Calibrating in-field PM air samplers with the FP-25

Flow measurement is critical to air monitoring processes, and quality data requires consistent airflow with minimal fluctuations. This is easy to achieve inside a lab where temperature, air pressure, and humidity are all carefully controlled. Of course, most environmental air monitoring takes place outside, where the instruments and data are at the mercy of varying weather conditions.

With the FP-25 flow standard, gathering quality data on ambient air sources is easier and more accurate than ever before. These flow devices can be used to calibrate any low-volume particulate air sampler on your deck.

In this article, we discuss challenges in air monitoring applications and present the history of solutions culminating in the development of the FP-25.

Air monitoring challenges

• Lengthy calibration times
• Atmospheric pressure conditions
• Extreme temperatures and high humidity

Ambient air technicians have tried a series of solutions. From positive displacement meters to standard portable mass flow meters to low-pressure drop mass flow meters, each device has fallen short in its own way.

Positive displacement meters have a 20-point calibration process, making them very time-consuming solutions.

Standard portable laminar differential pressure (DP) flow meters decreased calibration time significantly. However, volumetric flow standards are more effective if they have low pressure drops, as this minimizes reading fluctuation when operating at atmospheric pressures.

Alicat released the Whisper portable low pressure drop laminar DP flow meters, able to measure with pressure drops as low as 0.07 PSID. But these high-precision laboratory-based standards were built for indoor use. When used outside, they oftentimes took too long to equilibrate to ambient temperature, especially on hot, sunny days. Even after making the flow bodies smaller and switching from stainless steel to aluminum, these devices still had trouble with radiated heat. Water vapor posed an additional challenge, as this changes the density and viscosity of air and affects airflow.

The optimal solution would need to provide high-accuracy measurements even at extreme temperatures, as air monitoring can take place at 108°F (42°C) in the dry, hot Arizona summer, at 86°F (30°C) during the humid hurricane season in the Gulf of Mexico, or at -20°F (-29°C) on a winter day in Alaska.

The FP-25 calibrator kit for air quality monitoring

The FP-25 flow standard was developed specifically for outdoor use, using live readings of flow stream temperature, relative humidity, and pressure to continuously update the viscosity value of the gas being measured. These devices use multiple pressure sensors to capture both ambient barometric pressure and internal pressure of the flow stream and take accurate measurements in environments ranging from -30 to 60°C with up to 95% relative humidity.

Reduces temperature equilibration time

The FP-25 is equipped with two temperature sensors. The first is mounted directly in the flow stream, leading to improved accuracy of air temperature measurements. The second is a fast-responding external temperature probe that corrects the internal flow stream temperature to ambient conditions.

This new flow standard equilibrates quickly and can be used under direct sunlight on a hot day. Even if you just came out of a heated vehicle into sub-zero ambient temperature, the device requires just a few minutes for equilibration.

Calculates actual volumetric flows compliant with EPA mass flow standards

The FP-25 also contains two pressure sensors, to measure both the external barometric pressure and the internal flow stream pressure. The barometric pressure sensor (located in the instrument casing above the display) is used to monitor ambient atmospheric pressure. Meanwhile, the internal pressure sensor package measures both the differential pressure between the inlet and outlet of the stream and the absolute pressure of the stream.

The device uses raw data from these pressure and temperature sensors as well as a humidity sensor to calculate both actual (volumetric) flow and standardized (mass) flow readings each millisecond. These measurements are referenced to a set of standard temperature and pressure conditions (STP). By default, the FP-25 uses the mass flow standards set by the EPA: 25 °C and 1 atm.

The FP-25 uses the external temperature probe to reference the standardized flow readings against the ambient temperature, and the barometer to reference internal to ambient pressure. It then displays the actual volumetric flow rate, which reflects the volume of air that passes into the air sampler (at the local barometric pressure and probe temperature) per unit time.

Maintains flow accuracy below zero

Air monitoring happens year-round, so technicians in the northern US and Canada perform outdoor flow calibrations at temperatures as low as -30°C. It can be challenging to use electronics at subzero temperatures. At temperatures below freezing, the pistons of positive displacement meters freeze up, rendering them unusable. In the case of Venturis, pressure sensors become less responsive making measurements less accurate. Even standard Alicat devices only operate down to -10°C.

To solve this problem, FP-25 pressure sensors are carefully calibrated from -30°C to +60°C. Any measurement taken within this temperature range will be accurate to ±1% of reading.

Combats wind, dust, and rain

Because volumetric flow standards typically feature minimal pressure drop, they are also quite sensitive to wind – and wind additionally leads to dust. This issue was resolved using a few tricks from the manufacturers of air samplers. Air sampler inlets incorporate screens to filter out large dust particles, and their louvered designs prevent windy conditions from influencing the flow rate within their downtubes.

The Alicat FP-25 takes full advantage of the features of the air sampler inlet when it is mounted in line with the air sampler’s downtube, a configuration called “Direct Mode.” In this configuration, the FP-25 is positioned immediately below the PM10 inlet. But above the PM2.5, a very sharp cut cyclone (VSCC) is used. This protects the flow sensor from wind, dust, and rain, and it enables the FP-25 to monitor the very same flow conditions that the sampler sees under normal operation.

Equipped with Bluetooth communications

The FP-25 additionally has Bluetooth communications (compatible with Android mobile devices), so the instrument can stay mounted to the air sampler. This means fewer trips up and down a ladder to check readings in hot or freezing conditions, allowing air monitoring technicians to stay focused on calibration from within shelter.

Resists drops

The device is also built to survive a fall from a rooftop, just in case.

Technology that works for you

Alicat’s FP-25 flow device was developed by working closely with air monitor technicians across various jurisdictions. The result is a robust, durable, highly accurate, and easy-to-use air sampler calibration device. The FP-25 comes in a kit complete with all the equipment required to fully calibrate any EPA-approved air monitor.