When sampling for air pollutants, the accuracy of your flow rates is a major determining factor in the accuracy of your final concentration value. The simple equation for pollutant concentration is:
Concentration = mass of pollutant / volume of air
Sampling pumps for both ambient and personal evaluations will use a mass or volumetric flow control system to keep the air flowing through the system at a constant rate. The total volume of air sampled is calculated by multiplying this flow rate by the length of the sampling period.
Air volume = sampling time x flow rate
Therefore, if the flow rate in the device is not well-controlled, your volume of air could be highly inaccurate, which will give an inaccurate end concentration.
Ensuring Flow Accuracy
So how do you know if your sampling instrument has accurate flow? Most sampling instruments, will state that they contain either a mass flow controller or microprocessor flow control using a mass flow sensor. The instrument should also have a stated flow control accuracy.
If your instrument does not have a stated flow accuracy or description of the mass flow control methodology, it likely does not have a method of flow control. Mass flow sensors and controllers can be costly components of an air sampling system. Therefore, inexpensive air samplers may not contain them. While this keeps the cost down, this is done at the sacrifice of concentration accuracy.
Let’s compare a sampler that has a 5% accuracy vs. a 1% accuracy. If you’re sampling for particulate matter at 16.67 liters per minute (LPM) over 24 hours, you will have sampled 24004 liters (24 m3) total. Now if your sampler has 1% accuracy, this could give you flow rates between 16.81 LPM and 16.5 LPM. If it has 5% accuracy, this could give you flow rates from 17.5 LPM to 15.84 LPM. Let’s assume a mass concentration of 2000 µg. If we had a perfectly accurate flow rate, this would give an end concentration of 83 µg/m3.
| Sampler with 1% Flow Accuracy | |||
| Flow Extremes | Volume of Air Sampled | Concentration | |
| 16.5 LPM | 23.8 m3 | 84 µg/m3 | |
| 16.84 LPM | 24.3 m3 | 82.3 µg/m3 | |
| Sampler with 5% Flow Accuracy | |||
| Flow Extremes | Volume of Air Sampled | Concentration | |
| 15.84 LPM | 22.8 m3 | 87.7 µg/m3 | |
| 17.5 LPM | 25.2 m3 | 79.4 µg/m3 | |
As you can see from the above example, having less accurate flow control can have a large effect on your end measured pollutant concentrations.
For more information on flow control accuracy, or to discuss the differences between sampler accuracies with a Mesa expert, please contact us today.
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