Level Measurement is regarded by many as one of the oldest forms of instrumentation, and perhaps the most commonly used in everyday life.
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 river and let them decide if it was safe to take another step forward. The dipstick 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 kitchen 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 oil and gas, and process industries there are two methods of measuring level, namely: Direct Level Measurement, and Indirect Level Measurement. Sometimes these are referred to as Mechanical Level Measurement, and Inferential Level Measurement.
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 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, measure the wetted length of a stick or "string" submerged in the liquid.,
- Sight Glass (also known as Level Glass): gives visual indication of level through a transparent window. The window can either be part of the 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.
- Float Gauges: uses the principle that a buoyant element floats on the surface of the liquid and changes position as the liquid level varies. Mechanical or magnetic methods are used to communicate 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. Examples include:
- Hydrostatic Head. One of the most common used in the oil and gas industry relies on measuring the 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 application 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 nonintrusive. They 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.