Instrumentation Lessons

Displacer level instruments exploit Archimedes’ Principle to detect liquid level by continuously measuring the weight of an object (called the displacer ) immersed in the process liquid. As liquid level increases, the displacer experiences a greater buoyant force, making it appear lighter to the sensing instrument, which interprets the loss of weight as an increase […]

Torque Tubes An interesting design problem for displacement-type level transmitters is how to transfer the sensed weight of the displacer to the transmitter mechanism while positively sealing process vapor pressure from that same mechanism. The most common solution to this problem is an ingenious mechanism called a torque tube. Unfortunately, torque tubes can be rather

Displacement Interface Level Measurement Displacer level instruments may be used to measure liquid-liquid interfaces just the same as hydrostatic pressure instruments. One important requirement is that the displacer always be fully submerged (“flooded”).  If this rule is violated, the instrument will not be able to discriminate between a low (total) liquid level and a low

Pressure Sensor Accessories Multiple accessories exist for pressure-sensing devices to function optimally in challenging process environments. Sometimes, we must use special accessories to protect the pressure instrument against hazards of certain process fluids. One such hazard is pressure pulsation, for example at the discharge of a piston-type (positive-displacement) high-pressure pump. Pulsating pressure can quickly damage

Applications of Differential Pressure Transmitter The combination of two differential pressure ports makes the DP transmitter very versatile as a pressure-measuring device. This one instrument may be used to measure pressure differences, positive (gauge) pressures, negative (vacuum) pressures, and even absolute pressures, just by connecting the “high” and “low” sensing ports differently. In every DP

Differential Pressure Transmitters One of the most common, and most useful, pressure measuring instruments in industry is the differential pressure transmitter. This device senses the difference in pressure between two ports and outputs a signal representing that pressure in relation to a calibrated range. Differential pressure transmitters may be based on any of the previously

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

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)