Help with Wattmeters needed

I am in a "dispute" with a supplier over Wattmeter readings...the upshot being large discrepancies between diffferent types. I could really use the short term loan of any such meters that peeps have, in order for me to carry out controlled testing. I already own an AstroFlight "Whattmeter" and an Eagle Tree data logger, so any other flavours that you guys may have would be really useful.
If you can help, then either call me, or post here, or PM. I can collect, or there is Llysfaen indoor which I attend every Tuesday. I would not need them for longer than a day or two.
Cheers and TYIA.

Tim, got one of these http://www.giantcod.co.uk/wattmeter-lipo-tester-p-403900.html

Give me a call on 01745 823973 if you want to borrow.

Yep, that will do for starters TY Allan....just need to sort out collection - Ill wait a while and see who else might chip in, and maybe we can co-ordinate summat

Hi Tim,

Without getting Andy to "excited" no pun intended, can't you use the Watts = Volts x Amps calculation I have a clamp on ammeter that you can borrow, you may get a more accurate reading. I know you may also have to calculate losses in the wiring, motor speedo etc but this should give you a best guess to see which meter is the more accurate.

4 cell lipo pulling 20amps
i.e. Watts = V X A Watts = 14.4 x 20 = 273.6 watts

Its not the calculation of the power thats the issue Rich...its the apparant discrepancies between brands of meters which are sold specifically for the purpose. My trusty AF meter indicates some current figures way beyond the ones claimed by the motor distributor on a particular cell count and prop size. This particular distributor claims that serious differences exist in these meters, and I need to prove this one way or another. Running this expensive motor on his recommended prop would according to my two meters destroy it pretty quickly...yet he insists that his meter ( a Hyperion ) gives far lower figures and that all is well.
I should also add that following the testing, the subject may form the basis of a small article in the near future
Assuming your clamp on unit is happy up to circa 75A DC then it would be most welcome TY

The discrepancies that you are seeing Tim, are more than likely due to cheap and nasty current measurement techniques being used (I'm trying not to become too excited here Rich!). The majority of Devices out there in RC land use Shunt based measurements. That is, they measure the voltage drop against a "known" resistance. Remember V=IR, so if you know V, and you know R, you can work out I. Once you know I, you can work out the power.

However, if you don't know V or R accurately, then the calculated current will also be out. Measuring V is pretty easy to do accurately. Knowing the value of the Shunt resistance accurately can be a bit of a pain. Especially if you use a "cheap" method of doing so (like a big chunky bit of wire, or a cheap resistor). You will find that the resistance of these cheap shunts will vary quite dramatically with temperature, humidity, age, and also the amount of current flowing through it.

I would strongly recommend binning all your cheap meters, and rely on something like a clamp on meter like Rich suggests, or one that you know uses a temperature compensated Shunt (the Eagletree devices use pretty good ones, at least, my micrologger one does ) Even then, the readings are not going to be truly "accurate".

If you are doing comparative testing between units, for the test to be valid, make sure you perform them at the same temperature, and if possible, put a known current through them (rather than using a model power system, as that will vary the current drawn with things like temperature, air pressure, humidity, time of the month etc etc etc).

HTH
Andy

Can't believe it is so simple

Are we measuring pure DC, pulsed DC or AC current here? If it's AC, what shape is the waveform? Is it sinusoidal, square or sawtooth? How does the meter deal with that?

Is the meter to be placed In series with the battery lead or one of the phase wires to the motor?

With any shunt meter in the battery lead or just one phase, the motor currents will be changed from "As Is". Any answers will be highly suspect and more likely to be a "good guess"... or read as "Rubbish in Rubbish out"

The clamp on meter sounds the most likely way to me of getting a vague picture of "watts" going on.

There are only a few DC clamp ammeters on the market, I know graupner did one.

where do you get ac batteries?

Leigh wrote:where do you get ac batteries?

Good point but
I think you will find the current flow from the battery to the speed controller is not smooth pure DC but pulsed as the speed controller switches the currents on the 3 phases in turn. The conversion is done digitally and so the current flow from the battery is "notchy". The phase currents are always AC but usually are not a nice sinsoidal pattern. We tend to assume the battery cuurent is DC but using a DC meter may not get the true picture. Try a scope on the battery leads with the motor running at part throttle and you will see this.

Leigh, The same place you get the DC transformers

OK lets keep it simple. I want anyone witih a "purpose made" electric flight wattmeter device to lend it to me for a couple of days.

What meter? Oh that meter Don't want anybody getting sinusoidal on us No funerals in the festive season please

Got a Watts up branded one you can borrow to try if needed.

Yep, that would be great too Mel, the more the merrier TBH