Altimeters/barometers - portable

[Harry Bloomfield]

A question which puzzles me about them and their calibration.

I've not looked into them in depth, but one point puzzles me....

An altimeter is as I understand it, just a barometer, a pressure sensor, but instead of being marked up in pressure, they are marked up in altitude instead - the higher you go, the lower the air pressure.

Barometers in a fixed location, are calibrated against ASL and comparison to known local standards. My weather station I calibrated against a local met station, during a time of steady pressure throughout the country.

Altimeters, are likewise calibrated at a known height whilst on the ground, then offset recalibrated whilst in the air when air pressure can vary from area to area using ground station transmissions. I'm assuming none GPS altimeters and no reference to known heights via maps, nor GPS reported ASL.

So how can hand held altimeters, or wrist worn altimeters ever be accurate, when they rely upon varying air pressure. Some such items even include a display of air pressure.

 

[SFK]

So how can hand held altimeters, or wrist worn altimeters ever be accurate, when they rely upon varying air pressure. Some such items even include a display of air pressure.

I always used them for change in altitude over a day - not actual altitude.,
Useful when hiking up mountain or skydiving. Not for actual hight.

In aircraft where calibrated to sea level, I always saw them as being used to gauge gross hight to ensure you were high enough to fly over the Rockies and not into them when in whiteout.
Although head-wind was the next issue, and many cases of people reducing height based on flight time not considering increase in head wind and hitting the mountain they thought they had flown over.

SFK

 

[Harry Bloomfield]

Although head-wind was the next issue, and many cases of people reducing height based on flight time not considering increase in head wind and hitting the mountain they thought they had flown over.

As in the case of the Lancaster bomber converted to commercial passenger use, which crashed in the Andies soon after the war.

 

[bernardgreen]

https://support.garmin.com/en-GB/?faq=vVuRmL80i18S6eC2vGMpz9

Many pilots confidently expect the current altimeter setting will compensate for irregularities in atmospheric pressure at all altitudes, but this is not always true. The altimeter setting (broadcast by ground stations) is the ground station pressure corrected to mean sea level. It does not account for the irregularities at higher altitudes - particularly the effects of nonstandard temperature.

As a slight aside.. the frequencies allocated to 5G mobile phone channels in the USA interfere with the radar systems that provide critical height above terrain information to pilots when on final approach to landing.

 

[Harry Bloomfield]

As a slight aside.. the frequencies allocated to 5G mobile phone channels in the USA interfere with the radar systems that provide critical height above terrain information to pilots when on final approach to landing.

Yep - that hit the news last week..

 

[xm77]

When flying you set your altimeter for one of three settings
Height over the ground you took off from so it reads zero on your home airfield when on the ground known as a local QFE setting
Height over mean sea level for the area known as a QNH setting - so you are given the pressure setting for the area QNH and set the sub-scale of the altimeter to that figure.
Finally the international airways setting for flying in regulated airspace so everyone in the entire world uses the setting which is 1013.2mb

What your real actual height over the actual ground below you of course will vary considerably. It also depends on how the real atmosphere varies as you go up in terms of pressure and temperature compared to the "standard international atmosphere" which is what altimeters are built to. Likewise if you fly from a low pressure area into a high pressure area or vici versa the reading on your instrument will be different even if you keep to the same height over a imaginary flat plain below you.

There have been many accidents though incorrect settings of altimeters. The standard one still repeated is where one sets local QFE take off and you fly to another place which has land higher than your home airfield. you then forget to get the local QFE. Then you do an aerobatic display based on your incorrectly set altimeters display to ensure you are high enough(!) and suddenly find as you exit the loop that the ground is far far closer than it should be and getting closer every seconds and then......

 

[Harry Bloomfield]

I think I might have got to the bottom of it.... Feel free to correct me if I have it wrong.

It's a matter of proportion - the air pressure due to normal weather only varying by a tiny amount and altitude making a much larger difference to measure air pressure...

1020.0 mb at sea level, becomes 1001.30 at 100m ASL, so a fairly large change in pressure for just 100m increase in height. Near enough for mountaineering purposes, but for aeronautical use just not accurate enough, so they have to factor in the known local barometric pressure.

 

[justigator]

Finally the international airways setting for flying in regulated airspace so everyone in the entire world uses the setting which is 1013.2mb

Not quite...only above the transition altitude, which varies from place to place but is always high enough to be sure that there can be no risk of flying into terrain or a structure upon it.

 

[Harry Bloomfield]

I think I might have got to the bottom of it.... Feel free to correct me if I have it wrong.

It's a matter of proportion - the air pressure due to normal weather only varying by a tiny amount and altitude making a much larger difference to measure air pressure...

1020.0 mb at sea level, becomes 1001.30 at 100m ASL, so a fairly large change in pressure for just 100m increase in height. Near enough for mountaineering purposes, but for aeronautical use just not accurate enough, so they have to factor in the known local barometric pressure.

And my theory above was entirely wrong too - having bought myself a watch which shows barometric pressure and altitude. You calibrate the pressure so it matches known local accurate stations, if you then know your altitude, you can then calibrate it - but the calibration is only good, whilst that pressure remains stable. When the pressure changes the indicated height can also vary quite widely.

 

[charliegolf]

When the pressure changes the indicated height can also vary quite widely.

In the flying arena, this is what makes the promulgated pressure setting critical. If everyone under the same flying regime has the same setting, variations due to atmospherics will affect all equally, thus maintaining vertical separation and safety. Oh, it's generally accepted that 1mb is approx 30ft, so your 100m alt change would be more like 10mb.