Date: Thu, 16 Aug 2012 20:27:37 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Gauge Mounting Options
In-Reply-To: <C6F8E3AF-96DA-4F07-9CE0-B217FA11FAAA@gmail.com>
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At 05:12 PM 8/16/2012, turbowesty wrote:
>Some earlier threads have me exploring gauges (wow, there are too
>many options for my feeble brain!).
One you may not have run across...Darrel Boehler of Digitool fame
felt strongly that a cooling-system pressure gauge that would read
between about +15 and -2 psig was a very important thing to have
given our overcomplicated cooling systems. You should be able to
find his arguments in the archive.
> I'll pick away at my questions by doing some research before
> asking too many questions here (like why would I choose mechanical
> rather than electrical,
Mechanical:
Pro - no electricity needed. May be easier/cheaper to make accurate
and/or mechanically robust against vibration. No big pressure-sender
fittings sticking out of the engine to vibrate or be hard to find
space for. Unlikely to wear out in any human lifetime subject to
corrosion or really extreme vibration problems or
overstress. Simpler to construct gauges with pointer travel greater
than 90 degrees.
Con- there's a stiffish tube running to the gauge, either the
capillary tube from the temp sender or an actual pipe in the case of
oil or manifold pressure gauges, and one end is tied directly to the
vibrating motor. Having that tube fail on an oil gauge could be a
big problem, both evil mess and loss of engine oil. Temperature
capillaries are metal; oil pressure either copper or nylon, each with
its own advantages.
Electrical:
Pro - flexible installation, only wires to run. Potentially flexible
scale changing by changing sender and gauge face.
Con - at low end, more expensive than mechanical. Price for price,
likely less accurate than mechanical. Pressure senders on
automotive-type gauges are normally *in essence* a mechanical gauge
connected to a fuel tank sender and are the size of a large toddler's
fist and moderately heavy. Not a good idea to hang them off an
engine on several inches of plumbing adapters; better to mount firmly
with a tube to the pressure port if necessary.
> which are more accurate,
It's necessary here to distinguish between accurate and
precise. Suppose an input of 20.00 units. A gauge that reads 27.43
every time on that input is extremely precise and repeatable, but
very inaccurate; while one that reads all over the map between 17 and
23 is much less precise and dead on for accuracy.
Both mechanical and electrical gauges if well made should be pretty
good for repeatability unless/until they start getting
sticky. That's good news because repeatability may be more important
than absolute accuracy in an automotive gauge.
An electrical gauge is unlikely to be rated better than +/- 3% *of
full scale* for any given reading; hence the general rule with analog
voltmeters to avoid readings in the bottom third of the scale. I
believe that mechanical bourdon-tube movements can do better than
that if you spend enough ("enough" might be quite a surprising figure
- for example I'm pretty sure that a submarine bourdon-type pressure
depth gauge with 300-degree rotation and a ten-inch face does
considerably better than three per cent, and I'm not sure I want to
know how much I paid for it). An electrical gauge working from a
sender also has to account for the
properties/characteristics/behaviors of the sender and its matching
to the gauge.
Note: digital gauges and meters have different accuracy
considerations. They are usually quoted as (+/-% of reading)+/-(so
many least-significant-digits on the display) for a given set of
conditions. Their basic accuracy is typically much better than
analog gauges, but that doesn't mean that an automotive pressure
sender hooked to one will give any more accurate results - and
they're much harder to read quickly.
In my recollection, accuracy specs for automotive gauges may be found
deep within the bowels of VDO etc. but otherwise on the same shelf
with hens' teeth.
> do existing senders need to be replaced, and on and on)
Gauges and senders are matched to each other. For maximum accuracy
an individual sender will be chosen and the gauge face
hand-calibrated to match it for each increment of the scale. Not
going to happen unless you do it yourself or are working in the
stratosphere somewhere. Next-best is to take a bunch of senders and
select the one that reads closest to truth with a particular
gauge. Also not going to happen in the automotive world.
Yours,
David
