Glossary of HVAC Terms

Absolute Pressure: pressure measured relative to full vacuum

  • Referred to as pounds per square inch absolute (PSIA)

Atmospheric Pressure: pressure of the atmosphere at the Earth's surface NIST standard atmospheric pressure (1.01325 bar)

BAR: Unit of pressure (or stress)

  • 1 bar = 750.07 mm of mercury at 0°C, lat. 45°

Barometric Pressure: atmospheric pressure, often measured in millibars, in Hg (inches of mercury), or hectopascals

Burst Pressure: the maximum pressure that may be applied to the positive pressure port without rupturing the sensing element

Capacitive Sensing: detection and measurement of pressure through the change in voltage across a capacitor, one plate of which is a diaphragm which deflects slightly which changes in applied pressure

Compound Pressure: pressure measured from full vacuum (-14.7 PSIV) to gauge pressure, referencing atmosphere

Current Sensor: a current sensor is a device that detects electrical current (AC or DC) in a wire, and generates a signal proportional to it

Differential Pressure: pressure measured relative to a reference pressure

  • Referred to as pounds per square inch differential (PSID)

FS (Full Span or Full Scale): the range of measured values over which a transducer is intended to measure, specified by the upper and lower limits

  • Ex: 0 to 100 PSIG
    • FS is 100 PSIG/) to 5 VDC
    • FS is 5 VDC
    • 800-100 MB FS is 300 MB

Gauge Pressure: pressure measured relative to ambient atmospheric pressure

  • Quantified in pounds per square inch gauge (PSIG)

Manometer: an early instrument for measuring pressure

  • Originally a U-shaped tube containing liquid (water, oil, or mercury), one limb opening to the gas volume to be measured, the other closed or connected to a registering or recording instrument
  • Modern versions utilize diaphragms, bellows, or other devices for sensing relative pressures

Millibar (mbar): unit of pressure generally used in barometric measurements

  • 1 mbar = 100 N/m2, or 1~ dyn/cm2

Newton (N): the unit of force in the International System of Units (SI)

  • The force required to impart an acceleration of 1m/sec2 to a mass of 1 kg

Pascal (Pa): the standard unit of pressure (or stress) in the SI system

  • Equal to 1 newton per square meter (1 N/m2)

P/I: term common to process industries meaning pressure-in/current-out

  • 3-15 PSIG Input to 4 to 20 mA DC Output

Pressure Transducer: an electromechanical device for translating fluid pressure valves into voltages across a high-impedance (5k ohms or greater) load

Pressure Transmitter: an electromechanical device for translating fluid pressure values into currents (generally 4 to 20 mA) in a low-impedance load

Proof Pressure: the maximum pressure that may be applied without changing performance beyond specifications (typically 0.5% FS zero shift)

PSIA: pounds per square inch absolute

PSIV: pounds per square inch vacuum

Range: the spread between the maximum and minimum pressure between which the transducer has been designed to operate

Relative Humidity: relative humidity is a measurement of water in the air at a given temperature

Relative Humidity Accuracy: RH accuracy is the error between the actual RH and the RH indicated by the humidity sensor

Relative Humidity Repeatability: repeatability is the ability of the sensor to reproduce the output when moving in one direction, either from low to high RH or high to low

RH Sensor Interchangeability: interchangeability is the %RH error introduced when replacing a sensor tip with a new sensor tip

RH Long Term Stability: long term stability is the %RH error of the sensor over time

RH Sensor Recovery from Condensation: recovery after exposure to condensing conditions

  • Sensor should self-recover after the moisture on the surface evaporates

RH Sensor Recovery from Chemical and Physical Contaminants: sensing surface coated with a micro-porous metal electrode, allowing the polymer to absorb moisture while protecting it from contamination and exposure to condensation 

Span: the algebraic difference between the limits of the range

  • Ex: 0.1 to 5.1 Volts DC
    • Span is 5 VDC
  • Sometimes used to designate full scale output (ex: 5 VDC)

Vacuum: generally refers to pressures between 0 and atmospheric

  • Often measured in 0-30 Hg Vacuum
  • Referred to as pounds per square inch vacuum (PSIV)

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