German ATA question (equivalence to inches Hg)

[Guest deleted@83466]

I'm not sure why I never asked myself this before, but let's say we're flying along with our engine set at a manifold pressure of 1.0 on a Standard Day, at sea level.  We can translate that to a manifold pressure of 29.92 inches Hg.

 

Now, let's say either it isn't a Standard Day, or else, we simply aren't at Sea level anymore.  Does the 1.0 ATA tickmark on the guage still correspond to a manifold pressure of 29.92 " Hg, as an absolute measurement, or is it instead reflecting the current ambient pressure at my current altitude and temperature?

Edited September 17, 2018 by SeaSerpent

[unreasonable]

IIRC JtD informed me that ata on the German gauges actually stands for technical atmospheric pressure, which is not quite the same as atm, being ~0.968 of an atm and defined in a different way, so not quite 29.92"Hg.  

 

https://en.wikipedia.org/wiki/Atmosphere_(unit)

 

But whatever the measurement, it is measuring the pressure in the system  based on it's standard quantity, so the answer must be that if it is showing 1.0 it is showing the same pressure irrespective of the ambient pressure.  I think.... wait for an engineer to confirm or deny.

[ZachariasX]

ata is kg/cm2. It is almost an athmosphere like you posted @unreasonable. But not exactly.

 

The main difference for practical purposes is however the Germans were evil enough using it for not following the proper English way that is discounting ambient athmospherinc pressure and just counting „boost“, a unit that starts at 0 even though there is almost 30, well, in real English weather maybe 25‘‘ Hg pressure. It much rather starts with 0 if there is no air at all and will reach 1 over England while already running considerably on the blower.

 

Was it atm, it would also only show boost, masking Engish athmospheric conditions as well.

 

 

[Guest deleted@83466]

Ok, so say the outside pressure in 26 inches of mercury, and I'm sitting there on the field with the throttle wide open,  and I have no supercharging:  is my guage going to read roughly 1.0, or is it going to read less than that because the outside pressure is less than standard?

Edited September 17, 2018 by SeaSerpent

[Art-J]

Less, just like American ones will show less than 29.92 in such scenario. Manometer is manometer, it doesn't know what sea level is. It might be calibrated for some arbitrary reference value (theoretical sea level) and arbitrary unit used in this or that country, but in the end, it shows specifically what it can "sense" around it. 

[ZachariasX]

39 minutes ago, SeaSerpent said:

Ok, so say the outside pressure in 26 inches of mercury, and I'm sitting there on the field with the throttle wide open,  and I have no supercharging:  is my guage going to read roughly 1.0, or is it going to read less than that because the outside pressure is less than standard?

If you had an ata calibrated gauge (wound't make too much sense without a supercharger), you'd read 0.87 ata with the throttle fully open on a naurally aspirated engine. As @Art-J said, for American crates you'd have 26, and british "boost", you'd have -3.9.

 

Edited September 17, 2018 by ZachariasX

[Guest deleted@83466]

Thanks guys.  Excellent answers.  I understand now.