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Old 09-04-2015, 10:35 AM   #1
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Engine hour meter overestimates

I've become suspicious that my engine-hour meter overestimating the time, so I started recording the times I started and stopped the engine. It is high by about 12%. This past spring I calibrated my odometer with a laser Tach, adjusting the set screw in the back of the Tach to match the laser unit. Is it possible that the Tach adjustment has put off the hour-meter?


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Old 09-04-2015, 11:27 AM   #2
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Probably not related to adjusting the tach cal. But if you want to hear a rant, here it comes.

Most hour meters SUCK!! Ones on tachs fail, stand alone ones fail, half are not accurate, and the other half jam, then unstick randomly.

I put two new meters on my rig when I built it. New VDO tach with digital HM. New Datcon HM for gennie. The VDO HM display randomly gives garbage, but seems to clock accurately. The Datcon I caught in an overnight gennie run clocking zero hours. Next overnight run it clocked 10hrs in an 8hr run. Trying to make for the previous night, I guess.

I have a box of dead HM's from other peoples boats. All brands represented.

Digital electronic ones suck in their own special way. When they die, you have absolutely no clue what the hours were.

I thought the Datcon was used in aircraft when I bought it. Those guys would not put up with this crap, or do they?

Pilots- Who makes your HM's? Tired of this crap.
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Old 09-04-2015, 11:32 AM   #3
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Probably not related to adjusting the tach cal. But if you want to hear a rant, here it comes.

................................Pilots- Who makes your HM's? Tired of this crap.
I've had good results with the few Hobbs meters that I've used in various applications over the years.
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Old 09-04-2015, 11:35 AM   #4
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A few dead Hobbs in the box, too. Grrr.
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Old 09-04-2015, 11:40 AM   #5
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Chuckle!

So it's case of "Relax! Don't worry! Have a Homebrew"


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Old 09-04-2015, 12:18 PM   #6
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Take it to an airport that rents aircraft by the hour, they will pay ya big bucks for that one.



Most of the A/C I am acquainted with are reciprocating and use Hobbs, mine as well.
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Old 09-04-2015, 12:28 PM   #7
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The more I think about this...it's part of the tachometer, so I must have put it off when I adjusted the tachometer.


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Old 09-04-2015, 12:33 PM   #8
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What kind of tach is it? Most electronic ones start clocking as soon as ign is on AND there is an AC signal coming in. Got to have both. Should not matter what the cal adjustment is.

Old Stewart Warner mechanical tachs did not really have hour meters, but rev counters. Those rev counters matched hours at 1800rpm, and then varied by factor above or below.
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Old 09-04-2015, 01:17 PM   #9
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"Clock" type hour meters seem simpler. They simply measure the time the ignition is on. Mine is accurate. My engine's alarm goes on when the ignition is on and the engine is not running.
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Old 09-04-2015, 01:37 PM   #10
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I like the ones that run off a fuel pressure switch. Pretty much like a stopwatch started and stopped by fuel flow. Matched my logs exactly.
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Old 09-04-2015, 02:51 PM   #11
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Put one of these in the charter boat when the one in the Cummins panel died. Seems accurate and reliable at least for the last 900 hours. $20 was worth the gamble.

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Old 09-04-2015, 02:56 PM   #12
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What kind of tach is it? Most electronic ones start clocking as soon as ign is on AND there is an AC signal coming in. Got to have both. Should not matter what the cal adjustment is.

Old Stewart Warner mechanical tachs did not really have hour meters, but rev counters. Those rev counters matched hours at 1800rpm, and then varied by factor above or below.

It's a Motorola model. I had an earlier thread on this forum called Tachometer Adjustment and you gave me some input at that time. I might look into an electronic hour meter rather than a mechanical one with a reference card indicating the date of the install.


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Old 09-04-2015, 03:00 PM   #13
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Put one of these in the charter boat when the one in the Cummins panel died. Seems accurate and reliable at least for the last 900 hours. $20 was worth the gamble.

Ted

Yes. That's the ticket! How is it wired? Can I jumper it from the existing electronics in the panel when they are powered up by the alternator?


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Old 09-04-2015, 03:09 PM   #14
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Since we've owned Hobo we have lost both tacks (upper and lower helms) at different times. I added the Dacton meter after I lost the first one and mounted under the dash. It seems to be the most accurate.
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Old 09-04-2015, 03:12 PM   #15
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...How is it wired? Can I jumper it from the existing electronics in the panel when they are powered up by the alternator?...
We wired ours into the key switch.
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Old 09-04-2015, 03:27 PM   #16
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Yes. That's the ticket! How is it wired? Can I jumper it from the existing electronics in the panel when they are powered up by the alternator?


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Mine is wired to the key switch.

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Old 09-04-2015, 03:49 PM   #17
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They can be wired to the oil pressure switch as well or even on their own oil pressure switch using a T.

Then they only measure running time and not switch time if you forget and leave the switch on and go to dinner. Not that I have ever done that,

Aircraft tach. time is not really straight time as it is RPM dependent, the higher the rpm the faster the tach. time, the slower then the slower the tach. time.

This is not how most aircraft are billed when renting however, it is only used for maintenance purposes. It is cross checked against the elect. hour meter in case the renter gets creative with a screw driver and disconnects the Hobbs meter for some discount flying time.
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Old 09-04-2015, 06:19 PM   #18
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We had Hobbs brand meters exclusively in many of the planes I flew in my early years. For years, I thought the name of the instrument was a Hobbs meter. Only later did I learn that it's a brand of meter like Kleenex is a brand of tissue. But we always seemed to call it "Hobbs time" vs. tach time.

A Hobbs counted every hour when the engine was running, regardless of engine speed. The tach meter counted one hour of time when run for an hour at full 100% or 1/2 Hr of time if run for an hour at 50% RPM. It served more for maintenance requirements while the Hobbs was the meter used to determine the rental costs.
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Old 09-04-2015, 08:35 PM   #19
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Fly, Must be that echo in here, thought that sounded familiar.
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Old 09-04-2015, 10:15 PM   #20
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Fly, Must be that echo in here, thought that sounded familiar.
Yup!!! Like you said!
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