Difference between vented and non-vented water level measurement

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

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

River level monitoring with hydrostatic level sensor in sealed PVC stilling well - Replacing unstable absolute pressure sensors with high-resolution gauge reference transmitters for accurate long-term environmental water monitoring.
Dock bridge tidal flap submersible level transmitter with sea water compatibility - Marine bronze submersible level transmitter for reliable depth measurement in dock bridge applications with tidal flaps.

Non-Vented Level Measurement Device

A non-vented design has a vacuum sealed on the back of the diaphragm so it does not self-compensate for barometric changes, and has to be compensated by using a separate barometer.

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

S12S SDI-12 Coms IP68 Pressure and Temperature Sensor - Combined pressure and temperature sensor with a 1200 baud rate serial data interface as defined by the SDI-12 support group.
Absolute Unvented Reference Hydrostatic Liquid Level Sensors - Absolute unvented reference hydrostatic level sensors provide reliable liquid level measurement in challenging environments where traditional vented sensors are impractical.

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

BMS water tank 6mH2Og range 0-10V out submersible level sensor for commercial buildings - This submersible level sensor provides accurate and reliable water level measurement in BMS applications in commercial office buildings, with a 0 to 6 mH2O gauge pressure range and a 0 to 10V output signal.
3m range 4-20mA output submersible hot water tank level sensor - This submersible level sensor is suitable for use in hot water tanks. It features a 4-20mA output, 0-3m measurement range, and high-temperature resistance up to 125°C (257°F).