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.
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
This is the most common way to measure water level with a pressure sensor. There is no need to measure barometric pressure with a device that has a vented pressure reference, which simplifies the installation and reduces the cost because there is less instrumentation required.
Also this method is more accurate because the accuracy of only one sensor is considered, rather than the combination of two measurement devices.
Featured vented reference liquid level measurement products
Low power consumption SDI-12 lake water level sensor - Monitor lake water levels accurately with this low power SDI-12 sensor. Features durable PUR cable and PC logging software for easy data analysis.- Borehole water level transducer to determine recharge rates - Borehole water level transducer for lowering into a borehole to measure the changing water level, and determine the recharge rate.
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
This method is used less than the vented reference type, but it does have advantages for some applications.
It is not always straightforward to provide a vent path, since the cable maybe terminated underwater, or the air at the surface maybe different to barometric reference pressure.
Vented cables can be problematic if not adequately protected from moisture and humidity, causing calibration shifts, unstable readings and in some cases failure of the sensor.
Featured non-vented reference liquid level measurement products
SDI-12 interface, 2 bar absolute, submersible water level sensor - The S12C is a submersible level probe with a SDI-12 protocol signal interface and has the option to measure absolute pressure rather than venting to air pressure above the water surface.
Seawater depth sensor with I2C output for aquaculture underwater camera integration - Compact OEM customised I²C pressure sensor with an Acetyl (Delrin) body and ceramic diaphragm providing a corrosion-proof solution for measuring depth in a subsea camera used for aquaculture.
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.
Featured water level measurement products
Underground rain water storage tank level sensor and readout for 2 metre depth - Water level indicator for a 2 metre deep rain water storage tank located underground. 
LMP307 Submersible Level Transmitter - Submersible level transmitter for measuring liquid level with a 4-20mA current loop or a 0-10V voltage output plus optional ATEX approval for intrinsically safe use in hazardous areas in ranges from 1mH2O up to 250mH2O. The LMP307 is suitable for use on potable water with no leaching, and can be supplied with a drinking water certificate according to DVGW and KTW.
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Contact us about this Difference between vented and non-vented water level measurement page to request more information, or to discuss your application requirements.