Level is measured at the position of the interface between phases, where the phases are liquid/gas, solid/gas, or immiscible liquid/liquid. Level is simply a measure of height. It defines the position of the interface, that is, the surface where the two phases meet with respect to a reference point. This measurement is often converted to a volumetric or gravimetric quantity.
Direct Level Measurement
Direct methods employ physical properties such as fluid motion and buoyancy, as well as optical, thermal, and electrical properties.
Direct level measurement does not require compensation for changes in level caused by changes in temperature. Direct level measurements show the actual level of the interface.
Indirect Level Measurement
Indirect level measurement involves converting measurements of some other quantity, such as pressure to level by determining how much pressure is exerted over a given area at a specific measuring point, the height of the substance above that measuring point can also be determined.
For example, the formula used to determine the height of water in an open tank is:
h = P / .433 psi
where:
h = height,
p = pressure indicated on a gage,
.433 psi = pressure exerted by one square inch of water, one foot high.
For substances other than water, the liquid’s specific gravity (the ratio of the liquid’s density to water’s density) must be factored into the level calculation:
h = P / .433 psi (G)
where:
G = specific gravity
Temperature can also affect the accuracy of indirect level measurement. Substances have a tendency to expand when heated and contract when cooled. Gases are greatly affected by changes in temperature, while solids are affected very little.
Because indirect level measurement is sensitive to specific gravity and the effects of temperature, it is necessary to compensate for these factors to ensure accurate measurement.
Continuous Level Measurement
In many processes, continuous level measurement is required because it is necessary to know at all times the exact position of the interface in relation to one or more specific reference points. A gage or sight glass, can be used to continuously observe the position of the interface.
Point-to-Point Level Measurement
Certain processes require only that the level of a substance be maintained between two points. Frequently these two points are a high level and a low level.
When this is required, a point-to-point level measurement system is used. Such a system activates control devices only when predetermined levels are reached.
Selecting Measurement Devices
Some level measurement methods and devices are better suited to point measurement. When selecting a measuring device, it is important to consider the operating parameters and the physical and chemical properties of the process materials.