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Level Measurement is regarded by many as one of the oldest forms of instrumentation, and perhaps the most commonly used in everyday life. Although level measurement is usually thought of in terms of liquid levels, many level measuring instruments also lend themselves to the measurement of solids.
An often cited example of early level measurement is when our ancestors discovered the need to gauge the depth of water when attempting to safely cross streams and rivers. They "invented" the simple dip-stick; a stick prodded into the water in front of them that would allow them to gauge the depth of the river and let them decide if it was safe to take another step forward. Dipstick level measurement is still used daily by millions of people, for example to check the oil level in their car engine.
On a simpler note level is measured daily in thousands of kitchens throughout the world by cooks using a graduated scale etched on the side of a measuring jug. Measuring the level in the jug lets them know what volume of liquid they are about to add to their recipe.
In the process industries there are two methods of level measurement, namely: Direct Level Measurement, and Indirect Level Measurement. Sometimes these are referred to as Mechanical Level Measurement, and Inferential Level Measurement. The difference between direct and indirect level measurement is discussed below.
Direct Level Measurement (Mechanical Level Measurement)
Direct level measurement is generally simple, and almost always an economical solution. Though it does come with limitations, most notably its inability to easily provide remote indication and/or control. The basis for this type of fluid level measurement is the measurement of level from a datum line e.g. in the examples above the data line is the river bed, and the base of the measuring jug. Typical methods of Direct Level Measurement include:
- Dipsticks, and Lead Lines. These measure the wetted length of a stick or "string" submerged in the liquid,
- Sight Glass (also known as Level Glass, or reflex sight glass). These give visual indication of liquid level through a transparent window. The window can either be part of a vessel or tank wall (e.g. the graduations on the measuring jug in your kitchen), or the gauge can be external to the vessel in which case it is bolted on to the outside of the vessel. For more details see our page on Reflex Sight Glasses
- Float Gauges. These use the principle of buoyancy. Here a buoyant element floats on the surface of a liquid and changes position as the liquid level varies. Mechanical or magnetic methods are used to communicate the measured liquid level to a remote indicator.
Indirect Level Measurement (Inferential Level Measurement)
Indirect methods of level measurement, or inferred methods as they are sometimes called do not measure level directly but infer the level from measurement or detection of a physical property of the liquid to be measured. Examples of inferential level measurement include:
- Hydrostatic Head. One of the most common liquid level measuring techniques used in the process industries relies on measuring pressure produced by a head of liquid. The level can be easily calculated from liquid density, measured pressure, and vessel pressure.
- Time of Flight level detection. This principle relies on measuring the time taken for a signal to travel from its point of origin to the liquid surface and back to a detector. Knowing the location of source and detector, and speed of signal in the gas above the liquid allows level to be inferred. Typical sources of signal include ultrasonics, and microwave. Vessel shape, vessel internals and conditions of the gas above the liquid can limit the use of this method.
- Capacitance. The liquid and gas above it act as the dielectric in a capacitor. As the level varies the capacitance of the dielectric varies due to more or less liquid between the plates. Knowing the location of the plates and the capacitance of the liquid and gas allows the level to be calculated. This method requires the liquid to have good capacitive properties.
- Conductance. This method relies on the liquid conducting an electrical signal between a probe and receiver. Obviously, the conductive properties of the liquid may limit use of this method.
- Nucleonic. A beam of radiation is transmitted across a vessel and the strength of the beam is measured on the other side. The liquid to be measured absorbs radiation therefore the higher the level the less radiation is detected on the other side. Advantages include that the source and detector can be "strapped" to the outside of the vessel i.e. they are non intrusive. Nucleonic level measurement techniques also lend themselves to interface measurement i.e. can be used to measure the level of an interface between two liquids e.g. oil and water. Downsides include the regulatory requirements around use of nucleonic sources, i.e. licensing, transport etc. and the need to use specialist vendors for installation and maintenance rather than in-house technicians.
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For those who want to delve further into the techniques employed in level measurement and the differing types of level instrumentation, then the following books from Amazon will be of interest: