Dan the man writes:
>Up to this point, Tom O'Malley and I are in perfect agreement. We differ a
>little, though, on how well they compensate for temperature. His experience
>has been that they compensate extremely well, and my experience has been that
>they don't compensate so well. Why the difference, I can't explain,
snip!
Well the difference should be obvious! My car is a Spitfire and yours
is a TR6. Everybody knows that the '70s Spifires were crafted to
Mercedes-like standards. :-)
>What other factors? Let's say you go out to your car and start it up. Your
>gas tank is almost empty, and the water in the engine is cold. Both the fuel
>and the temperature gauges are reading very near the bottom of the scale.
>Assume that your alternator is working just fine, and producing exactly 14.6
>volts. Now you drive your car 15 miles to the nearest gas station and fill
>up. The fuel gauge now reads full, and the temperature gauge is in the middle
>of its range. With the alternator still producing 14.6 volts, the current
>through the gauges has doubled or more, because of the change in engine
>temperature and fuel level. Because of the additional current through the
>stabilizer, the contacts will stay closed for a shorter period of time than
>before, reducing the voltage to the gauges, causing them to read lower than
>they should. The stabilizer has actually introduced an error! In this case,
>will the effects of ambient temperature changes be compensated for? If so,
>will they be compensated for when the gauges read low? What is the
>temperature of the stabilizer strip compared to the strips in the gauges? Do
>they both change the same amount with respect to changes in temperature? I
>don't know!
Well, if YOU don't know, *I* certainly don't! :-)
Actually Dan...I can't see any way that the stabiliser could be load
compensated. Particularly with only one stabiliser feeding two
gauges. I suspect that's why folks started looking at solid state
regulators in the first place. Regarding your comment above, I would
guess that the gauges are designed to run non-linear. Probably
another good reason to keep 'em far away from true voltage regulators.
Cheers!
Tom O'Malley
'74, '77 Spit <precision!>
>
>Which brings us back to Tom's statement about solid state regulators. As Tom
>said, there will be no temperature compensation with these, unless they were
>deliberately designed for it. I don't know if any of the units commercially
>available have this compensation or not. Does anyone on the list know?
>Temperature compensation would certainly not be hard to do.
>
>> Dan Masters, have you got some insight on this? Anyone else?
>
>I have a fair amount of insight into this sort of thing. Back in the early
>eighties, I was given the responsibilty for developing and implementing a
>program at TVA for determining the accuracy of the instrumentation used for
>safety functions at our Nuclear power plants. The simplist calculation for
>instrumentation accuracy required over 40 manhours to perform, and the most
>difficult took over 200 manhours! The calculations were usually fairly
>simple, once we had the required data in hand, but knowing what data was
>required, and where to get it, could be quite a daunting task. I would love
>to do a calculation on the gauges in our cars, but there is no way I would
>ever be able to get the data.
>
>I have never looked at instruments the same since then. As an example,
>consider driving accross country at 80 mph, and driving back at 20. You
>odometer would show a longer distance coming back than going, because the
>diameter of the tires is larger at higher speeds than at lower speeds. At 80,
>you cover more distance with each rotation of the tires. Since the
>speedometer only records rotation of the driveshaft, the additional distance
>is not accounted for. Thoughts like that drive me crazy now!
>
>I would be very surprised if the fuel and temperature gauges in our cars come
>close to 10 % accuracy. With the exception of the speedometer, accuracy is
>not as important as repeatability, anyway. If you have 2 gallons of gas left
>when the gauge reads empty, that is no problem as long as you ALWAYS have two
>gallons left. Who knows what the temperature of the water actually is - it's
>only important to watch for a descrepancy from norm.
>
>Hope this helps, and is not more than you asked for.
>
>Dan Masters,
>Alcoa, TN
>
>'71 TR6---------3000mile/year driver, fully restored
>'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
> http://www.sky.net/~boballen/mg/Masters/
>'74 MGBGT---3000mile/year driver, original condition
>'68 MGBGT---organ donor for the '74
>
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