Jump to content

RPM measurements


blackrose

Featured Posts

13 hours ago, Mike the Boilerman said:

 

Car tacho is just as likely to be wrong! Who would ever know if it was?

Modern car tacho's are very accurate these days. The signal is derived from the ecu which needs accurate data (inc. RPM) to control the fuel system & ignition timing.  A belt driven alternator supplying accurate readings is a different kettle of fish. Belt, pulley wear & slippage due to loading are all factors.

  • Greenie 1
Link to comment
Share on other sites

2 minutes ago, Flyboy said:

Modern car tacho's are very accurate these days. The signal is derived from the ecu which needs accurate data (inc. RPM) to control the fuel system & ignition timing.  A belt driven alternator supplying accurate readings is a different kettle of fish. Belt, pulley wear & slippage due to loading are all factors.

+1

You beat me to a reply saying the same.

Link to comment
Share on other sites

Hey let's not forget it's just a boat engine we're talking about here, not an F1 car! The analogue gauge on any canal boat is never going to be perfectly calibrated. My gauge goes up in increments of 200rpm spaced about 5mm apart so when the needle points to 1000rpm I don't really know if it's 950 or 1050rpm. I expect most gauges on boats are the same. But my revcounter is reading 300rpm too low so I just want to correct that, I don't need it calibrated to an atomic clock!

Edited by blackrose
Link to comment
Share on other sites

On 25/11/2018 at 18:36, alan_fincher said:

I'm struggling to see why the alternator would apparently be slipping by a higher percentage at low revs than at high revs.

It's not going to need a  lot of power at low revs, but once charging properly I would have thought the slippage more likely?

Or perhaps not?

My strong hunch is that it is not properly calibrated, particularly as Mike seems to feel that it has always read lower than expected over many years.

Whilst I don’t think belt slip is a factor here, your comment isn’t right. An alternator on a modern engine can produce a lot of electrical power at idle - the power doesn’t increase at higher rpms unless perhaps the batteries are very low. So the power normally remains the same at low and high rpm. The mechanical power comprises rpm x torque which equates to belt speed multiplied by belt “pulling force”. So at high rpm you have high belt speed and low belt force. At low rpm you have low belt speed and high belt force. It is the belt force that gives rise to belt slip.

 So an alternator belt is more likely to slip at low rpm.

 

Edited by nicknorman
Link to comment
Share on other sites

7 hours ago, blackrose said:

 

I'm sure some would say the same thing about modern hand held tachos too.

Absolutely, I have a cheapo digital tacho and checked it against a calibrated and certified one. I was amazed that it came out within the claimed accuracy of +/-  0.05% + 1 digit.

Link to comment
Share on other sites

2 hours ago, nicknorman said:

Whilst I don’t think belt slip is a factor here, your comment isn’t right. An alternator on a modern engine can produce a lot of electrical power at idle - the power doesn’t increase at higher rpms unless perhaps the batteries are very low. So the power normally remains the same at low and high rpm. The mechanical power comprises rpm x torque which equates to belt speed multiplied by belt “pulling force”. So at high rpm you have high belt speed and low belt force. At low rpm you have low belt speed and high belt force. It is the belt force that gives rise to belt slip.

 So an alternator belt is more likely to slip at low rpm.

 

Fair enough, but, if so, why do loose car ones start to squeal only as the car pulls away?

Link to comment
Share on other sites

36 minutes ago, alan_fincher said:

Fair enough, but, if so, why do loose car ones start to squeal only as the car pulls away?

I think Nick explained this in post 55      " At low rpm you have low belt speed and high belt force".

Link to comment
Share on other sites

2 hours ago, alan_fincher said:

Fair enough, but, if so, why do loose car ones start to squeal only as the car pulls away?

Yes this was a trait in particular of older cars. I think because the pulley ratios chosen meant it could barely charge much at tickover so there was only significant load on the belt once the revs were up a bit. Either that or you need a fair bit of slip to get squeal. Maybe if the alternator is just slipping a bit at idle its not enough to squeal and the lower alternator rpm means it drops below an ability to produce much useful power. When you rev a bit the slipping increases and makes the squeal. Either way, it is basic physics that for a given amount of alternator output, the lower the rpm the higher the belt force, and it is the latter that creates slip.

 

On our boat we have a jap engine that idles at 850 rpm and a 175A alternator. It has a large crank pulley and a polyvee belt and it can produce about 150 or so amps at 850 engine rpm, needs a little more revs maybe 1000rpm to get the full 175A. Under those sorts of loads even with a correctly tensioned belt you can hear it complaining a bit, a bit of squeakery if not actual squealing. As soon as you take the revs up a bit to 1000 or so, this disappears. The travelpower is similar (different belt) - it can run the 2kw tumble drier at idle, but it doesn't like it. One has to be careful not to put too much load on the electrics with the engine at idle, or risk short belt life. Also not good for the cranskshaft pulley. But it is all happy above 1000rpm.

Link to comment
Share on other sites

7 hours ago, Flyboy said:

Absolutely, I have a cheapo digital tacho and checked it against a calibrated and certified one. I was amazed that it came out within the claimed accuracy of +/-  0.05% + 1 digit.

