Sunday, July 26, 2009

Make vs. Buy

I recently bought a nice turbine flow meter off Ebay. I was planning to use the counter on my DAQ board to measure the frequency of the turbine and convert frequency into flow. Unfortunately, the flow meter magnetic pickup turns out to have a millivolt peak to peak output and the counter expects a TTL (0-5V) signal.

So, says me, "Self, how hard can it be to cobble together some Radio Shack parts to fix this problem ..." After a little bit of work and a trip to the store, I ended up with a circuit that sort of works, based on a LM339 comparator IC. The schematic is below.

Now, this particular setup has a couple of features.
1) When the turbine is not spinning, the DAQ counter reads 60 Hz. This is actually not an issue, because 60 Hz is below the low limit of the turbine. I simply have to remember that if the DAQ board says the flow rate is 0.125 gal/min, its actually zero.

2) If i touch any part of the circuit between the DAQ card and up to and including the body of the meter itself, the DAQ counter reads about 20 kHz. This IS a problem, as the max frequency of the meter is about 1800 Hz. However, its not clear to me if this would be an issue in operation. I seem to remember something about grounding problems causing these kinds of issues....

Now for the point of the post. Omega sells a nifty little box that is designed to do exactly what I need in this case. It costs $70. So, should I have just bought one instead of futzing around with making my own? I mean, $70 is a pretty nice dinner for me and the Mrs. So far, I have about 3 hours into this tiny project. Call it 4 hours by the time i transfer the project from the bread board to a perf board and come up with some sort of enclosure. So, is the value of my time less or more than $17.50/hour?

1 comment:

LeadZoomer said...

Check out this site for a variety of Turbine Flow Meter