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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
  • Non-Vented Level Measurement Device
  • Help
    • Plugging the vent line
    • Non-vented sensor in ditches & springs

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.

Vented reference liquid level measurement products
  • 18.605 G Low Cost Submersible Diesel Fuel and Water Tank Level Sensor
  • LMK 487 Submersible Ballast Tank and Draught Level Pressure Transmitter
  • DCL 531 Modbus RTU RS 485 Submersible Stainless Steel Liquid Level Sensor

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.

Non-vented reference liquid level measurement products
  • SDI-12 interface, 2 bar absolute, submersible water level sensor
  • Absolute Unvented Reference Hydrostatic Liquid Level Sensors
  • LMK457 Marine Approved Level Transmitter
  • SLS-D SDI-12 Digital Interface Submersible Level Sensor

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.

Water level measurement products
  • LMP307 Submersible Level Transmitter Water Level Transmitters with 4-20mA Output
  • DCL 531 Modbus RTU RS 485 Submersible Stainless Steel Liquid Level Sensor
  • LMK382 Low Range IP68 Waste Water Level Transmitter
  • 18.605 G Low Cost Submersible Diesel Fuel and Water Tank Level Sensor

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Help Guides

  • Measuring negative pressure using a positive differential pressure range
  • Pressure Sensor Accuracy Specifications
  • Process Connections
  • Determining calibration error of Bourdon tube pressure gauge
  • Measurement Accuracy
  • Supply voltage and load resistance considerations for pressure transmitters
  • Measuring vacuum with negative gauge or absolute ranges
  • What is the difference between gauge and absolute pressure measurement
  • Measuring the difference in air pressure between rooms
  • How to Connect a 4-20mA Current Loop Pressure Transmitter
  • Advantages and Disadvantages of using a 4 to 20 mA Signal
  • Determining water tank volume using hydrostatic pressure
  • How do you measure the volume of a liquid using pressure
  • What is the difference between vacuum and absolute pressure
  • What does negative and positive gauge pressure mean

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