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Is this the lowest and highest pressure ever in/near Europe?

alioth wrote:

However, some EFIS seem to have a restricted range, and it would cost no more to make an electronic one go from 850 to 1100 than it would to make it go 950 to 1050.

It would. It’s about twice the range. This means the accuracy will be cut in half as a start (for twice the cost). To get the same accuracy, it has to be twice as good with twice the range. That is a factor 4 in improvement. The cost to achieve that? 4-5x probably. Then the number of bits has to be increased correspondingly. You need twice the bitcount when the range is doubled. But, how much does such a transducer cost to start with ? €5? €50? An EFIS cost €5000 and up (sky is the limit).

ENVA ENOP ENMO, Norway

This makes no sense at all.

The sensor measures (static) pressure, down to very low values (e.g., around 250mb at FL 360), with resolution sufficient for the altimeter (some fractions of a millibar)

The QNH setting in an EFIS is simply a value which is used to calculate the altitude from that pressure, So you could support a QNH of, say, 500mb and make the electronic display read 0ft at FL 180. The sensor will measure the exact same 500mb, just the value displayed on the altimeter is different.

So there is absolutely no limit to the QNH value which could be set in the EFIS; even a ridiculous range (500 – 1500mb) would cost nothing.

There also is not sensor limit to the lowest QNH – the sensor works at the aircraft’s aerodynamic ceiling, which will be the same pressure [at a given temperature] regardless of QNH.

There may be a limit to the highest QNH if the sensor has a “stop” at, say, 1100mb

Biggin Hill

LeSving wrote:

It would. It’s about twice the range. This means the accuracy will be cut in half as a start (for twice the cost). To get the same accuracy, it has to be twice as good with twice the range. That is a factor 4 in improvement. The cost to achieve that? 4-5x probably. Then the number of bits has to be increased correspondingly. You need twice the bitcount when the range is doubled. But, how much does such a transducer cost to start with ? €5? €50? An EFIS cost €5000 and up (sky is the limit).

The range of the transducer is not the QNH range but the full range of pressures the aircraft can be expected to be exposed to. The QNH range has negligible effect on the necessary resolution of the transducer. Mechanical altimeters for GA aircraft are typically certified to FL250, so assuming that as the highest altitude, the lowest pressure would be about 375 hPa. Extending the range in the other end from 1050 to 1100 hPa would mean a range increase of about 7%.

Last Edited by Airborne_Again at 18 Feb 07:30
ESKC (Uppsala/Sundbro), Sweden

Cobalt wrote:

The QNH setting in an EFIS is simply a value which is used to calculate the altitude from that pressure, So you could support a QNH of, say, 500mb and make the electronic display read 0ft at FL 180. The sensor will measure the exact same 500mb, just the value displayed on the altimeter is different.

Is it like: how mercury based thermometers can show degrees in Kelvins without having to liquefy helium?
I think there is the measurement range and some offset, the latter should be cheap in an EFIS

Last Edited by Ibra at 18 Feb 10:10
ESSEX, United Kingdom

The sort of pressure sensor used in “cheap” (i.e. GA) avionics looks something like this

data sheet

Administrator
Shoreham EGKA, United Kingdom

The range of the transducer is not the QNH range but the full range of pressures the aircraft can be expected to be exposed to.

Yes, sorry, brain disorder

ENVA ENOP ENMO, Norway

Peter wrote:

The sort of pressure sensor used in “cheap” (i.e. GA) avionics looks something like this

Speaking about it. Getting out of winter hibernation and starting to fly again today. Looked over the P2008 yesterday evening. It’s brand new, bought last summer, and already the G3X is broken, no altitude. Luckily the G3X is only for “situational awareness” and the little tiny “backup” instrument works just fine Would be interesting to open that Garmin, and see what kind of stuff actually is inside.

ENVA ENOP ENMO, Norway

LeSving wrote:

and already the G3X is broken, no altitude

I had the same problem with the G5. Mendelssohn’s (the Garmin dealer where we got the G5) did say the problem was uncommon, it was the first time they had a G5 sent to them for repair with a problem with the altimeter. The rest of the instrument worked fine (including the airspeed indicator, which suggests the ASI is using a different component to sense the static pressure, independent of the altimeter’s sensor).

Andreas IOM
108 Posts
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