Instrumentation

Force Balance Pressure Transmitters An important legacy technology for all kinds of continuous measurement is the self-balancing system. A “self-balance” system continuously balances an adjustable quantity against a sensed quantity, the adjustable quantity becoming an indication of the sensed quantity once balance is achieved. A common manual-balance system is the type of scale used in […]

Liquid Level Measurement by Level Gauges (Sightglasses) Level gauges are perhaps the simplest indicating instrument for liquid level in a vessel. They are often found in industrial level-measurement applications, even when another level-measuring instrument is present, to serve as a direct indicator for an operator to monitor in case there is doubt about the accuracy

Filled, Purged, Self-purged, Heat-traced Impulse Lines Filled Impulse Lines An alternate method for isolating a pressure-sensing instrument from direct contact with process fluid is to fill the impulse lines with a harmless fluid, which in turn directly contacts the process fluid. Filling impulse tubes with a static fluid works when gravity is able to keep

Bubbler System of Liquid Level Measurement An interesting variation of direct hydrostatic pressure measurement (discussed in privios article) is the use of a purge gas to measure hydrostatic pressure in a liquid-containing vessel. This eliminates the need for direct contact of the process liquid against the pressure-sensing element, which can be advantageous if the process

Piezoresistive | Strain Gauge Pressure Sensors Piezoresistive means “pressure-sensitive resistance,” or a resistance that changes value with applied pressure. The strain gauge is a classic example of a piezoresistive element, a typical strain gauge element shown here on the tip of my finger. In order to be practical, a strain gauge must be glued (bonded)

Liquid Level Measurement by Floats Perhaps the simplest form of solid or liquid level measurement is with a float: a device that rides on the surface of the fluid or solid within the storage vessel. The float itself must be of substantially lesser density than the substance of interest, and it must not corrode or

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