RPM measurements

[blackrose]

I just bought a hand held digital tachometer and took some engine readings from the main flywheel today which were quite interesting.

 

Izuzu 55 revcounter           Handheld tachometer       (Difference)

RPM in neutral                   

700                                     853                                    (155)

1000                                   1186                                  (186)

1200                                   1445                                  (245)

1400                                   1676                                  (276)

1600                                   1916                                  (316)

1800                                   2105                                  (305)

 

 

RPM in reverse gear

1000                                  1205                                   (205)

1200                                  1433                                   (233)

1400                                  1648                                   (248)

 

RPM Prop shaft (PRM 150 with 2: 1.09 reduction box)

Izuzu 55 revcounter                                 Handheld tachometer

1000   (actually 1205)                               574

1400   (actually 1648)                               807

Edited November 25, 2018 by blackrose

[TheBiscuits]

3 minutes ago, blackrose said:

I just bought a hand held digital tachometer and took some engine readings from the main flywheel today which were quite interesting.

 

Izuzu 55 revcounter           Handheld tachometer

RPM In neutral

700

1000

1200

1400

1600

1800

Is there a column of figures missing from that ?

[alan_fincher]

4 minutes ago, blackrose said:

I just bought a hand held digital tachometer and took some engine readings from the main flywheel today which were quite interesting.

 

Izuzu 55 revcounter           Handheld tachometer

RPM In neutral

700

1000

1200

1400

1600

1800

If that is meant to be some kind f comarison table, it hasn't really worked!

[blackrose]

Sorry I mistakenly posted as I was typing!

[TheBiscuits]

9 minutes ago, blackrose said:

I just bought a hand held digital tachometer and took some engine readings from the main flywheel today which were quite interesting.

 

Izuzu 55 revcounter           Handheld tachometer       (Difference)

RPM in neutral                   

700                                     853                                    (155)

1000                                   1186                                  (186)

1200                                   1445                                  (245)

1400                                   1676                                  (276)

1600                                   1916                                  (316)

1800                                   2105                                  (305)

 

 

RPM in reverse gear

1000                                  1205                                   (205)

1200                                  1433                                   (233)

1400                                  1648                                   (248)

 

RPM Prop shaft (PRM 150 with 2: 1.09 reduction box)

1000                                  574

1400                                  807

Either your revcounter or your tachometer is incorrectly calibrated.  Do like @Mike the Boilerman and get a third way of measuring

 

Or just check the signal connection to your revcounter.

[blackrose]

2 minutes ago, TheBiscuits said:

Either your revcounter or your tachometer is incorrectly calibrated.  Do like @Mike the Boilerman and get a third way of measuring

 

Or just check the signal connection to your revcounter.

I tend to trust the tachometer over the revcounter.

 

https://www.amazon.co.uk/gp/product/B00HWLTW90/ref=oh_aui_detailpage_o02_s00?ie=UTF8&psc=1

Edited November 25, 2018 by blackrose

[blackrose]

I had to take the in gear readings in astern so I didn't bother other moorers and I couldn't take it up to max revs, but if the tachometer is to be believed I'm getting about 300rpm more than the Isuzu revcounter suggests. That means the max revs I get on the Isuzu counter in gear (2300rpm) is actually more like 2600rpm.

 

Edited November 25, 2018 by blackrose

[Chewbacka]

The rev counter on the boat works by converting the frequency from the alternator into rpm.  This depends upon the pulley ratio and so when fitting the rev counter is must be calibrated to your engine.  So the accuracy depends upon how well the initial calibration was done.  It is possible that the fitter assumed the engine idle speed was 700rpm and calibrated accordingly without actually measuring it.

 

Added

If you look at your figures the difference is about 17% low and fairly consistent, which says to me that both instruments are 'stable' but one or both is not calibrated.

 

If your hand held rev counter is one of those that emits an invisible ir beam and counts the pulses reflected off a bit of tape then you could try pointing it at a 240V AC powered led light as they flicker at 100Hz which (if it works) would read 6000rpm on your meter.

 

 

Edited November 25, 2018 by Chewbacka

[TheBiscuits]

Of course you could also have a slipping belt on the alternator.

Edited November 25, 2018 by TheBiscuits

[MtB]

56 minutes ago, blackrose said:

I tend to trust the tachometer over the revcounter.

 

These are two different names for the same function.

[Chewbacka]

13 minutes ago, TheBiscuits said:

Of course you could also have a slipping belt on the alternator.

But as the error is a consistent at about 17% and as belt slip would vary with load and speed I think that slipping is not the problem

[TheBiscuits]

1 minute ago, Chewbacka said:

But as the error is a consistent at about 17% and as belt slip would vary with load and speed I think that slipping is not the problem

Fairy nuff.  I hadn't worked out the percentage differences.

 

My money is on both the engine revcounter and the digital tachometer being out.  There was a suggestion here recently to use a guitar tuning app to set a revcounter, so that may be a valid third check.

[MtB]

1 minute ago, Chewbacka said:

But as the error is a consistent at about 17% and as belt slip would vary with load and speed I think that slipping is not the problem

 

And if the angular velocity is reading 17% low, the engine hours meter usually on the tacho is probably also reading similarly low. 

 

The main lesson to learn from this is not to trust the calibration of instruments, but to find a way to check if the result is important. People are far too ready to accept at face value what an expensive-looking instrument is telling them, with no independent corroboration. 

