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using a Thommen TX 15000 altimeter / barometer

Greetings,  I recently obtained an old (like new) Thommen altimeter that uses barometric pressure to calculate altitude.  I know there are newer more advanced products on the market, but I liked the looks of it and wanted to give it a try.

 I'm hoping that there are a few users here that know something about the use of this older instrument.

 I have the manual and have read it, thus I have a basic understanding of the unit.  However, I'm a bit confused on how to set up the unit.   Would the barometer be set to the local reading?  Local airport barometer in my area presently is 30.36.  However, I am 25 feet above sea lever and my corrected to mean sea level reading is 30.42".   So, would the instrument be set at the local reading, or the calculated MSL reading?  It's not easy to calibrate given the very small window and screw in the back of the unit. Also, if I set the barometer to the local reading it would not be dead center at 12:00 o'clock on the face.  Would after setting the unit to the local barometric pressure would you move the dial to the known altitude and then use that as a base even though it is not dead center on the dial face?

In any case, any insight or past experience with this unit from other members would be greatly appreciated. 

Thanks, Dan 

 

Welcome to Trailspace, Dan.

I'm going to try and bump up your questions here, in case some of our other members can help.

Good luck!

Alicia

I used a Thommen altimeter for quite a while in the 70’s and 80’s.  I never used it more than casually as a barometer.  When using it as an altimeter I would reset the dial every time I arrived at a known elevation.  Small changes in barometric pressure such as with an advancing weather front can throw the readings off otherwise.  They are great little instruments.

I think you incorrectly identified your altimeter.  I think you refer to the TX16, which is good up to 15,000'.  Thommen produced other models; one good up to 21,000' and one for the 9,000 meter peaks.

I have the manual somewhere...  But just as good, this link provides access to the English version of the multi-national instruction pamphlet.

The only adjustments the user performs is rotating the bezel (notched silver ring) and/or the glass face.  

Ed 

Thanks for the info.  Actually, there is a small window on the rear of the unit ( both of mine).  Inside you can see a very small screw for adjustment.  With this you can indeed adjust the barometric reading for the local known reading.   In this way the instrument can used more accurately.  I was hoping someone had first hand knowledge is the use of these.  I recently used mine on a trip and did not find it to be all that accurate when compared to known altitudes. 

 As far as adjustments go, I would assume it needs to be calibrated to a known reading from time to time much like a pilot would do with the cockpit types.  I am a pilot and know well how to use one in the aircraft, hence my initial question on the used of them.

I assume since the manual does not address these screws on the back of the device that they are not intended to be tampered with by the user.

The instructions describe re-calibrating using the bezel whenever coming upon positions with known elevations, like lakes and peaks.  I find many lakes are not at the exact elevation the map states, perhaps that is what you are experiencing.  In that regard locations bench marked with USGS data plaques, such as dam spillways, peaks and other certified land features are the most reliable calibration points.

I had only one trip where the elevation reading was unreliable; it was on a winter trip in the Sierra in the mid 1980s (the El Niño year that took out all the piers along the California coast).  The trip lasted weeks, the storms were intense, and came one after another, causing the barometric pressure to fluctuate dramatically, resulting in readings of up to 90 foot variance from actual ground elevations.  No barometric pressure based instrument is very reliable under those conditions.  Additionally the temperature fell considerably below the -20⁰C (-2⁰F) limitation of the device. Hey winter mountaineering is a challenge...

Unless you are a weather hobbyist, the precise barometric pressure isn't necessary. If used as the manual describes, the altimeter will obtain accurate elevation readings, as long as the barometric pressure hasn't fluctuated to wildly since the last time a  benchmark calibration was performed, AND as long as the actual elevation change isn't too large between calibration points. (Don't expect an accurate reading atop a 14er if the last calibration point was at the trailhead, 8,000 lower.)  As for barometric readings, the device is limited as a weather barometer.  The manual alludes to this limitation, noting its use as a weather device in the field is limited to registering relative change in pressure over time while stopped at a given spot.  You can also derive weather related changes in pressure by noting the deviation in the elevation reading that occurred while traveling, comparing it to the actual elevation of the next way point with a documented elevation.  The assumption being the loss of accuracy between way points is the result of weather related air pressure changes.  But if your device is randomly drifting off calibration, it may be broken. 

Ed 

I have used my Thommen for quite a while. However, having used a number of other devices, using them on all 7 continents. And yes, I have used altimeter devices to get my plane off and back onto the ground (sold the plane after our move back to California and discovering that having a plane and moving to California are not financially compatible). The plane got flown from Mississippi to California, then to Canada (for climbs in the Bugaboos, back to Texas and other places). Then settled back in Calif.

Times have changed. A basic altimeter, barometer, plus your compass, are ok. The problem is the bulk. I don't know what your exploration approach is. But GPS receivers have become much easier to use when you get out into the back country A decent GPS is more accurate, more compact, and less expensive. You will get your altitude, tracks, etc.

Currently, I tend to use my Kestrel 5500 and GPS devices more than most other devices to deal with weather and navigation. And this is in locations like the Andes. It's a lot cheaper.

thommen

September 25, 2020
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