How to measure Watts?

[force9BOAT]

Hello,

I have a question about how to measure the power output from a turbine.  With my multi-meter I can measure no-load volts or I can measure current when attached to a load but I don't know how to measure both at the same time with a load attached.  In other words, I don't know how to determine how many watts my turbine is producing.  I've read descriptions of other turbines where the builder would claim that he is getting 400 watts or 800 watts or something like that.  How do they come to those measurements?

Also, another question... eventually I would like to have some sort of control board where I could see batter voltage and net current flow at the same time.  Can anyone recomend such a device or devices that could be used for that beyond a simple handl-held multi-meter.

Thank you,

Rob

[TomW]

9;

Well, very few folks actually have a "wattmeter" to test their output.

Standard procedure is to simultaneously measure battery voltage and current into batteries.

Then you simply multiply Battery voltage by the Current into the batteries.

Example:

Battery voltage at 12.5 volts with charging current into batteries at 20 amperes. 12.5 times 20 would give you 250 watts out of the alternator and into the batteries.

Thats the nutshell version.

Cheers.

TomW

[scottsAI]

Hello force9BOAT,

The easy answer is buy a second meter.

Harbor Freight tools has a simple DVM for $4. I own several.

Work great as panel meters too. (can't buy panel meters for $4.)

Multiply the two meters readings together to get watts.

What this does not tell is the power dissipated in the generator due to voltage miss match with the load.

Hard answer is get a watt meter:-) many problems with this answer!

House hold stuff, the Kill-O-watt meter $30 is a great buy. Expects ~120v don't think it will work here.

Have fun,

Scott.

[hiker]

try using an old auto amp meter--they have them in diffrent ranges..0-20 amps 0-60amps.

they may not be the most accurate--but there close enough for me..

[terry5732]

If your power is going into a resistive load rather than batts, use the volt ohm formula - volts squared divided by ohms = watts. Or for test purpose you can use a known resistor as load and measure voltage across it for this formula.

[drdongle]

Or with you can mesure volts with your test setup, then mesure amps ( same speed and load), then multiply volts by amps for watts. ( VxI=W),Ex. 12x2=24)

[finnsawyer]

One thing engineers have to keep in the back of their mind is how their measurement techniques affect the circuit under test.  For instance, an analog voltmeter is usually rated at 20,000 ohms per volt for dc, maybe 5,000 ohms per volt for ac.  What this means is you take the full scale reading and multiply by the rating.  This gives a resistance of 200,000 ohms on the 10 volt dc scale, for example.  So, if your load resistor is 100,000 ohms you have changed the circuit by adding the voltmeter.  If the load is 10 ohms or a 12 volt battery, it probably doesn't add any significant loading.  In a similar fashion an ammeter has some small shunt resistance.  When current flows through it, a voltage drop appears across it.  If this voltage is small enough we don't have to take it into consideration.  If neither usage of the meter causes concern for the circuit under test then the measurements can be done using one meter with a great deal of confidence.  If that is not true then using two meters has changed the circuit anyway and the measurements are wrong.  In most practical situations ten per cent accuracy is good enough.

[Opera House]

There is a fairly simple circuit you can build using  a SG3524 or other pulse width switching regulator to calculate DC watts.  I'll have to dig that out and post it.  Saves the trouble of using a four quadrant multiplier.

[DanB]

Those cheap meters are pretty handy -

but beware of them, my experience is that their reading varies a lot, depending on the condition of the battery.  I have one of the more expensive models (it has a backlight.. $8.00) and Ive also noticed that readings change if the backlight is on!  So while they are handy - they  cannot be trusted to be accurate - mine will vary over 10% depending on its battery.