From: jgd@dixie.com (John De Armond)
Subject: Re: Tire gauges (was: Re: ME33A Laser wearing too fast...)
Date: Tue, 29 Dec 92 19:20:36 GMT
Organization: Dixie Communications Public Access.  The Mouth of the South.
Distribution: na

xxkozak@lims01.lerc.nasa.gov (GEORGE KOZAK) writes:

>>Any suggestions where to get an accurate gauge?  Everything I see for sale
>>retail is the same old junk.  In theory, all but the cheapest rod-type
>>gauges (is there a name for these?) should be relatively accurate,
>>since they depend only on the geometry of the thing.  My wife, unfortunately,
>>insists she can only read dial-type gauges; those I've seen have been
>>pretty cheesy, and I don't want to spend $20 for a good one unless it can
>>handle glove-box temperature extremes without dying...

The rod gauges are among the most accurate types commonly available to 
the consumer.  Both in theory and in practice, they beat the el-cheapo
(regardless of price) dial gauges hands-down.  Assuming a well made
gauge with no stiction, the gauge is almost by definition linear and
very repeatable.  I have tested many such gauges in my lab against 
a W&T lab pressure standard have yet to find one lacking any obvious 
problems such as a bent plunger not accurate to better than one minor
division.

Bordon tube gauges, on the other hand, particularly the cheap kind sold
to the automotive market, have many problems.  Inherent to the mechanism
is a nonlinearity that manifests itself as an error term that is a
maximum at mid scale.  IF the gauge is hand-calibrated, the error can
be distributed across the range as ripple but it still remains.  The
mechanism can be damaged by excessively rapid pressure application or
removal.  Precision gauges are typically protected by a snubber, a 
device I've never seen on automotive gauges.  Unless the needle pivot
is equipped with ball bearings, there is stiction that causes repeatability
problems even if the gauge is tapped before reading.

>    Schrader makes an excellent digital gauge.  It comes in two  ranges,
>0-50 and 0-100 psi.  It's accurate to +-1% and very rugged.  I use the
>low pressure one and it beats the pencil and round gauges for ease of use
>and accuracy.  It calibrates itself each time you use it and "beeps" when
>the correct pressure is reached.  All this for $32.99.  They also sell 
>another model for $29.99 that's 0-100 psi.  

A couple of corrections in terms.  These gauges use a thermal conductivity
mechanism to measure pressure.  The unit does NOT calibrate itself when
used even though that is what they claim.  The controller merely 
rezeros the thing to accomidate current atmospheric conditions.  These
gauges are subject to many error terms including contamination of the 
sensor by oil, dirt, tire mounting lube, etc.  I bought one of these
things on sale at K-mart (closeout) awhile back for $10.  I have
not been impressed.  Lastly, don't confuse the display precision with
the instrument's accuracy.  Also note that pressure instrument tolerances
are normally expressed as a percent of full scale.  So the 100 psi unit
is only claimed to be +-1 psi.  Also note that either rod or dial-type
analog gauges have MORE resolution because one can interpolate between
minor divisions.

John

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