Hydrostatic Level Sensing

Side/Bottom Mounted for Liquids in Unpressurised Vessels

Principle

Hydrostatic level sensing offers a straightforward method to measure liquid levels in a tank. This method is effective for any liquid type, as long as it has no suspended solids. By attaching a pressure transmitter to the lowest point of the tank, the weight of the liquid above creates a head pressure measured by the transmitter, which provides an accurate liquid level reading.

Advantages

Easy installation in above-ground tanks or vessels
Accessible side mounting for maintenance
Highly accurate in most conditions
Compatible with a wide variety of fluids

Disadvantages

Not suitable for solids or liquids with suspended solids
Not suitable for pressurised vessels

Considerations

Install the transmitter as low as possible to read the full tank level.
Know the specific gravity (SG) of your liquid. For example, diesel fuel with an SG of 0.7 will read 30% less pressure than water.

Side/Bottom Mounted for Liquids in Pressurised Vessels

Principle

In pressurised vessels, using a regular pressure transmitter is impractical due to the added pressure from the vessel itself. A differential pressure transmitter, which compares the pressure at the top and bottom of the tank, provides an accurate level reading by calculating the difference.

Piping

A second connection at the top of the tank is required for the transmitter to compare pressures. Install a relief valve under the high tapping point to prevent flooding the transmitter pipe, which would cause an incorrect ‘zero’ level reading.

Wet Leg

For vessels with moisture, consider using a ‘wet leg’. Condensation can drip down the pipe from the top of the tank, leading to incorrect readings. Filling this pipe with liquid during installation ensures accurate readings.

Drop-In Sensing for Liquids

Principle

Hydrostatic level sensing measures the column of water above the sensor. A drop-in sensor, suspended from the top of the tank by a waterproof cable, can measure depth in tanks, pits, rivers, or other liquid bodies that are difficult to access from the sides.