Progress?

So, I was out of town for a week on my day job. But right before I left, I was able to try out the Omega amplifier that I bought. The good news is that it works perfectly. The bad news is that the signal it outputs has the same amplitutde as its power source, which in this case is 24V. My TTL counter on the DAQ board didn't mind the 24V one little bit. However, when I went back to use the A/D section of the DAQ, it didn't work. A quick inspection revealed burnt resistors... probably from trying to sync all the current from 24V. So, the board went back to the manufacturer. And I have a non-working test stand.....

I took the downtime as an oportunity to build a Thermocouple amp that I've had partially completed for a while. I now have a four channel amp that outputs 1 mV/degC from Type K thermocouples.

Still no fire...

Sucess is Bittersweet

Last night after ordering the Omega amplifier, I went to go put away all my electronics gear. Just for kicks, I thought I'd try to get the LM339 based circuit to work one more time. An Eddisonian inspiration led me to try grounding one of the magnetic pickup lines at the amplifier, and wouldn't you know it, the noise problems went away.

So, the circuit below seems to work perfectly in my application. To recap, I have the following

- A Cox AN8-4 turbine flow meter

- A magnetic pickup which outputs a 10 mV minimum peak to peak signal

- A Measurement Computing 1208FS

I'm sure that it never would have worked had I not bought the Omega unit.

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?

And here it is ...

Tonight, I drilled the injector holes and welded up the motor. The injector holes were surprisingly simple to drill at 0.06", 0.022" and 0.013". I did find that the middle sized drill bits were only good for about 6 holes in the stainless.

With the exception of the spark plug bung, I'm very happy with how it turned out. Welding is definately simpler than trying to design, fab, and assemble a bolted joint.

A night's work