What is the Process Variable?
The process variable is a measurable quantity that reflects the state or condition of a process. Common process variables include:
- Flow
- Pressure
- Temperature
- Level
- Quality (e.g., % O?, % CO?, pH)
Definitions and Units of Process Variables
Flow
- Definition: The volume or mass of fluid passing through a point per unit time under specified temperature and pressure conditions.
- Measurement Methods: Positive-displacement meters, rate meters (e.g., turbine, ultrasonic).
- Units:
- Mass flow: kg/hr
- Volumetric flow: liters/min, gallons/min, m³/hr
- Gas flow: Normal cubic meters per hour (Nm³/hr)
Pressure
- Definition: Force exerted per unit area.
- Formula: ( P = \frac{F}{A} )
- Units: bar, Pascal (Pa), kg/cm², pounds per square inch (psi or lb/in²)
Level
- Definition: The height difference or amount of fluid within a container or vessel.
- Units: meters (m), millimeters (mm), centimeters (cm), inches (in), percentage (%)
Temperature
- Definition: The degree of hotness or coldness of a substance or body.
- Units:
- Degree Celsius (°C)
- Degree Fahrenheit (°F)
- Kelvin (K)
- Rankine (°R)
Quality
- Definition: Measures related to composition or characteristics of the fluid, such as chemical composition or physical properties.
- Parameters include: pH, % CO?, % O?, conductivity, viscosity.
- Units vary with parameter (e.g., % for gases, pH is unitless, Siemens/m for conductivity).