Theory of Operation: FP-25 Field Flow Calibrator for PM Air Samplers

Alicat’s FP-25 and FP-25BT flow devices can be used to calibrate any low-volume particulate air sampler on your deck. These NIST- traceable field flow calibration devices are multi-parameter mass flow meters that display pressure and temperature. The FP-25 uses multiple pressure sensors to capture both ambient barometric pressure and internal flow stream pressure, resulting in all-weather accuracy.

These flow standards operate on the principle of differential pressure-based laminar flow measurement. The barometric pressure sensor monitors ambient atmospheric pressure and is housed within the instrument casing above the display.  The internal flow stream pressure sensor monitors pressure inside the FP-25’s flow body work in concert to generate flow and pressure readings. This sensor package monitors both differential pressure and absolute pressure within the flow stream directly. The FP-25 also uses two fast-responding temperature sensors, one mounted directly in the flow stream and one located at the tip of the external temperature probe. The FP-25 uses this raw data from the multiple pressure, temperature and humidity sensors to calculate both actual (volumetric) and standardized (mass) flow readings a thousand times every second.

Poiseuille’s equation for laminar flow measurement


At its heart, the FP-25’s flow measurement is based on laminar flow differential pressure technology. The FP-25 measures differential pressure, similar to Venturi products.  A stack of plates and spacers, called a laminar flow element, is used to laminarize the flow and create a pressure drop. A sensor measures the differential pressure across a section of the laminar flow element with known pressure drop, another measures the absolute pressure of the fluid and the flow stream temperature sensor measures the temperature of air in the flow stream. By laminarizing flow, the Poiseuille equation can be used to determine the raw volumetric flow.

To the first order, the Hagen-Poiseuille equation can be used to determine the raw internal volumetric flow rate as follows:


Q = Volumetric flow rate

K = A constant factor determined by the internal geometry of the restriction

∆P = Differential pressure across the LFE

Ƞ = Absolute viscosity of air

Equation (i) demonstrated the dependence of flow rate on the gas viscosity. Because the actual viscosity of a gas varies with changes in temperature and relative humidity, the FP-25 continuously updates the viscosity value with live readings of flow stream temperature, relative humidity, and pressure to provide accurate flow readings all the way to 95% RH, at temperatures as high as 60°C.

Standardized Flow Measurement

M=Q(Ts/Ta)(Pa/Ps)Once the raw internal volumetric flow rate has been determined, the FP-25 references the raw internal volumetric flow readings to standard temperature and pressure conditions (STP). The FP-25 ships with a default STP of 25°C and 1 atm for standardized (mass) flow readings, which matches the US EPA’s standard. Since the FP-25 is a laminar flow device, additional measurements have to be incorporated to determine the actual standardized flow measurement. To determine the standardized mass flow rate (M), two correction factors must be applied to the volumetric flow rate (Q): the effects of temperature and absolute pressure on density.
Together, the conversion to mass flow can be written as:


Ts = Absolute temperature at standard conditions (STP) in kelvins

Ta = Absolute temperature at flow conditions in kelvins

Pa = Absolute pressure at flow conditions

Ps = Absolute pressure at standard conditions (STP)

In the Alicat FP-25, discrete absolute pressure and temperature sensors are placed in the laminar region of the flow stream. This information is sent to the microprocessor to make the appropriate calculations to determine standardized mass flow.

Actual volumetric flow using the external temperature probe

Once the standardized flow rate has been determined, the FP-25 device references the standardized flow readings to ambient temperature using the temperature probe, and to ambient pressure using the barometer. This actual volumetric flow rate reflects the actual volume of air that passes into the air sampler (at the local barometric pressure and at the probe temperature) per unit time. Whenever the FP-25 is displaying volumetric flow referenced to the temperature reading of the external probe, the word Probe will flash on the right side of the display, alternating with the relative humidity value. Actual volumetric flow always appears at the bottom left of the FP-25 screen in Standard Mode.

Technology that works for you

The Alicat FP-25 flow device was developed in coordination with air monitor technicians and has resulted in a durable, accurate and easy to use device. The FP-25 comes in a kit complete with all the equipment required to complete calibration of any EPA approved air monitor.

Shruti Thakur, MS Applications Engineer | India Liaison
Shruti holds a Master’s degree in Environmental Engineering and a Bachelor’s degree in Chemical Technology. She specializes in process improvement and design for environmental engineering applications including water treatment, air quality, and EPA regulated interests.

For more information about Air Monitoring contact Shruti or the Alicat Environmental Team at

WordPress Video Lightbox Plugin