Date: Mon, 13 Oct 2008 10:57:54 -0700
Reply-To: Keith Hughes <keithahughes@QWEST.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Keith Hughes <keithahughes@QWEST.NET>
Subject: Re: Gauges
In-Reply-To: <486361F1C8E7456FA884412B630048D1@mike2d93581d7f>
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Mike wrote:
> Nothing hard about it! In the frame of the question posed to the list,
> I am correct, in regards to automotive guages, INCLUDING tire pressure
> guages.
> See the following references;
Well, first let's recap shall we? You wrote "1 Bar = 1 atmosphere = 14.7
PSI (@ sea level). What's so hard about that?" That was incorrect, and I
pointed that out to you. It's irrefutable, even using your own references.
All the subsequent discussion resulted from your attempt to equivocate
with "Sea level pressure IS assumed as the "0" reading on every pressure
guage designed for automotive use", an assertion for which you still
provide no reference. Gauges with a needle stop pin *may* be ambient
gauges, referenced to sea level, or they may not. If they are ambient
gauges, the needle will be spring loaded against the pin if you're above
sea level. OK, so;
> http://www.nationmaster.com/encyclopedia/Pounds-per-square-inch
>
> "Psig (pound-force per square inch gauge) is
> a unit of pressure relative to atmospheric pressure at SEA LEVEL. By
> contrast, psi measures pressure relative to a vacuum (such as that in
> space). Most pressure gauges, such as tire gauges, are calibrated to
> read zero at SEA <http://www.nationmaster.com/encyclopedia/Sea-level>
> LEVEL, because most applications require the difference of pressure."
>
Don't know who these folks are, but they have their terminology wrong.
Gauges that are referenced to sea level are properly called "ambient"
gauges, although they read in UNITS of psig. See below.
> http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech103.html
>
> # "In determining pressure ratio, the absolute pressure at the
> compressor inlet (P2c) is often LESS than the ambient pressure,
> especially at high load. Why is this? Any restriction (caused by the
> air filter or restrictive ducting) will result in a “depression,” or
> pressure loss, upstream of the compressor that needs to be accounted
> for when determining pressure ratio. This depression can be 1 psig or
> more on some intake systems. In this case P1c on a standard day is:
> 14.7psia – 1 psig = 13.7 psia at compressor inlet
> # Taking into account the 1 psig intake depression, the pressure ratio
> is now:(12 psig + 14.7 psia) / 13.7 psia = 1.95.
> # That’s great, but what if you’re not at SEA LEVEL? In this case,
> simply _**s**ubstitute the actual atmospheric pressure in place of the
> 14.7 psi in the equations above to give a more accurate
> calculation**_. At higher elevations, this can have a significant
> effect on pressure ratio.
> # For example, at Denver’s 5000 feet elevation, the atmospheric pressure
> is typically around 12.4 psia. In this case, the pressure ratio
> calculation, taking into account the intake depression, is: (12 psig +
> 12.4 psia) / (12.4 psia – 1 psig) = 2.14
> # Compared to the 1.82 pressure ratio calculated originally, this is a
> big difference."
>
> ****Notice where your statement regarding Denver pressure being 11
> psia is NOT correct, it's stated here as 12.4.
>
OK, wow, you got me! It's been a few years since I measured atmospheric
pressure in Denver, and my memory ain't what it used to be. The example
was provided to illustrate the issue of having to correct for local
atmospheric pressure, and that point is as valid for 12.4 psia as it is
for 11 psia. Please note, however, that the quote you provided above,
right after the spot where you capitalized "sea level" as though that
somehow supported your point, it states that you have to substitute the
ACTUAL atmospheric pressure if you want an accurate ratio.
> http://www.worldpath.net/~thompson/misc/cfm%20mystery.htm
> <http://www.worldpath.net/%7Ethompson/misc/cfm%20mystery.htm>
> "Air at SEA LEVEL has an atmospheric pressure of zero pounds per
> square inch as measured by a pressure gage (psig) and 14.7 pounds per
> square inch on the absolute scale (psia)."
