If you're a real weather nut, you can |
| ! There are other things, mountaineers can pin point | | | | use the table below to estimate the air pressure at |
| where they are - if you are on a knife edge ridge, if | | | | sea level from the altitude and barometer readings |
| you know your height you can read off a contour on | | | | from your location. Remember:- this is an estimate, |
| the map and fix your position. If you're camping in | | | | many factors affect the actual air pressure - keep |
| the wilds, you can watch the air pressure overnight | | | | calibrating and keep your eyes open!!! (If you really |
| and see if tomorrow is going to bring that glorious | | | | want to know more check the net, there's some |
| day you're hoping for! Whatever you do from cycle | | | | really good geeky weather sites out there).m mbar |
| touring to parachuting, wouldn't it be good to know | | | | ft mbar |
| just that bit more about where you are and what | | | | 0 0 0 0 |
| you've done ?Ok, I've bought one, how do I use it | | | | 100 11 200 7 |
| to measure height ?The basics are dead simple, if | | | | 200 22 400 13 |
| accuracy isn't a matter of life or death or you are | | | | 300 33 600 20 |
| just interested in the relative change in altitude | | | | 400 43 800 27 |
| (relative change is the difference between A & B, | | | | 500 54 1000 33 |
| not the actual height of point A or B), then you just | | | | 600 65 1200 40 |
| read your height or change of height, straight from | | | | 700 76 1400 46 |
| the watch face - nothing to it. Zero it (or set it to | | | | 800 87 1600 53 |
| the nearest contour on your map) at the bottom and | | | | 900 98 1800 60 |
| read off your climb at the top, dead easy and with | | | | 1000 109 2000 66 |
| these great watches, accurate to a metre or two. | | | | 1100 120 2200 73 |
| Even if you don't zero it, the altitude reading will be | | | | 1200 130 2400 80 |
| approximately right for any particular place (say | | | | 1300 141 2600 86 |
| within 10m) and the height gain will be accurate to | | | | 1400 152 2800 93 |
| within a metre or two.But the spec says it measures | | | | 1500 163 3000 99And this ones got a compass |
| height accurate to within one metre ?Well, what they | | | | too!!!Some of them do, they're not the best military |
| mean is that it will measure the change in height to | | | | marching compasses, but they do the job. The |
| within one meter, to measure the absolute height to | | | | pointers tend to be small, but with the backlit display |
| this accuracy you have to callibrate the meter and | | | | you can see enough to check your course and since |
| because the weather affects the air pressure, to | | | | it's on your wrist it's always instantly available and |
| maintain accuracy you'll have to re-callibrate | | | | never gets in they way of your ice axe. Keep your |
| whenever you can throughout the day. This sounds | | | | marching compass handy for real navigational |
| like a chore but it isn't. All you do is, every time you | | | | decisions, but check your watch often to stay on |
| reach a point where you know the height, say at the | | | | track you've set.The Final PictureAt the end of the |
| top of a mountain, a lakeside or any other point | | | | day, alti-watches are just another tool, they can be |
| where you can read the height off a map, you reset | | | | just fun, a serious training aid, or really get your ass |
| the actual height into the watch. This take a couple | | | | out of the deep stuff. If you are going to use it |
| of button presses and a few seconds. Even when | | | | seriously you need to practice using it and keep |
| you need the utmost accuracy, say when going into | | | | checking it against all the other inputs you have |
| the cloud at 2,000ft on the Cairngorm Plateau, you | | | | around you. Always keep it visible, in good weather |
| only need to do this every few hours at most. I've | | | | on your wrist, in bad weather put it on over your |
| set mine at the sea's edge and been meter perfect | | | | jacket - it's waterproof! A good navigator uses all his |
| on on the top of Ben More Assynt 4 hours later !And | | | | tools together and thus will spot it if one of them is |
| the weather?Well, set the watch to barometer mode | | | | misleading him. Personally my alti-watches (I'm on my |
| and check the air pressure right now. If its high, | | | | second!) have saved me some serious grief and kept |
| 1020Mbar and above it'll likely be dry. 980Mbar and | | | | me off bad ground more than once, I wear it every |
| below and it'll probably be grey and wet. I say | | | | day and watch the weather so I know whether to |
| probably, because it isn't foolproof, but it's a pretty | | | | take the bike to work or not!I can't say you have to |
| good rough guide. If you watch the weather over | | | | get one, but if you spend time outdoors I think you'll |
| time, say overnight you'll get to see the trend | | | | use it, and at todays prices why not just give it a |
| (shown on the little greaph on the watch), this shows | | | | try.Have fun, but keep safe out there!The Author R |
| whether the pressure is rising falling or maybe just | | | | Brammer is an avid walker climber snowboard and |
| staying the same. Generally, if the pressure is falling, | | | | founder of the Tech equipment retailer I share with |
| it's getting worse, if it's on the up, the sky will | | | | the readers my knowledge of the products I use to |
| probably be clearing soon. It takes a while to get the | | | | get the most out of your adventure sport. |