AdamR
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| posted on 23/11/09 at 04:00 PM |
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Mechanical gauges - how do they work?
I'm starting to think about instrumentation, and one of the options I am looking at is using mechanical gauges for water temp, oil pressure and
possible oil temp too. Mechanical gauges typically have nice full sweep needles and I also like the idea of the not having to worry about whether the
sender is working properly or compatible with the gauge.
My understanding of the water temp gauges is that you typically have a sealed capillary filled with a gas. The sensor plugged into the cooling system
expands/causes the gas to expand and pushes the needle round in the gauge.
Do mechanical oil pressure gauges work in the same way? Or do you have to plumb in a capillary to carry the oil to the gauge itself? Can't quite
believe it's the latter.
Any other comments in favour or against? I guess if the gauge/capillary/sender is all sealed together then I guess these are a real pain in the
backside to fit/remove.
[Edited on 23/11/09 by AdamR]
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blakep82
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| posted on 23/11/09 at 04:05 PM |
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i bought a mechanical oil pressure gauge years ago, had a copper pipe which was to go from the engine to the gauge. not for me thanks! went electic
after that
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cd.thomson
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| posted on 23/11/09 at 04:08 PM |
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haha, yes on mechanical oil pressure gauges if you disconnected the pipe from the back of the gauge, you'd end up with a very messy dashboard 
Craig
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dogwood
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| posted on 23/11/09 at 04:08 PM |
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quote:
Originally posted by AdamR
Or do you have to plumb in a capillary to carry the oil to the gauge itself? Can't quite believe it's the latter.
Better beleive it !!
just ordered my set of gauges, and deliberately gone for electrical ones because I didn't like the idea of oil etc behind the dash..
David
David
[Edited on 23/11/09 by dogwood]
FREE THE ROADSTER ONE…!!
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britishtrident
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| posted on 23/11/09 at 04:23 PM |
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The disadvantage of capillary temperature gauges is the capillary is very easily damaged if working on the engine. If that is the case the unit needs
refilled by a speicalist.
Bourdon tube oil pressure gauges never cause any problem --- Minis used them for years. But the tube at the engine end can get damaged when
taking the engine in and out but unlike the temperature gauge you can fix that yourself.
You can get oil presure and oil temp or oil pressure and water temp combined into one unit.
These days it is probably better to go digital or plasma.
[Edited on 23/11/09 by britishtrident]
[I] “ What use our work, Bennet, if we cannot care for those we love? .”
― From BBC TV/Amazon's Ripper Street.
[/I]
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02GF74
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| posted on 23/11/09 at 04:30 PM |
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hahah - good job I save me text - tried to correct double post and both disappeared 
you are correct.
they work by having a bellows to push the needle via levers - don't know the exact details.
temp gauges contain some fluid that expands a bellows - the movement pushes the needle.
oil pressure gauge has the oil travelling through a long thin tube to push the bellows - yes, oil does travel up the tube and if the nipple at the end
of the tube falls off, you will get oil in your cockpit - no need to ask how I found that out, a steady drip as opposed to Dallas type oil rig gushing
- you can drive on quite happily for a long time before you start to worry or drown.
it is easy enough to cut the copper line and resolder the nipple on.
advantage of mechanical gauges is that the needle pivot is centred - looks better IMO, they require no electrical wiring or power to operate and will
operate regardless if the engiine is running or ignition is off.
they tend to be cheaper too. (actaully not sure about that).
downside is since the gauge and sender is one piece, you need to rip the dash out to replace a guage but how ofter would you need to do that?
also the temp gauge capillary is quite long so you would need to loop and secure it from flapping around.
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Johneturbo
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| posted on 23/11/09 at 04:45 PM |
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How do electrical oil presure gauges work!?
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blakep82
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| posted on 23/11/09 at 04:47 PM |
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quote:
Originally posted by Johneturbo
How do electrical oil presure gauges work!?
sender screws into the side of the block, varies resistance in the sender, wire from sender to gauge completes circuit fuse-gauge-sender-block?
unless its a trick question?
________________________
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don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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Johneturbo
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| posted on 23/11/09 at 04:56 PM |
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quote:
Originally posted by blakep82
quote:
Originally posted by Johneturbo
How do electrical oil presure gauges work!?
sender screws into the side of the block, varies resistance in the sender, wire from sender to gauge completes circuit fuse-gauge-sender-block?
unless its a trick question?
yes, you pass the trick question of the day
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johnston
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| posted on 23/11/09 at 05:04 PM |
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I worked on a rally car with a nice mechanical oil pressure gauge.
Nice man that built it used plastic pipe and routed it over the manifold inside some heat wrap.
One day it decided to melt mid stage, oil light never came on so driver kept going thinking a faulty gauge. When he stopped the obviously the air
flow keeping the flames down also stopped and up she went melting the engine bay!!!!
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02GF74
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| posted on 23/11/09 at 05:04 PM |
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when current is passed trhough a coil in a magnetic field, it will move.
attach a need to the coil, choose the number of windings and magnets appropriately and then if there is a resistor whose value varies with
temperature, you have all the ingredients for a gauge.
this can be used to measure fuel level, pressure and temperature.
it can be a bit more complex than that as usually you may want some damping to prevent fast fluctuations of the needle, but that is another thread
really, just hope nobdy asks that question, my fingers are getting tired.
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AdamR
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| posted on 23/11/09 at 05:21 PM |
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What I really want is full sweep
Stack Pro Control gauges, but alas rather pricey! 
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02GF74
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| posted on 23/11/09 at 05:26 PM |
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cheap as chips!!!
what is interesting is how visible the red "stack" text is under different lighting conditions.
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britishtrident
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| posted on 23/11/09 at 05:49 PM |
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Just to clarify a few things car electric gauges for things like water and oil temp and battery condition tend to be bi-metal thermal types which
work on heating effect nothing to do with magnetism. Although less accurate than most other types they became industry standard because they are
cheap and very robust.
Old fashioned oil pressure and calipary temperature gauges are generally bourdon gauges although many have diaphragm type mechanisms.
[Edited on 23/11/09 by britishtrident]
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blakep82
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| posted on 23/11/09 at 05:49 PM |
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quote:
Originally posted by 02GF74
cheap as chips!!!
what is interesting is how visible the red "stack" text is under different lighting conditions.
photo shop i'd say, changing the color balance, ie taking the red out of the red logo to increase the green and blue
________________________
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don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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jeffw
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| posted on 23/11/09 at 08:24 PM |
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over damped, under damped or critically damped.... 
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blakep82
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| posted on 24/11/09 at 01:52 AM |
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quote:
Originally posted by Johneturbo
quote:
Originally posted by blakep82
quote:
Originally posted by Johneturbo
How do electrical oil presure gauges work!?
sender screws into the side of the block, varies resistance in the sender, wire from sender to gauge completes circuit fuse-gauge-sender-block?
unless its a trick question?
yes, you pass the trick question of the day
AWESOME!
________________________
IVA manual link http://www.businesslink.gov.uk/bdotg/action/detail?type=RESOURCES&itemId=1081997083
don't write OT on a new thread title, you're creating the topic, everything you write is very much ON topic!
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MikeRJ
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| posted on 24/11/09 at 06:23 PM |
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quote:
Originally posted by britishtrident
Just to clarify a few things car electric gauges for things like water and oil temp and battery condition tend to be bi-metal thermal types which
work on heating effect nothing to do with magnetism.
All the dashes I have looked at on modern cars almost exclusively use air cored magnetic or stepper motor gauges. The magnetic ones are quite clever;
they have two windings to cancel out changes in supply voltage, so no regulator needed.
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