[Jeff Horsburgh]

Update!

I switched to a 12-bit ADC that I had laying around from another project (this one). You can supply it with a range of voltage up to about 5.5 V i think, but it has it’s own voltage reference. That cleaned up the noise I was getting, and I’m now getting voltage readings across the resistor in my current loop that put me very close – it’s right on at 0 gal/min at the bottom end and within about 0.05 gal/min if I set my top end at 50 gal/min. I did have to apply a small correction factor to the output voltages (let’s call this a single point calibration), which I’m assuming is due to the fact that my shunt resistor is not exactly 240 ohms (its a 5% resistor). I just got some precision resistors that will probably take care of that.

So, now on to some live testing in the lab. I’ve got the guys downstairs in our hydrualics lab plumbing one of the 2 inch meters into a line that should give me up to 200 gal/min of flow, which is right near the top end of the manufacturer’s stated accurate operation range of the meter.

Your information about the depth sensors is interesting. I’m dubious about how well I will be able to do with flows at the very low end. For testing, I’m not sure yet how much control I will have over getting really low flow through the meter in the lab, but I’m going to see what I can do.

[Jeff Horsburgh]

Thanks for the reply @neilh. The meters are commercial grade (read that as $$$$$), and USU will eventually work them into their building automation (SCADA) system. But, they don’t have it deployed yet in all the buildings and so I need a cheap way to collect high frequency data from the meters and get them on USU’s network until that happens – hence my attempt with the Arduino Ethernet.

The meters are ultrasonic and have an actuation flow of 1/16 gal/min. So, anything less than that should be output as 4 mA on the 4-20 mA output from the meter. Obviously, there are limits to the accuracy of the meters themselves at flows that low. The top end of the meter output is configurable (e.g., I can set the max flow rate that is output from the meter as 20 mA).

So, theoretically, if I choose to integrate a stable voltage reference of 4.096 V via a breakout (I ordered one today), I set the top end of the meter output at 50 gal/min (which is probably reasonable for these buildings), and calculate the theoretical resolution I should get with the 10-bit ADC on the UNO, I get 1/16 gal/min. This might be OK for my needs. I’m going to test tomorrow on a meter that they just installed this week, so I’ll have some real data to have a look at.

As an aside – I would love to use the Mayfly. I’ve got one, and I’m working with Stroud on a project right now that will be using them. I really like the specs. Two things pushed me toward the UNO:

1. The ability to input 12 V on the barrel power jack and supply 12 V to power my 4-20 mA current loop using the Vin pin
2. I think I need Ethernet to reliably get them on USU’s network because WIFI is a little iffy in some of the spaces where these meters are located (although, I did get an XBee WIFI module working nicely with the Mayfly).

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