Tank Expert Systems An alternative to using a compensating leg to subtract gas pressure inside an enclosed vessel is to simply use a second pressure transmitter and electronically subtract the two pressures in a computing device: This approach enjoys the distinct advantage of avoiding a potentially wet compensating leg, but suffers the disadvantages of extra […]

Differential Capacitance Pressure Sensors In this design, the sensing element is a taut metal diaphragm located equidistant between two stationary metal surfaces, comprising three plates for a complementary pair of capacitors. An electrically insulating fill fluid (usually a liquid silicone compound) transfers motion from the isolating diaphragms to the sensing diaphragm, and also doubles as

Transmitter Suppression and Elevation A very common scenario for liquid level measurement is where the pressure-sensing instrument is not located at the same level as the 0% measurement point. Consider the example of a pressure sensor measuring the level of liquid ethanol in a storage tank. The measurement range for liquid height in this ethanol

Liquid Level Measurement by Pressure A vertical column of fluid generates a pressure at the bottom of the column owing to the action of gravity on that fluid. The greater the vertical height of the fluid, the greater the pressure, all other factors being equal. This principle allows us to infer the level (height) of

Remote Seals and Chemical Seals Isolating diaphragms have merit even in scenarios where pressure pulsations are not a problem. Consider the case of a food-processing system where we must remotely measure pressure inside a mixing vessel: The presence of the tube connecting the vessel to the pressure gauge poses a hygiene problem. Stagnant process fluid

Process | Instrument Suitability On a fundamental level, pressure is universal. Regardless of the fluid in question; liquid or gas, hot or cold, corrosive or inert, pressure is nothing more than the amount of force exerted by that fluid over a unit area: P = F/A It should come as no surprise, then, that the

Radar Level Measurement Instruments Radar level measurement instruments measure the distance from the transmitter (located at some high point) to the surface of a process material located farther below in much the same way as ultrasonic transmitters – by measuring the time-of-flight of a traveling wave. The fundamental difference between a radar instrument and an

Compensated Leg Systems The simple and direct relationship between liquid height in a vessel and pressure at the bottom of that vessel is ruined if another source of pressure exists inside the vessel other than hydrostatic (elevation head). This is virtually guaranteed to be the case if the vessel in question is unvented. Any gas

Hydrostatic Interface Level Measurement Hydrostatic pressure sensors may be used to detect the level of a liquid-liquid interface, if and only if the total height of liquid never drops down into the sensing range of the interface level instrument. This is critically important because any single hydrostatic-based level instrument cannot discriminate between a changing interface

Mechanical Pressure Sensing Elements Mechanical pressure-sensing elements include the bellows, the diaphragm, and the bourdon tube. Each of these devices converts a fluid pressure into a force. If unrestrained, the natural elastic properties of the element will produce a motion proportional to the applied pressure. Bellows resemble an accordion constructed from metal instead of fabric.