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Oil gauge accuracy

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Hi everyone.

I have a Smiths capillary gauge and I have suspicions about it’s accuracy.  I see that they make electrically operated gauges too, like other manufacturers.  I want to keep Smiths ideally as they match the main instruments.  Does anyone know which is more accurate and more reliable - electrical or mechanical?  I think mine is under reading, and it also seems to have the tiniest amount of oil mist getting inside the instrument, so I suspect a tiny fracture in the pressurised part of the mechanism.

In a similar vein, my Smiths voltmeter gets warm to the touch, surprisingly so, and much more than the others with the lights on.  Is that normal in a voltmeter?

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Posted (edited)

Old-style analog voltmeters use either a 'hot wire' gauge or a two-coil balanced-armature. They do get warm but they also include temperature-compensation so this is not a problem.

Generally, capillary-gauges are more-consistent in their readings than electric ones - the electric ones, even with voltage-regulators [the old Smiths/Lucas bimetal ones are horrid!] can be affected by battery voltage variations whereas a capillary one won't be.

As regards oil-pressure gauges the important thing is not the actual numbers-on-the-dial (they're only generally accurate to +/- 10%) but to learn what is 'normal' and watch for any departures which will indicate that there's a possible problem. A while back the oil-pressure gauge on my 90TD5 warned me that a Chinese "Blue-Box" oil-filter I'd fitted was really restricting the oil-flow - replacing it with a genuine JLR filter [made in France] probably saved me an engine.

See

https://forums.lr4x4.com/topic/103319-blue-box-oil-filters/

Edited by Tanuki
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I agree with above, capillary gauges tend to be more accurate. But it all depends on calibration, so as long as you know the gauge you're looking at, you can interpret the reading. Why do you doubt the accuracy? If there really is a leak, I'd expect the mist to quickly build up with use .An electrical one, while easier to calibrate, is susceptible to bad connections and voltage fluctuations and bound to give a different absolute reading, without necessarily being more accurate.

As an aside, in my Lotus Excel the oil pressure gauge needs to be mounted exactly right, or it will not return to 0 with engine off, or alternately harldy move with running engine. It's also a Smiths, I set it to get a reasonable reading and am happy to use it as a relative indication and not an absolute reading.

Filip 

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I have doubts about the accuracy as the gauge reads lower than nominal figures on my Tdi, and has ever since the engine was rebuilt with all new bearings.  The pump was stripped and inspected, and ultimately replaced with another that seemed equally good inside.  Removing the washer from the oil stat helped - this seems a variable fitted item anyway, and warm oil pressure did pick up a bit after it took it out (oil temperature is still good, even on a hard run).  I didn’t replace the cam bearings, but they seemed good, passed the inspection by the engineering shop and most remanufacturers say they rarely need replacing unless they slip from position.

Im getting about 20psi at idle (15 with the oil stat washer fitted) and about 45-50 at mid revs once the oil has fully warmed up.  Book figures are 25 and 55, which my old gauge and 12j matched exactly.

I broke the old Smiths gauge that ran before this one (can’t remember how, but probably something like cracking the internals by overtightening the hose, which did tend to weep at the connection).  I may have damaged the replacement swapping the faces over, as the new one had the ivory face and I wanted the old one’s black face - it’s possible that is why I get a small amount of oil mist deposits inside the new one and I may have ruined its calibration by bending the needle while swapping the faces over.  Not sure, but a suspicion.

I’m surprised the mechanical versions are more accurate.  I take the point about not being perfectly accurate, and just learning where the needle is under normal conditions, but I’m a bit OCD on these sorts of things, so I’ll replace it at some point.  The new electric version has internal voltage stabilisation, so doesn’t use the big Lucas unit on the back of the speedo.  It should be pretty stable with a healthy alternator.

 At least it sounds like the voltmeter, fitted during the car’s rebuild, is ok - it gets about as warm as a heavily used mobile phone screen, very warm but not quite “hot”.  I’d imagine that’d drain a lot of power if it wasn’t connected to the battery via the ignition switch gauge circuits.

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Just remove the glass and tweak the needle round a bit.

I did it with my lightweight's fuel gauge and it never worried me again ;)

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48 minutes ago, Bowie69 said:

Just remove the glass and tweak the needle round a bit.

I did it with my lightweight's fuel gauge and it never worried me again ;)

😂😂😂. Did it give you infinite range?  Could save a fortune on fuel....

I’d be quite happy to do that if I knew I could reasonably accurately calibrate it.  The oil misting is quite small and hasn’t worsened over 40,000 miles, I don’t think, so I don’t know if it’s related or even still occurring.  The trouble is finding an accurate gauge to calibrate it from, unless there is a company that can calibrate these thing cheaply.  I suppose rigging up a line to a tyre or inner tube and using a known tyre pressure gauge to check the tube pressure would work...

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Well, it meant that a full tank actually showed as full, and empty was just off the bottom of the scale.

It worked, don't knock it ;) 

Honestly, any automotive gauge is NOT accurate in any shape or form without spending a fortune. Some junky old smiths gauge that has been sitting around, or worse being bounced around in an old series for 30 years is unlikely to be even remotely close.

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This one was brand new from Holden when fitted about ten years ago or so, so age and abuse wasn’t a factor, just me!

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I can understand you want the gauge to read exactly right, but with the values above I'd say you're more than close enough. Especially considering the gauge has been opened and the faces swapped.

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Thanks, fellas.  I’m very fussy, perhaps unhealthily so, about these things, but the discussion has been useful.

If anyone has a 52mm Smiths white on black face clock, by the way, let me know - they’re incredibly expensive at Holden, but I want all the instruments to match and I’m sick of trying to figure out how to set the stereo clock every time I have a power interruption to it - they’re far too damned complicated!

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For better accuracy you could go for actually calibrated industrial stuff but it gets spendy, this is the cheapest calibrated gauge RS sell that looks even close to suitable, claims +/-1.6% accuracy:

https://uk.rs-online.com/web/p/analogue-positive-pressure-gauges/0176208/

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