[Jen-in-Wellies]

1 hour ago, TheBiscuits said:

Either your revcounter or your tachometer is incorrectly calibrated.  Do like @Mike the Boilerman and get a third way of measuring

 

Or just check the signal connection to your revcounter.

A third way would be an oscilloscope / frequency aanalyser app on a phone. Several free ones around. I used one to double check a boat gauge. They use the phones microphone to pick up the engine noise. Get the engine up to around 1500rpm. At tickover you may be picking harmonics of the speed as the engine speed is below the frequency range of the mic. You'll see one, or more sharp peaks that will be related to engine rpm, directly, or a multiple of depending. Convert from Hz to rpm. We found this easy to use.

 

Jen

[blackrose]

14 minutes ago, Mike the Boilerman said:

 

These are two different names for the same function.

 

Indeed, but we need different terms to differentiate them and I think you and everyone else knows what I mean. I have already named each meter specifically in my original post. 

 

For any other pedants, in this thread "rev counter" refers to the Isuzu revcounter and "tachometer" refers to the handheld tachometer.

Edited November 25, 2018 by blackrose

[Chewbacka]

Just now, TheBiscuits said:

Fairy nuff.  I hadn't worked out the percentage differences.

 

My money is on both the engine revcounter and the digital tachometer being out.  There was a suggestion here recently to use a guitar tuning app to set a revcounter, so that may be a valid third check.

Another checking method would be to put 48 bits of reflective tape evenly around the circumference on the turntable of a vinyl record payer, set it to 45 or 33.333 rpm and measure the rotation.  48 bits at 45rpm will measure 2160rpm on your meter.

[TheBiscuits]

6 minutes ago, TheBiscuits said:

There was a suggestion here recently to use a guitar tuning app to set a revcounter, so that may be a valid third check.

 

3 minutes ago, Jen-in-Wellies said:

A third way would be an oscilloscope / frequency aanalyser app on a phone. Several free ones around. I used one to double check a boat gauge.

 

I insist we are great minds thinking alike ... not that other thing.

 

It was probably your previous post I was thinking of.

[Floating Male]

Whilst I have no first hand knowledge of the tacho you have bought these are usually spot on. They use a timing system that is derived from a crystal clock, so there is normally no 'adjustment' as such.

One review on the amazon site seems to corroborate this as he tested with a digital scope.

The engine tacho however will be analogue and need to be calibrated. This is essential because as noted the pully ratio needs to be acounted for. If this has not been done correctly then there will be your problem.

[Chewbacka]

6 minutes ago, Mike the Boilerman said:

 

And if the angular velocity is reading 17% low, the engine hours meter usually on the tacho is probably also reading similarly low. 

 

The main lesson to learn from this is not to trust the calibration of instruments, but to find a way to check if the result is important. People are far too ready to accept at face value what an expensive-looking instrument is telling them, with no independent corroboration. 

I would be surprised if the engine hours is out as usually the rev counter will have a quartz 'clock' that drives both the hours counter and gives a reference signal to compare to the alternator signal to calculate the rpm.  That said it is easy to check, just idle the engine for exactly one hour......

[blackrose]

32 minutes ago, Chewbacka said:

If your hand held rev counter is one of those that emits an invisible ir beam and counts the pulses reflected off a bit of tape then you could try pointing it at a 240V AC powered led light as they flicker at 100Hz which (if it works) would read 6000rpm on your meter.

 

 

 

Thanks, I don't have any 240v led lights on the boat but I'll try to find one at work.

[MtB]

On my engines, all I need is the stopwatch function on my iPhone and I count the bangs over a five or ten second period. Then a brief bit of arithmetic.

 

[blackrose]

7 minutes ago, Floating Male said:

Whilst I have no first hand knowledge of the tacho you have bought these are usually spot on. They use a timing system that is derived from a crystal clock, so there is normally no 'adjustment' as such.

One review on the amazon site seems to corroborate this as he tested with a digital scope.

The engine tacho however will be analogue and need to be calibrated. This is essential because as noted the pully ratio needs to be acounted for. If this has not been done correctly then there will be your problem.

 

Thanks. Athough MtB is correct that we shouldn't trust any uncalibrated meter, my intuitive feeling is that the handheld tachometer is more likely to be correct simply because I've never understood why my engine revs are so low. 

 

Edit: even after having my prop repitched to produce more revs a few years ago the max revs were still low. These new measurements make more sense.

Edited November 25, 2018 by blackrose

[Jen-in-Wellies]

An optical tachometer could also be used to measure how fast your ecofan is turning, but an oscilloscope/ guitar tuning app won't be able to.

 

Jen

[TheBiscuits]

2 minutes ago, Jen-in-Wellies said:

An optical tachometer could also be used to measure how fast your ecofan is turning, but an oscilloscope/ guitar tuning app won't be able to.

About as much use though 

[blackrose]

14 minutes ago, Floating Male said:

 

The engine tacho however will be analogue and need to be calibrated. This is essential because as noted the pully ratio needs to be acounted for. If this has not been done correctly then there will be your problem.

 

I've no idea how to adjust the Izusu revcounter. The plan was to check the calibration of the hand held tacho somehow and if correct just mark the offset on the Isuzu gauge.