HYDROSTATIC:
The simplest form of hydrostatic measurement is that in an open vessel. The pressure at the bottom of a vessel is directly proportional to the liquid height. By connecting one end of a pressure sensing instrument to the lowest liquid level in the vessel and the other end to a point above the liquid where vapour is guaranteed the difference in pressure can be obtained. Once the difference in pressure is known a simple calculation can be performed between density of liquid, height and pressure, which will indicate the height of the level of the liquid. A computer can be used to constantly monitor the change in pressure and so continually monitor the change of liquid level. Care has to be taken to avoid entrapped air in the pressure tubes, which would affect readings, also a drain valve is required to allow sediment to be flushed away from the piping.
Bubble tube systems:
In this method a bubble tube is placed into the liquid in the vessel. A gas flow is passed through the tube, and a constant differential regulator is used to maintain a constant flow rate and avoid variations in pressure drop. A pressure measurement element is then used to measure the pressure difference, which can be used to obtain the height of the liquid.

Suitable liquids for measuring:
Water
Food/Beverage
Oils/Chemicals
Solvents
Viscous fluids
Slurries

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ULTRASONIC:
This works by inserting a transducer at the top of a tank. This transmits pulses down to the liquid surface, generated by a transmitting crystal. The pulse is reflected by the interface between the fluids. The time taken for the reflected pulse to return to the receiver is measured by the electronics and is proportional to the distance between the face of the transducer and the liquid, hence determining the liquids height. The transmitter/receiver can be positioned either above the interface or below it, so that the pulse can be sent through either the liquid or vapour face.

This method can also function as a fixed point detector, for the detection of a liquid-vapour interface. A transmitter and receiver are required, which are separated by a short gap, when this gap is filled with a liquid, ultrasonic energy is transmitted across the gap. However if there is vapour between the gap then this will not actuate the receiver and so no detection is made.

Suitable liquids for measuring:
Aggressive and corrosive liquids without foam layers

Link: Ohmartvega.com Lesman.com Cdnova.com Psm-sensors.co.uk

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CAPACITIVE:
This method works using the fact that the electrical capacitance between two electrodes varies with the di-electric constant of the material between them. This allows a continuous level measurement device to be created. It works by inserting one electrode down the centre of the tank, whilst the outer metallic wall acts as the second electrode. Large cylindrical vessels make the most suitable vessels for this type of measurement because the electrodes are always the same distance from each other. The tank then acts as a cylindrical concentric capacitor, having a capacitance which alters as the liquid level rises or falls, the level can then be measured using a capacitance bridge. The central electrode is usually coated in plastic so that it is conductively insulated from the process fluids.

Suitable liquids for measuring:
Non-Conducting fluids
Oil/Water interface detection.

Link: Lesman.com

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CONDUCTIVE:
Generally used for fixed-point level detection, this method can be applied using the conductive properties of the fluid involved. Two electrodes are placed in the tank, one at a low level and one at a high level. Whilst the liquid remains in between these two levels no current will flow, however once the liquid reaches the pre-determined level of the high electrode the electrodes are connected and a circuit completed so a current will flow. This can be detected and so indicate that the level of the fluid has reached a certain height, by powering an alarm or indicator.

Suitable liquids for measuring:
Highly corrosive materials
Any Conducting fluids - such as:
Water
Sewage
Acids
Milk
Beverages
Paper
Detergents
Wine

Links: Hawker-electronics.co.uk

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RADIOACTIVE:
This works by using a single radiation source mounted on one side of the vessel, and a number of Geiger Muller tubes are installed on the opposite side. As Gamma radiation passes through the liquid in the vessel they are attenuated. This reduces the pulse output of the Geiger Muller tube, and hence indicates the liquid level in the tube.

Suitable environments for measuring:
High temperatures and pressures
Corrosive environments
Volatile and biohazardous environments

Link: Ohmartvega.com

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MAGNETIC/FLOAT:
Float activated liquid level indicators are probably one of the most simplest types of liquid level measurement devices. They operate by having a buoyant part that floats at the interface of the two fluids in the tank, which make them ideal for liquid-gas interfaces.
Chain or Tape Float Gauge:
The float is connected to the level indication device by means of a chain or tape. A counterweight is required to counterbalance the float so that the chain or tape always remains taught. There is then a large-scale gage board on the outside of the tank where the counterbalance/indicator illustrates the level inside the tank. Alternatively the tape can be drilled with holes and connected to indicator dials so that as the liquid level moves up and down the tape moves either forward or backwards through the indicator dials which subsequently alter to indicate the level of the liquid.
The float itself is usually a circular hollow metal shape, with guide wires to prevent it from moving from side to side.
Lever and Shaft Mechanism
This method is often used in pressurised vessels, this involves the float inside the vessel between the interfaces of the two fluids being connected to a lever and a rotary shaft which transfers the movement of the float inside the vessel to the outside. So on the outside the turning motion of the shaft alters the position of a dial, which corresponds to the height of the liquid inside the vessel.

Magnetic
The operation is based on magnetic sensing of levels. A float rides on a stainless Steel Probe in which Level Sensors are enclosed and sealed. Float moves with the rise and fall of liquid level and actuates the sensors magnetically. The signals from the sensors are received by the Main Control Unit. This unit indicates Liquid Levels and provides necessary outputs of Alarm and Automation. An example of this can be seen in the diagram here.

The magnetic method can also be used successfully for level-sensing electrical switches. These work, by having a magnet connected to the float, and another permanent magnet, which operates the switch. When the liquid level is low the magnet is no where near the magnetic switch, however as the liquid level rises the magnet on the float attracts the permanent magnet and operates the switch.

Suitable liquids for measuring:
Almost all liquids: - The most common being:
Water
Waste water
Effluent
Oils and Chemicals

Links: Hawker-electronics.co.uk

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