Calibrating Air Sampling Equipment
An air sampling train is used to collect a sample of hazardous gas or aerosol to assess exposures.
A sample of air is pulled in from the breathing zone through a sampler, here a respirable sampler.
The air then passes through collection media, here a filter for respirable sampling, and tubing.
The sampler selectively allows only the contaminant of interest to reach the collection media.
An airflow pump, typically mounted at the waist, pulls air through the entire system.
The concentration of hazard is typically calculated as the mass of the hazard collected
on the sampling media divided by the volume of air sampled.
The mass collected is obtained from analytical methods,
whereas the volume of air sampled is a function of how you operate the sampling pump.
Volume is calculated as airflow rate multiplied by the time sampled.
Thus, the hazard concentration is a function of the terms in red: mass, airflow rate, and time.
Now, let’s look at the different sampling errors from these three key parameters.
The error introduced by mass of the hazard is primarily a function of the analytical method,
such as gravimetric weighing in particulate sampling.
The error introduced by time depends on the type of sampling you are conducting.
If the sample time is off by one minute over an eight-hour workshift, or 480 minutes, the error is only 0.2%.
However, it is 7% over a 15-minute sampling period. So, one must be careful with short-term sampling.
The airflow rate is often the largest source of sampling error.
Air sampling pumps have distinct behavior that one needs to become familiar with.
Also, airflow rate may change over the sample period.
As mass on the collection media increases, airflow rate may drop.
If you do not let the pump warm up before you set the flow, the airflow rate may be too high when sampling.
There are numerous types of airflow calibrators used in the field of industrial hygiene.
They include volume meters, flow rate meters, and velocity meters.
The most common type of airflow calibrators used in air sampling are frictionless volume meters and rotameters.
A soap burette is also known as a manual bubble meter.
The open end of the burette is dipped into a soap solution until a bubble is formed.
The bubble moves upward with the air through the burette. The user times how long the bubble takes to pass a defined volume.
Airflow is then calculated as the volume divided by the time.
This meter can be a primary standard, if the volume of the burette is traceable to NIST.
To calibrate airflow, the soap bubble meter is connected to the end of the sampling train.
The sampling train is operated with the same collection media as will be used in the field. Here, a filter cassette with a filter is used.
The sampling pump is warmed up for several minutes.
Then, the airflow is measured using the soap bubble meter.
Electronic soap bubble meters are also available.
A plunger mechanism draws a film of soap across the base of a cylinder tube with a traceable diameter.
Electronics then measure the time for the bubble to traverse the height of the cylinder,
and the airflow is calculated as the volume traversed by the bubble divided by the time to traverse that volume.
This is known as an intermediate standard because the electronics are involved in determining
the traverse time, and they need to be mounted in a known position.
The airflow calibration set up with the electronic bubble meter is shown here.
The top of the electronic bubble meter is connected to the sampling train.
The same procedures mentioned for the manual soap bubble meter are used to determine airflow.
Sometimes, it's difficult to connect the meter to the inlet of the sampling train.
Here, for example, is a respirable cyclone and a filter cassette assembly.
Given difficulties in attaching tubing to the inlet of the cyclone, the entire sampling assembly is placed within a bottle.
In this way, the bubble meter can be connected to the bottle rather than the cyclone.
Again, the same procedures mentioned for the soap bubble meter are used to determine the airflow.
In frictionless piston meters, air moves a piston through a constant diameter cylinder.
The time for the piston to traverse the cylinder is measured electronically.
The airflow rate is then calculated as the distance the piston traverses multiplied by the area of the cylinder divided by travel time.
This device is an intermediate standard because the manufacturer recalibrates the electronics.
Rotameters are another type of airflow measuring device.
In a rotameter, airflow moves vertically through a marked tube of variable area.
A float moves upward until the airflow balances gravity.
It is important to read the position of the float with the rotameter in the vertical orientation.
The markings on the rotameter itself must be calibrated at or close to temperature and pressure expected in the field.
The user must also correct for actual temperature and pressures if outside of calibration.
The airflow calibration set up with the rotameter is shown here.
The top of the rotameter is connected to the sampling train.
The same procedures mentioned for the soap burette are used to determine the airflow.
The only exception is that care must be taken to ensure that the rotameter is vertical.
Typically, the rotameter will have a bubble level to ensure that the rotameter is upright.