I was thinking of using manometer to measure pressure using a capacitive sensor to measure the length of the fluid column.
Water has a high dielectric constant but suffers from evaporation. Oil has a slow evaporation but has a low dielectric constant. Would a thin layer of oil on top of the water work long term over a period of months?
Are there any non-toxic (food safe) liquids with a high dielectric constant with low evaporation? Would something like glycerin work, or would it change in volume and dielectric constant depending on the ambient humidity?
Would a thin layer of oil on top of the water work long term over a period of months?
Oil certainly does slow evaporation of water, but does not eliminate it. You would have to experiment with the oil layer thickness to find something suitable for the project.
What are you trying to measure? If you are simply measuring low air pressure differences, there are other methods. Why the concern for food-safe materials?
jremington:
What are you trying to measure? If you are simply measuring low air pressure differences, there are other methods. Why the concern for food-safe materials?
I'm trying to measure the evaporation rate of a water tank (or pond) that will come into contact with people/animals. My plan is to use a bubbler level sensor and measure the (static) air pressure. I haven't found any electronic pressure sensors that work at very low pressures (mm of water).
If the loss is entirely due to evaporation, for reasonably large bodies of water and open tanks that is pretty well understood. You may be able to estimate it quite accurately from evaporation rates published by local weather stations, or from some simple formulas. Here is one resource: ftp://ftp.fao.org/fi/cdrom/fao_training/FAO_Training/General/x6705e/x6705e02.htm
I don't quite understand the "mm of water" specification. Surely your tank/pond is deeper than that. Do you want to accurately estimate the height of a water column to within a few mm? If so, there are plenty of sensors capable of that, but they aren't cheap. I've used the Omega PX309 for many years -- they are accurate and extremely reliable. The 70 mBar model should easily be capable of measuring 1 mm difference in water column height (maximum height 70 cm). http://www.omega.com/pptst/PX309.html
jremington:
The 70 mBar model should easily be capable of measuring 1 mm difference in water column height (maximum height 70 cm). http://www.omega.com/pptst/PX309.html
$US 345.
Hopefully I can build something at 1/10 the cost.