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Any simple way to check a tach's accuracy.

Old 06-05-2007, 08:10 AM
  #26  
karolh
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Default RE: Any simple way to check a tach's accuracy.


ORIGINAL: Gulliver

Another $.02

The light bulb calibration method does assume that the calibration curve is linear, which it may not be. Might explain your deviation at the high end after you tweaked it.
I was just going through an old copy of RC Airplane Buyers Guide, and in the Meters section and saw a picture of my old Royal Tach. It was turned on and guess what it as reading .....035, while several others shown were reading 036.

Mine sometimes fluctuated between 034and 035. Go figure

Karol
Old 06-05-2007, 05:43 PM
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Default RE: Any simple way to check a tach's accuracy.

another vote for the Fomeco.
i also have a mechanical tach that was made by Kavan some years ago...........it is a precision instrument, and Dad and i used it to tach all our racing engines and Free Flight comp engines.
over a week ago i checked the RPM's of my 91FS Surpass....the mechanical tach read within 50 rpm of the Fomeco.
Old 06-05-2007, 10:09 PM
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Default RE: Any simple way to check a tach's accuracy.


ORIGINAL: pilotpete2

Hey All,
First use a fluorescent lamp, not an incandescent lamp.
With a 2 blade setting it will read 3600 on 60 Hz. and 3000 on 50 Hz. The reason there is no conversion needed is simple, any lamp "flickers" at twice the line frequency, so at 60 Hz the lamp actually goes on and off 120 times a second, or 7200 times a minute giving you the 3600 reading in 2 blade mode.
Pete
Be careful, the new florescent lamps with electronic ballast's will give false readings.
Old 06-06-2007, 08:07 AM
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Default RE: Any simple way to check a tach's accuracy.

ORIGINAL: Pile-O-Wood
Be careful, the new florescent lamps with electronic ballast's will give false readings.
Yeah, I just ran into that one too, the new work shop units over my building table, not so good[]
Pete
Old 06-07-2007, 07:34 PM
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Parkerm
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Default RE: Any simple way to check a tach's accuracy.

The US Standard of 60 hz is a target not an exact frequency. I have seen consumer power frequency dip as low as 57 and go as high as 62 hz. So just because your tach doesn't read EXACTLY 3600 when pointed at a bulb does not mean the tach is bad, and it doesn't mean it needs adjusting. You are more than likely seeing the actual hz of the power grid at that particular moment.

Old 06-09-2007, 09:44 AM
  #31  
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Default RE: Any simple way to check a tach's accuracy.


ORIGINAL: Parkerm

The US Standard of 60 hz is a target not an exact frequency. I have seen consumer power frequency dip as low as 57 and go as high as 62 hz. So just because your tach doesn't read EXACTLY 3600 when pointed at a bulb does not mean the tach is bad, and it doesn't mean it needs adjusting. You are more than likely seeing the actual hz of the power grid at that particular moment.

Where did you see that?
When I was in engineering school I was told the frequency was very carefully controlled as most of the electric clocks depended on it. That was years ago and now the clocks usually are crystal controlled so they are not dependant on it. But I don't see why they would drop the control. There are still clocks that have a synchronous motor
Old 06-09-2007, 06:26 PM
  #32  
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Default RE: Any simple way to check a tach's accuracy.

More than just the old synchronous motor ac clocks, also every cheapo ac powered digital clock and clock radio (they just have a digital counter that is controlled by the 60Hz freq). when our power goes out for a day or so the clocks in the the stove, microwave, yada yada go waaaaay fast when running off our generator.
The frequency on the grid does vary slightly, goes flat when load is high, my understanding is power grid adjusts daily at night to eliminate any cumulative error from building up. that's why cheapo ac electric clocks are so accurate long term, technically they ain't clocks at all there is no internal clock, the power grid does the timekeeping
Pete
Old 06-09-2007, 07:18 PM
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Josey Wales
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Default RE: Any simple way to check a tach's accuracy.

I like my Hangar 9 tach....gives the same readings as my buddys Fromeco tach
Old 06-09-2007, 10:31 PM
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Default RE: Any simple way to check a tach's accuracy.

I just purchased this unit from LHS a few weeks ago. It's a Hanger 9 combination tach and volt meter for TX and RX. Great unit! Had to replace my others because both failed, old they were.


http://www.horizonhobby.com/Products...?ProdID=HAN111
Old 06-10-2007, 07:51 AM
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Default RE: Any simple way to check a tach's accuracy.

It was actually noticed in the oilfield in the 1980's. As rod pumped wells were converted from gas engines to electric prime movers using 3 phase power. It was assumed that the measured strokes per minute would be constant. Several people noticed that the strokes per minute would vary + or- 0.5 if it was measured at different times over several months. Variations in Hertz were not considered as a reason because everyone assumed a stable 60 hz power supply. It became an OFM (oilfield mystery). A few years later computerized rod pump controllers were perfected. As a technician for these controls, I noticed that the computer recorded and stored the lowest Hz it had ever seen and the highest Hz it had ever seen. There was no provison for recording the time or the duration of these highs and lows but by checking and resetting these these parameters I was able to establish that the dips are more numerous than the peaks. Mystery solved, we now know why the strokes per minute vary.

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