Capacitive water level sensor

- J
- Jim Thompson
  
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posted
17 years ago

Wed, Jun 14, 2006 3:24 AM

I doubt that humidity has much effect on resonance frequency... Q maybe, resonance frequency, no. Likewise temperature over a body of water should be relatively stable.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

- G
- Glen Walpert
  
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posted
17 years ago

Wed, Jun 14, 2006 2:30 PM

Considering the most basic form of the small-signal speed of sound in an ideal gas equation:

C = sqrt[k*pressure/density]

Where C is the small-signal speed of sound, k is the ratio of specific heats (specific heat at constant pressure divided by specific heat at constant volume) and pressure and density are of the gas without disturbance (subsitute ideal gas law relations for other forms).

Humidity has an effect on the small-signal speed of sound only in that it changes the density of the air, the exact same reason temperature changes the small-signal speed of sound. Since the atomic wt of water is very close to the mean atomic wt of air this is a very small (but easily calculated) change, unlike the change in speed due to temperature. Since the density of atmpspheric air, an ideal gas for all practical purposes, is proportional to 1/AbsTemp and the small-signal speed of sound varies as sqrt[1/density], it also varies as sqrt[AbsTemp]. So temperature compensation would be required for an accurate measurement unless temperature is controlled.

The decrease in Q due to the increased viscosity of humid vs dry air is very small, probably hard to detect without a lot of care.

For real entertainment, consider the non-linear finite-amplitude case :-).

- K
- Ken Moffett
  
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posted
17 years ago

Fri, Jun 16, 2006 1:14 AM

"Electric dabbler" wrote in news: snipped-for-privacy@bt.com:

Phillip,

I made a level sensor for a project for our Psychology Department with a QProx chip. Two bands around the container. The chip controlled a solenoid feeding the tank. Never could see a change in level, but it was being refilled. Incredable sensativity to the level of alcohol level in the container.

formatting link

see top right picture.

Email me at work > snipped-for-privacy@macalester.edu < and I'll send you the schematic.

Ken