Instrumentation Glossary

The difference in pressure between two measurement points. Commonly used to measure flow rate, filter loading, and liquid level. Expressed in inches of water column (in. w.c.), Pa, or psi.

Dwyer's trademarked differential pressure gauge using a patented Magnehelic movement. Extremely reliable, no liquid fill, reads directly in inches of water column. The industry standard for HVAC filter monitoring.

Dwyer's combination differential pressure gauge and switch. Provides visual indication like a Magnehelic plus two adjustable SPDT switch contacts for automatic control or alarms.

A unit of pressure equal to the pressure exerted by a 1-inch column of water at standard conditions. Approximately 0.0361 psi. Commonly used for low differential pressure measurements in HVAC.

The pressure exerted uniformly in all directions within a fluid at rest or in motion. In HVAC, it is the pressure that tends to burst or collapse a duct.

Pressure measured relative to atmospheric pressure. A gauge pressure of 0 psi means the pressure equals atmospheric. Abbreviated as psig.

Pressure measured relative to a perfect vacuum (zero pressure). Absolute pressure = gauge pressure + atmospheric pressure (~14.7 psi at sea level). Abbreviated as psia.

An instrument for measuring pressure, typically using a liquid column (U-tube) or electronic sensing. Dwyer offers both inclined/vertical liquid manometers and digital manometers.

A flow measurement device where a float rises in a tapered tube as flow increases. The float position indicates flow rate directly on a scale. Simple, reliable, no power required.

The measurement of flow expressed as mass per unit time (e.g., g/min, kg/hr). Unlike volumetric flow, mass flow is not affected by changes in temperature or pressure, making it ideal for gas flow measurement.

The volume of fluid passing a point per unit time (e.g., GPM, LPM, SCFM). The most common way to express liquid flow rates.

A unit of gas flow rate referenced to standard conditions (typically 70°F, 14.696 psia). Used to compare gas flows regardless of actual operating temperature and pressure.

A flow measurement principle that detects flow by measuring the cooling effect of a fluid on a heated sensor. No moving parts, suitable for both liquid and gas flow detection.

A flow switch that uses a paddle or vane inserted into the flow stream. When flow moves the paddle, a switch contact changes state. Simple and cost-effective for liquid flow detection.

A temperature sensor that uses the predictable change in electrical resistance of a metal (usually platinum, PT100 or PT1000) with temperature. More accurate and stable than thermocouples over moderate temperature ranges.

A temperature sensor made of two dissimilar metals joined at one end. The voltage produced at the junction is proportional to temperature. Types include J, K, T, E. Wide temperature range, rugged, low cost.

Proportional-Integral-Derivative controller. A feedback control algorithm that continuously calculates an error value and applies corrections. Used for precise temperature regulation in process applications.

A mechanical thermometer using two bonded metals with different thermal expansion rates. As temperature changes, the bimetal strip bends, moving a pointer across a dial. No power required.

A switch configuration with one common terminal and two output terminals (normally open and normally closed). When actuated, the common switches from one output to the other. The most common switch type in Dwyer level and flow switches.

A level measurement method that uses the pressure exerted by the liquid column above the sensor. Pressure = density × gravity × height. Used in submersible transmitters.

Non-contact level measurement using ultrasonic pulses. The sensor emits a pulse and measures the time for the echo to return from the liquid surface. No wetted parts.

The most common analog signal standard in industrial instrumentation. 4 mA represents 0% of range, 20 mA represents 100%. The live-zero (4 mA) allows detection of broken wires (0 mA). Immune to voltage drops over long cable runs.

An analog voltage output signal where 0V = 0% of range and 10V = 100%. Common in HVAC and building automation. Simpler wiring than 4–20 mA but more susceptible to voltage drop over long distances.

An enclosure rating indicating protection against dust, rain, splashing water, hose-directed water, and corrosion. The X suffix adds corrosion resistance. Common for outdoor and washdown applications.

A transmitter that draws its operating power from the 4–20 mA signal loop itself. No separate power supply required. Also called 2-wire transmitter. Simplifies installation.

A CO₂ sensing technology that measures the absorption of infrared light at a wavelength specific to CO₂. More accurate and stable than electrochemical sensors. The standard for demand-controlled ventilation (DCV) applications.

An HVAC strategy that adjusts fresh air supply based on actual occupancy, measured by CO₂ levels. Reduces energy consumption while maintaining indoor air quality. Requires CO₂ transmitters.

A unit of concentration. In air quality, CO₂ is measured in ppm. Outdoor air is approximately 400 ppm. ASHRAE recommends maintaining indoor CO₂ below 1100 ppm for acceptable air quality.

A unit of air velocity. Used to measure airflow in HVAC ducts and fume hoods. Typical duct velocities range from 500–2500 FPM. Fume hood face velocities are typically 80–120 FPM.

A measure of a valve's flow capacity. Defined as the flow rate in US gallons per minute of water at 60°F that produces a 1 psi pressure drop across the valve. Higher Cv = more flow capacity.

A device that moves a valve to an open or closed position (or any position in between) in response to an electrical or pneumatic signal. Can be on/off or modulating.

A device that ensures a control valve reaches and maintains the exact position commanded by the control signal, compensating for friction and other forces. Used with modulating control valves.