I have noticed a phenomenon on my MPS Delta 40, and I'm not sure if there's anything that can be done about it. The MPS is in use as a fill-level sensor, with the high-pressure port connected to a tube running to a port on my reservoir, and the low pressure port open to the air. The reservoir is fitted with a pressure equalisation membrane.

I note that the fill level reading I'm getting is varying with the ambient air temperature, and hence air density. It's particularly relevant for me, since my radbox sits near a window, so the ambient temperature at the MPS Delta 40 can vary quite a lot depending on if the window is open or closed, but I think the same problem would also apply to someone running one inside a PC case with the temperature variations that can occur there.

I finally tracked it down today, here's what I did:

- I calibrated the MPS delta carefully with the window open. At the time I calibrated, the temperature of the air where the MPS is was 14 degrees C
- I watched the fill-level reading for about 15 minutes, to ensure it remained steady.
- I closed the window, allowing the temperature of the air near the MPS delta to rise to the ambient temperature in the room which was 24 degrees C
- I watched, and within 5 minutes my fill level reading had changed from 91% (which is correct) to 100%
- I re-opened the window allowing the air temperature to fall, and the fill level returned to the correct, 91% indication over a period of 5 to 10 minutes.

During the process, I ensured that the actual res level did not vary (checked by measuring with a ruler), and I observed that water temperature did not vary by a significant amount (less than 1 degree).

I have repeated the process of opening and closing the window to verify that the effect happens consistently, and it does.

I concluded that what is happening is that the water column pressure is remaining constant, however when the colder, denser air is sensed by the "low pressure" port on the MPS, the difference between water column pressure and ambient pressure is higher. As the air temperature increased when the window was closed, the "low pressure" port would be sensing lower pressure, less dense air, and as a result the pressure difference between that and the water column pressure would be higher and the apparent res level increased even though the water column height did not change.

My res level is about 166mm of water, which should exert about 16 hectopascals (millibars) of pressure. There is a small variation with water temperature, but between the reference temperature (4c) and actual water temperature (~30c) there is only about a 1% difference in density, so I will ignore that.

So at a standard 1013.2 millibar atmospheric pressure, the MPS delta should see a pressure of 1029 millibars on the high-pressure input, and 1013mb on the low pressure side. That should correspond to a 91% fill level.

At a zero fill level, both sides should be 1013

Therefore, each 0.175 mb change in water column pressure (16 millibars divided by 91 percent) corresponds to a 1% change in fill level sensed.

Equally, it seems to me that each 0.175mb change in ambient air pressure should correspond also to a 1% change in the sensed fill level. When I tested as described above, although the fill level hit 100% (and hence could have actually gone higher without showing) the rate of change was slowing down at that point, so we'll assume it changed by about 10% of fill level, or about 1.75 millibars.

If my memory serves correctly, the change in pressure at sea level is about 0.15mb for every degree C, which means a 10c change in air temperature would correspond to about a 1.5 millibar change in pressure, which would be in the right ballpark to describe the effect seen.

Anyway, I'm rambling a bit - I don't know if there's actually anything that can be done to fix this problem. I don't expect there is, short of somehow controlling the temperature of the air being sensed to keep it constant.

The other thing that I'd like to know is if there is some sort of temperature compensation going on inside the MPS firmware, if so, that could be the problem.

I know you can run the MPS delta with two tubes to the res - one to the bottom of the water column, and one to the air space at the top of the res. Obviously, that requires you keep the res airtight so the differential pressure can be properly sensed. Perhaps if someone runs on configured that way, they can try varying their ambient temperature quite markedly while keeping everything else constant and report back if their sensed fill level varies at all - if not, I might change over to that configuration.

Great idea - thanks Shoggy. I will do this as soon as possible and report back.

First - everyone please ignore my ramblings above about density, it seems Shoggy is right and this is a fault in my MPS delta 40 rather than being an inherent characteristic of these sensors. Please don't let it put you off buying one - I really think I got tricked by the way in which this one is failing into drawing the wrong conclusion about them.

Shoggy:

I did as you asked and removed the MPS from the system and tried with both ports open to the air.

I plugged it in via USB so I could observe the "unscaled value" indicated in the configuration page.

It seems my conclusion that it is sensitive to temperature was correct, because there is definitely an effect when I open or close the window near it. When it's quite cold near the device, the reading gradually reduces to zero or near (with not much variation - it stays between 0 and 10), however if I close the window and let it get warm, the reading starts increasing gradually over a period of 10 or 15 minutes and eventually it settles down around 120 (unscaled units).

Although, with no water pressing on the "high pressure" side, this doesn't result in a change in fill-level percentage (it stays at 0%) that must be because of the way the calibration curve works - I note that the % reading only starts to increase from 0% at around 150 unscaled units, however at the top of the graph (near 100% fill level) I can see that a change of 120 units would correspond to the difference between for example 90% and 60% fill levels approximately.

So I think you are right - the unit is faulty, and seems to be responding to temperature but not in the way I had imagined.

Please can you let me know what I need to do to get it replaced/repaired. Ideally, if you are able to cross-ship a replacement with me, that would be perfect.

Thanks

Problem now fixed after following instructions in the PM. Thanks Shoggy.