Clamp Meter

[John Orentas]

Originally posted by AMICUS in another thread.

 

Quote: "Mangleing the Topics, but..........

I did'nt know about Rapid so had a bit of a shufty, and lo & behold

 

http://tinyurl.com/5d85o

 

John, is this suitable for boating kit? DC amps 300 FSD 0.1A Resolution.

Bout the best price I've seen"

 

This clamp meter is affordable by everyone, it will show you at a glance how much current is going in or out of an individual battery and how much current is being generated by your alternator. And all without disconnecting a single wire.

 

How much current does you fridge draw during that mysterious start up.

 

How many amps does your starter motor take, don't take someone else's word for it, measure it.

Edited April 4, 2005 by John Orentas

[John Orentas]

If anyone decides to order one of these, please let us know how well it works and how you get on with it. I already have one but a different manufacturer (and 4 times the price).

[David Schweizer]

What is your view about this one John - it's even cheaper

 

http://www.screwfix.com/app/sfd/sea/search...&cd=1&x=11&y=11

 

Just realized it only meassures AC - not much use on a 12v DC system

Edited April 4, 2005 by David Schweizer

[John Orentas]

Yes, everyone and his dog sells the AC types. The principle of how the DC ones work is quite complex and the few that where available were very expensive.

[Hondaman]

The principle of how the DC ones work is quite complex and the few that where available were very expensive.

<{POST_SNAPBACK}>

 

But very useful. I bought 2 from Maplin (2 'cos 1 got nicked ). They do half price specials from time to time (but I had to go and miss out with both ).

 

The AC part is not reliable with an Inverter because of the weird waveform from most.

[Alan Saunders]

. . . How much current does you fridge draw during that mysterious start up.

 

How many amps does your starter motor take, don't take someone else's word for it, measure it.

<{POST_SNAPBACK}>

This digital device will not give reliable information on instantaneous loads. The current rises and falls more rapidly than the sampling rate of the instrument or the human eye/brain! The 'Data-hold' feature my obtain a maximum reading but without the time factor it is meaningless except as a comparative measurement. Only a storage oscilloscope and a calculator, or a meter with an integration function could give you data in any useful form.

 

At £50 it could be useful to someone who knows how to use it. See my previous post on using existing cables as shunts. Most 'trained engineers' (including first degree level) and no 'electricians' I have met can use a multimeter (digital or analogue) effectively.

 

Alan

[ChrisPy]

This digital device will not give reliable information on instantaneous loads.  The current rises and falls more rapidly than the sampling rate of the instrument or the human eye/brain! The 'Data-hold' feature my obtain a maximum reading but without the time factor it is meaningless except as a comparative measurement.  Only a storage oscilloscope and a calculator, or a meter with an integration function could give you data in any useful form.

 

 

Alan

<{POST_SNAPBACK}>

so an analogue meter is the obvious answer for AC?

[John Orentas]

Well Alan you have have obviously reached a state of grace that none of us mortals could ever aspire to.

 

But had you been observing the many opinions about the start up of the new high efficiency fridges you would know that people where not talking about spikes, transients and things but a high current loading for a measurable amount of time, perhaps a couple of seconds.

 

We are talking about basic 12 volt electrics here, forget your storage oscilloscopes and meters with integration functions, leave them at work.

[DHutch]

Well Alan you have have obviously reached a state of grace that none of us mortals could ever aspire to.

 

-

 

ah well john, we cant all know it all!

 

Im really not sure why you would want to know the startup current of a fridge anyway? it only lasts a few secs so doest really come into the calulations of battery usage. as long as its not enought to trip the MCB all if fine?

 

 

Daniel

[John Orentas]

If you recall Daniel some people were saying that a fridge will take 20 amps for a couple of seconds and so were mistakenly sizing the wiring to cope with it.

 

16mm cable for a fridge ?

[Alan Saunders]

Well Alan you have have obviously reached a state of grace that none of us mortals could ever aspire to.

 

But had you been observing the many opinions about the start up of the new high efficiency fridges you would know that people where not talking about spikes, transients and things but a high current loading for a measurable amount of time, perhaps a couple of seconds.

 

We are talking about basic 12 volt electrics here, forget your storage oscilloscopes and meters with integration functions, leave them at work.

<{POST_SNAPBACK}>

John, you seem to have missed the point of my argument! I agree with the above except about my state of grace unless you mean that I know how much I don't know! All that kit, and the clamp meter, is left at work!

 

I will rephrase: Don't try to measure surges unless you have the knowledge and equipment to do so - e.g. you are an electrical design engineer, which I am not.

 

Size the fuses (ideally 'slow-blow') or CB's according to the manufacturers data and use a cable of at least the same rating. As Daniel says 'as long as its not enough to trip the MCB all is fine'. To which I would add - so long as the fuse is not getting hot - evidenced by discoloration or distortion or failing after a considerable period of use. And so long as the voltage drop in the cable during normal operation (not start-up) is acceptable.

 

I would be very surprised if the compressor is drawing 20A two seconds after start-up. I would be unsurprised by a larger current falling, rapidly at first, to normal over a two second period. If this surge is a problem then I would expect the manufacturer to provide a soft-start modification. Has anyone asked the manufacturer for a solution? If I had one of these 'fridge's that's what I would do.

 

Alan

[Alan Saunders]

so an analogue meter is the obvious answer for AC?

<{POST_SNAPBACK}>

Chris, an analogue meter is better at averaging a varying voltage or current than a basic DVM. Some DVM's do provide an analogue display. This applies to both DC & AC - the variation we are trying to 'damp-out' or average is much slower than 50hz. If you were trying to measure a 50hz ripple on a DC voltage or current then you could simply use the AC ranges of the meter.