Exactly what I've been saying! Note that a calibrated gauge pressure
gauge (not an "ambient" gauge - see below) will always read zero at
ambient pressure (when properly calibrated). Note also that air at sea
level is NOT 1 Bar.
> http://www.ptcmetrology.com/Metrology_pressure.html
>
> "Pressure is usually measured, either as absolute pressure (psia),
> _/**or relative to atmospheric pressure(psig) such measurements are
> called gauge pressure*/_*. An example of this is the air pressure in a
> car tire, which might be said to be "35 psi," but is actually 35 psi
> above atmospheric pressure. Since atmospheric pressure at SEA LEVEL is
> about 14.7 psia, the absolute pressure in the tire is therefore about
> 49.7psi.
>
> Atmospheric pressure is the pressure exerted by the air around us. The
> pressure varies both with altitude, and weather patterns. Standard
> atmospheric pressure at SEA LEVEL is defined as 1 atmosphere equal to
> 760 millimeters of mercury (760 Torr) and 101,325 Pascals. 29 117/127
> inches of mercury 29.92 inHg 14.6959 psia or 0 psig (pounds-force per
> square inch, absolute or gauge lbf/in ^2"
>
And standard atmospheric pressure is STILL not 1 Bar! Squirm all you
want, you can't redefine standard units, not even non-SI units.
> I've cited a few references supporting my position; can you you cite
> some references against my position? I don't believe that you can.
>
Well, the examples you cited don't even support your position. As for a
few references, how about;
http://en.wikipedia.org/wiki/Pressure_measurement
"Although pressure is an absolute quantity, everyday pressure
measurements, such as for tire pressure, are usually made _/**relative
to ambient air pressure*/_*. In other cases measurements are made
relative to a vacuum or to some other ad hoc reference. When
distinguishing between these zero references, the following terms are used:
* *Absolute pressure* is zero referenced against a perfect vacuum,
so it is equal to gauge pressure plus atmospheric pressure.
* *Gauge pressure* is _/**zero referenced against ambient air
pressure*/_*, so it is equal to absolute pressure minus
atmospheric pressure. Negative signs are usually omitted.
* *Differential pressure* is the difference in pressure between two
points."
Or, you might check this out, if you want confirmation that gauges
require zeroing during calibration, unless they are pinned and preloaded
- i.e., ambient gauges:
http://msl.irl.cri.nz/training_&_resources/Technical_guides/TG13.pdf
Or:
http://www.kinequip.com/pressure_terminology.asp
Gauge Pressure—_/**Atmospheric pressure serves as a reference level for
other types of pressure measurements. One of these is "gauge pressure."**/_
Gauge pressure is either positive or negative, depending on its level
above or below the atmospheric pressure reference. For example, an
ordinary tire gauge showing 30 pounds (actually, 30 psi) is showing the
excess pressure above
atmospheric. In other words, what the gauge shows is the difference
between atmospheric pressure and the pressure of the air pumped into the
tire. Gauge pressures can be either positive (above atmospheric) or
negative (below atmospheric). _/**Atmospheric pressure represents zero
gauge pressure**/_.
Or this:
http://www.iqsdirectory.com/pressure-gauges/
Types of Pressure Gauges
* Absolute pressure gauges measure any pressure above vacuum (zero
pressure).
* Air pressure gauges
<http://www.iqsdirectory.com/air-pressure-gauge/> measure the air
pressure of pneumatic equipment.
* **Ambient gauges**, also called “sea level gauges,” are the most
commonly used gauge. Ambient gauges are preset to read zero
pressure at standard atmospheric pressure (14.7 psi).
* Combination gauges measure both pressure and vacuum.
* Commercial gauges, also referred to as “general purpose gauges” or
“equipment gauges,” are low-cost measuring instruments designed
for applications that do not have severe conditions. Commercial
gauges may be ruggedly constructed but are not typically
economical to repair.
Now, continue wizzing into a stiff breeze if you want, but I'm not
wasting any more time or bandwidth on this.
Keith Hughes
'86 Westy Tiico (Marvin)
