[Healeys] Conclave tire pressure

[Bob Spidell]

How does a pencil/slider-type of gauge know what the ambient pressure 
is, and how does it compensate? I'm guessing it reads absolute pressure 
regardless of ambient pressure; has anyone tried this 'experiment' with 
one of those?

What would a Bourdon tube-type gauge, set to zero at Mean Seal 
Level--good ones have a 'calibration' knob, or the needle can be reset 
(I have one of those)--read at altitude without being reset (I'm 
guessing less than zero)? IOW, a Bourdon-type gauge is effectively a 
barometer, and will read less at altitude and give a higher 
relative--not absolute--pressure if not reset to zero.

I presume the digital gauges--which I've found to be the most 
repeatable, if not the most accurate--do have ambient pressure 
compensation. Richard, did you reset your gauge to zero at altitude, or 
do you use a digital gauge?

I did google it, from a reliable source:

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=167

This phrase: "... if tire inflation were set with a tire pressure gauge 
at sea level (where the atmospheric pressure of 14.7 pounds per square 
inch is used as ambient atmospheric pressure by the gauge), the same 
tire pressure gauge would indicate the pressure has increased at higher 
elevations where the ambient atmospheric pressure is lower"

I read this to mean the GAUGE, calibrated at Std. Temp and 
Pressure--760mm of mercury at 15degC--will read inaccurately unless 
reset for ambient air pressure. This is demonstrated in aircraft 
altimeters, which are essentially barometers but have the addition of a 
'calibration' mechanism, known as a Kollsman Window. So an altimeter, 
which is a pressure gauge with a means to adjust to both altitude and 
temperature needs to be adjusted quite often to give an accurate reading 
of altitude.

bs

ps. I'm not waging a semantics war here, I've often thought about these 
things myself and would like clarification.


On 5/24/2021 4:13 PM, richard mayor via Healeys wrote:
> Sheesh.   Temperature was not a factor. It was roughly the same 
> temperature in Big Bear as it was in Portland when I checked the 
> tires.    I race cars.  I know that tire pressure goes up when you 
> race.  I did not race the Healey before taking the tire pressures.
> This is not rocket science.  Google it.   Altitude affects tire 
> pressure.  Just like it affects the pressure in your ears.
>