 

HTH - Alan

[John Orentas]

Alan

 

I don't believe that a fridge draws those silly currents either but there is no doubt a very high start current, that and the requirement for a very low voltage drop supply is probably the price you pay for a very high efficiency motor. Remember a compressor motor always starts on full load.

 

I must insist though that a clamp meter is the best instrument to have on a boat, there is rarely the type of subtlety that you imply on boat electrics and it is invaluable to be able to measure the current in a particular conductor.

 

They are now available at a very afford able price.

Edited April 6, 2005 by John Orentas

[Richard Martin]

Slightly off subject but perhaps someone may be able to help.

 

We have a Mikuni diesel central heating system and find that it will only fire up when the engine is running.

 

Is this similar to the fridge question or does it perhaps mean that our batteries are not getting fully charged.

 

Perhaps I should invest in one of these clamp meters to check them.

[Big COL]

Slightly off subject but perhaps someone may be able to help.

 

We have a Mikuni diesel central heating system and find that it will only fire up when the engine is running.

 

Is this similar to the fridge question or does it perhaps mean that our batteries are not getting fully charged.

 

Perhaps I should invest in one of these clamp meters to check them.

<{POST_SNAPBACK}>

 

Richard

 

It seems to be indicating that the voltage is low when the engine is not running. This could be due to undersized cables causing a volt drop, which is unlikely as it would have to be as low as 10.5 volts. It is more likely to be your batteries not holding a full charge. Check the voltage across your batteries, at full charge you should have at least 13 volts. If you have 13 volts at the batteries then check the voltage at the Mikuni, also check for loose/dirty connections in the heater circuit.

[John Orentas]

Yes it must be related to the voltage, there may not necessarily be anything wrong with the wiring however, you will inevitably get at least one volt extra when the engine is running. With the engine not running you will have in the region of 12 - 12.5 volts. with the engine running and the alternator charging you will have 13 - 14 volts, it is quite likely that something is a bit marginal on the Mikuni that needs the extra volt.

 

If you have a multimeter check the voltage at the Mikuni when it is trying to start, it should be holding up around 12 volts, if it is much less than that the wiring may be suspect.

[DHutch]

Yeah, check the voltage accros the battery (with the engine not runing) - then check the voltage at the Mikuni,

- if this is much lower then you eather have a bad conection, or the wiring it not upto the job.

- if it not, but the battery voltage low the batteys maybe old.

 

- Anyone here knot what the minium voltage for the Mikuni is?

 

 

Daniel

[Richard Martin]

Thanks for the all the replies, most useful.

 

I do not currently have a meter, what should I get, I am new to 12 Volt electrics so will need to get a book and start learning, any suggestions as to a good book for the novice so I can start learning.

[NB Willawaw]

I had a similar problem with a diesel heater on a boat. It seemed to be turning itself off when the engine was switched off. For a while, I thought it was caused by a integral timer circuit which is designed to turn the heaters off after 30 mins (a throwback from their auto vehicle heritage). I even found out how to modify the heater, but when I went to do it, the mod had already been done.

 

In the end, it was all due to the battery voltage dropping below a threshold when the engine (alternator) was stopped and the fact that the engine wasn't being run long or often enough to charge the batteries up to their full potential.

 

The boat went on shore charge for 24 hours and everything was fine after once the cause was recognised.

 

Its funny how little mistakes and wasted effort like this mean you never make the same mistake twice. School of hard knocks I guess.

 

Mark

 

 

Slightly off subject but perhaps someone may be able to help.

 

We have a Mikuni diesel central heating system and find that it will only fire up when the engine is running.

 

Is this similar to the fridge question or does it perhaps mean that our batteries are not getting fully charged.

 

Perhaps I should invest in one of these clamp meters to check them.

<{POST_SNAPBACK}>

[John Orentas]

Richard.

 

 

The Reading College site is ok but there are a few glaring errors in it. For a link see the thread "Repair or replace" Britch.

 

For a clamp meter see the first post on this thread.

[DHutch]

I do not currently have a meter,

 

Yeah, you dont need a clamp meter (i dont have one), but a good multimeter such as the SkyTronic 600.035 is essential, i use mine all the time for everything. I also have a cheap analogy one (occastially analogy ones are usfull as the a have a "realtime" display)

 

 

Daniel

[dor]

  Thanks for the all the replies, most useful.

 

I do not currently have a meter, what should I get, I am new to 12 Volt electrics so will need to get a book and start learning, any suggestions as to a good book for the novice so I can start learning.

 

 

Try

 

http://www.motoren.ath.cx/

 

Near the bottom of the list of links on the left hand side:

 

Electrical:

12 Volt Handbook

Alternator Handbook

 

They are in pdf format, quite large, but make good reading.

[DHutch]

Wow, just been reading those guides! they go on for pages (over a hundered each infact)

 

- Starting right from the basics of what volts and amps are.

- There a bit old, and in parts hard to even read, but really good.

- They should be stickied with "Fitout good ideas" etc

 

 

Daniel

[dor]

I have finally put them in the links section (or at least - "subject to the moderators".)

 

This is actually the 4th time I've posted links to this site. I found the electrical books excellent - a really good guide to 12v electrics.

 

I suspect the other guides - gearboxes etc, are useful as well.

[Richard Martin]

Thanks for the link to the 12V guide just had a quick look but seems to be ideal to dummies like me.

 

Also thanks for the suggestion as to meter, I shall read book and then get meter.

 

Out of interest how much power does a 12V TV + Video take, my teenage daughter watches it most of the day while we are cruising, will the batteries still be charging even with her watching all the time ?