Waterlevel ATO needs barometric differential

[KRavEN]

I've been running the 4 port waterlevel ATO for a while now and I've noticed that my level goes up or down depending on the barometric pressure. If I had last calibrated while we had a high pressure system in my area and then a storm comes through the difference can be a couple inches. This makes sense because the expansion is really just 4 pressure sensors.

I don't use all the ports on my expansion so I was thinking that the high level could be computed as the difference between an unconnected port and the 100% level that is set on a port during calibration.

For those that use all 4 ports I was thinking I could work up a webservice that returns the barometric pressure for a specific zip code and then add a periodic call to that service to the RA code.

Anyone else noticing the same thing or have any thoughts or suggestions?

[rimai]

I'm in CA, so I'm not sure if we have that much pressure difference, but I've been using mine to throttle my DC return pump and create a bubble free and noise free system for quite a while. It think it's been almost 1 year already.
My tolerances are pretty tight. I have about 3", if I'm not mistaken, of travel in my overflow chamber, which means from 0 to 100%, water only travels 3". If water is too low, air gets sucked in and if too high, it hits the emergency float switch. I've never had to recalibrate it yet.

[KRavEN]

I'm in Dallas, we get huge low pressure systems and thunderstorms and the barometric pressure can swing quite a bit. I originally though maybe I was getting bubbles in the tube causing the pressure to rise so I hooked up 2 tubes in different areas of my sump and they both track within 1-2% of each other so I don't think that's the issue.

[rimai]

Did you get measurements of the unused ports overtime?
I'd load them in a custom variable and use the chart to plot a graph overtime and see if the sensor is sensitive enough to measure air pressure difference.
The other thing I can think of would be air expansion, but I doubt it would interfere that much.

[sabo]

Your right thinking this, but after doing the sums I don't think you should get so much of a swing. Let's see if my maths is right.

One atmosphere is a fathom.
One fathom is 6 feet of water.
Six feet of water is 1800mm
One atmosphere is 1013hpa
So 1.77 mm of water difference per hpa.
For 50mm of water difference you need 28hpa front/ storm to come through.

Now I think about it, with some of the wild storms you guys get in some parts of the states, it wouldn't surprise me at all.

[sabo]

Actually, I think my memory sucks. I thought a fathom was an atmosphere, but I think that's wrong. But I'm on my phone at the pub, so this will have to wait.

[KRavEN]

Well, WL port 4 has never been hooked to anything so if I could isolate that on the portal web chart I can see the swings.