Applicable Model Families: 5700, 5850E, 5850EM, 5850EMH, 5850I, 5850TR, SLA5800, SLAMF
There are a variety of scenarios where sensor factors could be used, including:
1. Factor Application to Existing Calibration with a Different Gas
The MFC was calibrated for nitrogen but you want to run a different process gas. The factor can be applied in the PLC.
2. Factor Application to Existing Calibration with Change in Gas Mixture
The gas mixture is slightly different from before causing accuracy issues.
3. Re-Calibration/Verification Using a Surrogate Gas with a Factor
Calibrating with specialty or hazardous gases can be expensive. Using a surrogate can reduce calibration expenses.
4. Re-Calibration/Verification Using Process Gas with No Factor Required, Factor = 1
Calibrating on actual process gas will get you the highest accuracy possible.
Download this helpful list of gas correction factors for nearly 200 gases.
Download the List
Factor Theory and Derivation
When a thermal mass flow controller (MFC) or meter operates with a different gas than the one it was calibrated for a scale shift will occur in the relationship between the output signal and the mass flow rate. This happens due to variations in heat capacities or specific heat between the two gases.
Specific heat is the amount of heat energy required to raise the temperature of one unit of mass of a substance by one degree Celsius/Kelvin. It measures how much energy a substance can absorb before its temperature increases. The specific heat is often denoted by the symbol Cp and is expressed in units of joules per kilogram per degree Celsius (J/kg·°C) or joules per kilogram per Kelvin (J/kg·K). Specific heat varies with temperature, pressure, and composition. In thermal wire-based sensors, specific heat is typically measured at 80°C, as these sensors operate at approximately 59°C above ambient.
Mathematically, it can be defined as:
The scale shift between the specific heat of two different gases can be approximated by using the ratio of the molar specific heat or by a sensor factor.
Derivation of the sensor factor from mass flow relationships is shown below:
Download this helpful list of gas correction factors for nearly 200 gases.
Download the List
The specific heat of most gases is not strongly pressure, and/or temperature. However, gas conditions that vary widely from reference conditions may cause inaccuracies. Additional considerations in sensor factor accuracy include reference database consistency of specific heat, Reynolds Number effects (flow path geometry, gas density/viscosity), and thermophysical similarity of surrogate/process gas. The highest accuracy will always result from calibration on actual gas.
Changing to a New Gas
To change to a new gas, multiply the output reading by the ratio of the sensor factor for the desired gas to the sensor factor of the calibration gas.
See example calculation:
Changing Gas Mixture
To calculate flow for a new gas mixture, use the following equations:
Operating the thermal mass flow controller or meter at gas conditions, operating pressure, or flow range different from the factory built and calibrated conditions may also require new sizing of the orifice and restrictor. Such resizing will require support from the Brooks Technical Services Team to ensure the internal components of the flow meter are compatible with the new process conditions.