Totalizing power using a KW transducer

I am wondering if its possible to totalize KW hours using just a KW transducer with a 4-20ma output. Usually these are pulse output with some kind of pulse totalizer and then a 4-20 output that just displays the total. Id like to eliminate the totalizer and use 4-20 instead of a pulse.

I am thinking of using this product.

http://www.veris.com/file_uploads/h8040_d.pdf

Would you just take a reading every so often, assume it was the same for that whole readings period, and add them up? Or is this not proper for power? The machines power will mostly be from motors which will be on or off for long periods of time.
Thanks

Matt

Matthias von Zorn said:

Would you just take a reading every so often, assume it was the same for that whole readings period, and add them up?

Click to expand...

I've used this technique and it works well, as long as you aren't using ot for billing purposes.

Nope, this is just so they can get an idea of the machines power usage. How about doing the same for a gas totalizer? I would imagine that as long as my period is realatively short, it should be easily doable.

If you keep the time between updates short, you minimize the inaccuracy due to changing analog levels. You can further minimize the error by calculating the average of the current sample and the previous sample.

Another possibility is to sample every time the analog value changes and multiply the previous kw value by the elapsed time. Of course, if there is a lot of noise riding on the analog signal, you'll be updating every program scan.

You can totalize any anaolog signal the same way. (I'm not sure what good psi-hours will do anyone, though!)

Thanks for the link to that sensor supplier. They seem to have some neat stuff. Have you used them before?

Yea, we use them all the time. Mostly for current switches for motors. Much nicer than shaft sensors, etc. They have a setable range for over and under current, so they work on a variety of applications.

We do something very similar where I work. We use both pulse and analog inputs.

For most of our power monitors we connect them discrete. 1 pulse = .1 or 1 Kwh. This gives very accurate Kwh readings. We then back into the kw demand reading with a little math. It is just an average over the sample period.

For flow readings we use analog meters. We take samples of our 4-20 ma values. We stack them in an array and come up with an average value for the period. We use readings every 60 seconds stacked for a 15 minute period. The more samples you take the more accurate the totalized flow readings. You then calulate your totalized flow and add it to a counter. We also calculate the average flow rate.

All of these functions are built into loops. I can add meters easily. We do data collection on a web based server. This allows anyone in the plant to look at energy consumption.

This approach does work. Once you get the first one done the rest are a snap. To me the key is data collection and good access to the data. What good is knowing the energy use if nobody uses the info.

RSL