Tachometer Reads High

Discussion in 'Maintenance Bay' started by Rcmutz, Mar 3, 2021.

  1. Rcmutz

    Rcmutz Line Up and Wait

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    I have a Mitchell Tach that has started to read high by 100-200 RPM. Happened once the air temperature has really turned cold and density altitudes below SL. Does anyone have an idea of why this would happen? I am aware that as a Tach gets old it could begin to read low, but have not heard or read much about high reading tachs.
     
  2. Mtns2Skies

    Mtns2Skies En-Route

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    Mine does the same. I found it out when my prop got dynamically balanced and they had an electronic tach reader. Reads about 100-150rpm high. I just note it and carry on.
     
  3. Rcmutz

    Rcmutz Line Up and Wait

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    Mine was reading fine up until just a week or so ago. I was thinking the very cold air the engine is producing more power, and with a fixed pitch prop it could result in a higher RPM. This Tach was just overhauled about 6 months ago. Wonder if there is a problem with the cable?
     
  4. Silvaire

    Silvaire En-Route

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    That's what they do. In fact it turned out that mine had been reading high for a long time and when it really went wonkers and began erratically reading up to 400 rpm high I bought a new one and immediately gained 5 mph because I had been cruising around at 2000 rpm all those years thinking I was at 2250 :cool:
     
  5. thomasdr72

    thomasdr72 Pre-takeoff checklist

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    Mechanical tachs start doing that when they want to be replaced with a new digital tach. EI has one that will cost you about $450 plus install. Never have to worry about tach cables or tach oil seal leaking on the engine again...
     
  6. Dan Thomas

    Dan Thomas Final Approach

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    They read high because the light oil on a tiny bearing in there gets stiff in low temperatures, especially if the tach is old and the oil has dried out some. The tach cable drives a magnet inside an aluminum drum, and the drum drives a shaft attached to the indicator needle on the face of the tach. The magnet drags the aluminum cup around against a very light hairspring. The aft end of that needle shaft is supported by a tiny bearing in the tach's input shaft, which is spinning the whole time the engine is running. The oil in question is in that little bearing, and stiff oil tends to pull the needle shaft farther around the scale that it should.

    Tachs usually read low as they age. The magnet gets old and loses some of its strength. And before someone says "hold on a minute, that aluminum cup isn't ferrous and the magnet can't drive it," you don't understand eddy current. The magnet's movement generates an electrical current in the aluminum, which forms its own magnetic field that interacts with the magnet. Faster magnet spin generates more current in the cup, more magnetic field in the cup, and more interaction, pulling the needle farther around the scale.

    Magnetism generates electricity in copper wire, too, in case anyone forgot. It's not ferrous, either. And you rely on that fact for your ignition spark, your alternator or generator's output, your older avionics with their tuned tank circuits, for the strobe light powerpack transformer, and other stuff. We're surrounded by that phenomenon and take it for granted. Every motor in your house, every electron that comes from the power company....

    Don't buy a new mechanical tach. Get an electronic one. In Canada we have to check magnetic-drag-type tachs yearly for accuracy, and they have to be within 4% of true in the middle of the cruise RPM range. That regulation doesn't apply to electronic tachs, since they're crystal-controlled and won't drift. And an electronic tach could cost less than a new mechanical one.
     
  7. Rcmutz

    Rcmutz Line Up and Wait

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    Dan thanks for the explanation. It was weird because after the temps and Tach warmed up, it began to read normal again. If I had fuel flow, I could of checked it against that at the time to see I was cruising at probably 2300 RPM true with indicated at redline of 2700.