A hydrostatic level probe typically uses pressure sensor to measure the water level. The main component of a pressure sensor is the sensing diaphragm which converts the difference in pressure between either side of the diaphragm into a measured signal.
The front of the diaphragm measures the water level pressure (hydrostatic pressure) plus the barometric air pressure pushing down on the water.
Contents
Vented Level Measurement Device
In a vented design the back of the diaphragm is exposed to the same barometric air pressure so that it will self-compensate for barometric changes.
- Pressure on front of diaphragm = Water Depth + Air
- Pressure on back of diaphragm = Air
- Actual Measured Pressure = Water Depth
Non-Vented Level Measurement Device
- Pressure on front of diaphragm = Water Depth + Air
- Pressure on back of diaphragm = Vacuum
- Actual Measured Pressure = Water Depth + Air
Help
Plugging the vent line
Can a vented transducer be converted to an absolute transducer simply by plugging the vent line?
An absolute transducer has a vacuum reference, by plugging the vent line you will instead be trapping in atmospheric pressure, which is called a sealed gauge reference.
You can still use this, but bear in mind that the trapped air pressure will vary if the temperature changes.
Also as the atmospheric pressure changes, so will the level/depth readings by the same amount of pressure change.
These effects on readings will be proportionally greater the lower the pressure range of the device.
Non-vented sensor in ditches & springs
Should we use a non-vented water level sensor in ditches and springs?
We would recommend a vented rather than a non-vented because it allows the sensor to self compensate for changes in barometric pressure. This is particularly important when measuring water level in ditches and springs where the water is much shallower, since the changes in barometric pressure will have a greater impact on the water level reading accuracy if they are not compensated for.
Also the range will need to be much greater for a non-vented type, e.g. a vented (gauge) 3.5 mH2O water level range would need to be a 13.5 mH2O for a non-vented (absolute) range, so the accuracy will be degraded by 4 times before you have even considered the errors associated with barometric changes.