No need to be amazed!

As I mentioned earlier these devices have no adjustment as such. The 0.05% will be the temperature coefficient of the crystal clock (over the stated operation ambient temperature) and the 1 digit is the resolution of the display. (Actually its nice to see accuracy quoted correctly!)

They are similar in accuracy to your digital watch. I dont see people geting them calibrated regularly.

A digital tacho is one of the few measuring devices that normally has no analogue components that affect the reading. They pretty much work or dont! (Slight simplification?)

Instruments that can get out of calibration generally have an analogue aspect to the measuring system. eg multimeters, temperature guages, anything with a moving pointer.

 

As a retired head of a calibration laboratory I did sit down and contemplate all the possible sources of error in the standard alternator driven rev counter.

I got to eight and decided that a drink was needed and the project was abandoned!!! Consider +/- a few % a good deal!

 

Back to OP post.

I would suggest you set the rev counter to match your digital tacho. Best to do this at as high a speed as possible and ideally try to get an engine speed that matches one of the scale points on your rev counter. This will help get the best fit.

Then check now and again and see if it varies much. If it does investigate. If it doesnt pour a glass of wine and relax.

 

John

Link to comment
Share on other sites

On 25/11/2018 at 16:41, TheBiscuits said:

There's a small adjuster on the back of the unit.  Twist it.  If it gets worse, twist it the other way!

I can't see anything on the back of the revcounter that I could twist to adjust it? There's nothing inside any of those holes to turn either, just electrical connections.

IMG_20181202_113148_2.jpg

IMG_20181202_113252_4.jpg

Link to comment
Share on other sites

34 minutes ago, WotEver said:

Top right, labelled ‘4’, is that a cover you can remove?

So I actually have to break a hole in the back cover to get to it? I'd have thought they'd make access a bit easier than that. How on earth is one supposed to know it's there?

Edited by blackrose
Link to comment
Share on other sites

3 minutes ago, Alan de Enfield said:

 

I think the adjuster may be in the hole marked "on" and has a scale 1,2,3, beneath it.

 

No, that's got an electrical connection inside it. I'm afraid it's not a very good picture but I checked very carefully and I couldn't see any adjuster.

Link to comment
Share on other sites

To my mind, there are far too many holes on the back there for them to all have securing screws down them.

What can you see if you look down any of the obvious holes where there are not screws holding the thing together.

 

For example bottom right with "on" and "1 2 3(?)" definitely looks like something you might be able to insert something into, although it looks more like a multi-position switch rather than a potentiometer type adjustment.  (Just possibly to do with number of poles on the alternator - I seem to recall I had one with a selector switch for that?)
EDIT: OK, you say it isn't, I can now see!

It's unlikely to be that one, I think,  - but you are looking for something maybe with a mall slot in it that you can turn to make a variable adjustment.  Probably with a small fine blades screwdriver.

Edited by alan_fincher
Link to comment
Share on other sites

6 minutes ago, Tony Brooks said:

Or is the thing at the pottom of three a little push switch. I think mine was adjusted by multiple pushes of a little button but can't be sure now.

I don't think so but I'll have another look.

2 minutes ago, alan_fincher said:

 

For example bottom right with "on" and "1 2 3(?)" definitely loos like something you might be able to insert something into, although it looks more like a multi-position switch rather than a potentiometer type adjustment. 

That's an electrical connection

Link to comment
Share on other sites

5 minutes ago, blackrose said:

I don't think so but I'll have another look.

That's an electrical connection

Yes, I saw we cross posted, and acknowledged.

What is down each of the other holes that are not screws holding the whole thing together?

I think it would be an unusual rev counter that had no mechanism for fine adjustment to allow for differences in pulley ratios.  UNless the engine manufacturer fits a bespoke type they conside to give a result that will be "near enough".

Just now, bizzard said:

Bend the needle a bit. :closedeyes:

May not then be terribly accurate at zero RPM!

Link to comment
Share on other sites

All the holes apart from the screws holding it together have small electrical connections inside. I think the one marked 3 is just a hole. I've got other stuff to do today so I'll have another look next weekend, but I'm pretty sure there is no adjuster.

Edited by blackrose
Link to comment
Share on other sites

3 minutes ago, blackrose said:

All the holes apart from the screws holding it together have small electrical connections inside. I think the one marked 3 is just a hole. I've got other stuff to do today so I'll have another look next weekend, but I'm pretty sure there is no adjuster.

Some VDO Tachos have the adjustment hole in the side of the case.

Link to comment
Share on other sites

4 minutes ago, blackrose said:

All the holes apart from the screws holding it together have small electrical connections inside. I think the one marked 3 is just a hole. I've got other stuff to do today so I'll have another look next weekend, but I'm pretty sure there is no adjuster.without a method of calibration it is unlikely to be that accurate.

well if there really is no adjuster, my conclusion would be that it can't be adjusted! ?

That would, in my experience, be an odd situation for an alternator driven rev counter, as without any way of calibrating for the mechanical variables involved, I would have thought some inaccuracy was almost guaranteed.

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.