In part one of this series (May 2003), we explained the need for personal sampling, reviewed passive monitors and personal sampling pumps, and examined OSHA's substance-specific standards and their monitoring requirements. There is not much more to explain about the use of passive monitors, so this article focuses on pump sampling systems, which can be used for sampling particulates as well as gaseous contaminants.
Obtaining equipment. The purpose of the personal sampling pump is to draw air at a known flow rate through a collection media, and any device that will do so is acceptable. What distinguishes one manufacturer's pump from another are the features it offers. For example, some pumps include a built-in digital clock that displays the running time of the sample. Others have a feature that allows the pump to shut itself down at a preset time.
There are two features that many people may find very beneficial. One is a pump that affords a broad range of flow rates. Some sampling methods dictate flow rates as low as a few hundred milliliters per minute while others require flow rates as high as a few liters per minute. It is advantageous to have a pump that allows sampling over a wide range of flow rates so you will be equipped to do sampling no matter what flow rate is required by the method. Some manufacturer's pumps can be easily switched between providing a low flow rate or a high flow rate. On the other hand, if all you will ever sample for is metals where the sampling method stipulates a flow rate of 2 liters per minute regardless of the metal you are sampling for, then having a broad flow rate range is less important. Instead, the important thing is to be sure the pump you select is capable of maintaining a flow rate of 2 liters per minute.
Another useful feature is a pump that provides a constant flow rate when sampling for particulates. The flow rate decreases as the filter begins to load up with the particles you are collecting. Some pumps sense the pressure drop caused by the loading and compensate for it to maintain a constant flow.
My counsel is to invite a variety of vendors in to explain the various features of their products and then select the pump whose features best suit your particular needs. Another consideration is whether to purchase or to rent. While this decision is often based primarily on economic considerations, the convenience of having immediate access to equipment you own should not be overlooked. For example, you may want to address employee concerns about an unusual release as soon as you hear about it, rather than two days later when your rental equipment arrives. On the other hand, renting equipment provides a relatively easy and inexpensive way to get started immediately. Monthly rental fees for pumps can be less than $300, making them affordable even for low-budget operations.
Choosing a laboratory. As a practical matter, only laboratories accredited by the American Industrial Hygiene Association (AIHA) should be used to analyze personal air samples. A listing of all accredited labs can be found on the AIHA Web site (www.aiha.org). These laboratories have demonstrated they have not only the equipment and staff necessary to analyze industrial hygiene samples, but they follow generally accepted analytical methods such as those validated by NIOSH and have a quality assurance program to ensure the accuracy of the analytical results. Part of the accreditation process includes a site visit from AIHA representatives who observe the lab's facilities and operations.
There are two very practical reasons for identifying a laboratory before performing air sampling. First is that sampling media are provided by the laboratory as part of the analytical fee you are paying. Second, the laboratory will provide you with a manual of sampling methods which removes much of the mystery of sampling. Methods manuals typically include a matrix showing the sampling parameters for each substance. These parameters include the type of collection media required, the pump flow rate, the minimum sample volume required and any special handling requirements. For example, some samples must be refrigerated after collection. It is fairly common to submit samples to the laboratory via an overnight service such as Fed Ex, or they can be hand-delivered if within driving distance. The submittal paperwork should include a chain-of-custody log that is signed each time a sample is transferred from one person to another.
A field data collection form. A data collection form is a useful tool for recording information in the field. Typical information that should be noted includes: the identity of the employee being sampled, the date the sample was collected, the identification number you have assigned to the sample, and the pump serial number or other identifier such as an inventory number that can be used to identify the specific pump used to collect the sample. The form should also include: the signature of the person who collected the sample, the time the sampling pump was turned on and the time it was turned off, the pump flow rate setting, the calibrated flow rate at that setting and a description of what the employee was doing during the sampling period in sufficient detail to refresh your memory when you receive the results from the laboratory.
Friendly data collection forms that meet your specific needs can be easily prepared using the table feature of a word processing program to produce a columnar form with fields for related elements such as employee name, sample number, time on and off, etc. Since total time and flow rate are multiplied to obtain the sample volume, placing those three columns side by side can simplify completion.
Calibration. The volume of air drawn through the collection media must be accurately measured since the air volume is critical to determining the concentration of the contaminant being measured. Air flow rate in the field is often measured with a rotometer. A rotometer is a slightly tapered glass tube that contains a small lightweight float, most often a small plastic ball. When suction is provided at the top of the tube, air flowing in from the bottom causes the ball to rise in the resulting air current. The tube is scribed with lines that allow the height of the ball to be measured; the faster air flows through the tube, the higher the ball will rise. Many personal sampling pumps have a rotometer built in as an integral part of the pump's case. This is another feature that many people find convenient.
The numbers on the scribed markings may provide an approximate flow rate, but the rotometer must be calibrated to determine the exact flow rate at a particular setting. For example, adjusting the flow control to a setting of 2 may produce a flow rate of approximately 2 liters per minute, but calibration is required to determine the flow rate more exactly. When reading a rotometer, the reading should be taken from the largest part of the float, since most of the floats used are spherical. The reading should be made the using the ball's diameter; in other words, reading the center of the ball. Such readings are usually reported as "cob," meaning center of ball - for example, 3cob.
The first generation of calibration equipment consisted of nothing more than an inverted 1,000 millimeter burette. A burette is a glass cylinder whose volume is known very accurately. Since the intended function of a burette was to dispense liquids, one end was tapered to a point through which a liquid could be dispensed by opening a stop cock or relaxing a tubing clamp. Burettes were marked with gradations along their length indicating the volume at various levels. The markings included a zero level and a full capacity, which for a burette commonly used for calibrating personal sampling pumps was 1,000 milliliters or one liter. When used as a calibration tool, the burette is inverted and supported by a ring stand so that the tapered end is at the top. A piece of plastic tubing is connected from the tapered end of the burette to the air inlet fitting on the pump. A media of the same type that would be used to collect the sample was placed in line to mimic the same restriction to flow that would be encountered when sampling. The pump is used with that medium. The pump is switched on and the flow rate is adjusted to rotometer reading.
For example, for a cob reading of 1, air is drawn in through the large open bottom end of the burrette and flows up to the pointed end connected to the pump when a beaker containing a soap solution like that used by children for blowing bubbles is raised so the soap solution touches the lip of the opening at the bottom of the burrette. A thin soap bubble disc forms over the burette's cross-section and it is slowly drawn up the inside of the burette by the air flow created by the pump's suction. When the bubble passes the zero mark on the burette, the person doing the calibration starts a stop watch. When the soap bubble reaches the 1,000 millimeter mark, the person stops the stopwatch. The time it takes for the bubble to move through the volume of 1,000 milliliters is used to calculate the flow rate. For example, if the bubble takes 60 seconds to displace 1,000 milliliters or 1 liter volume of the burette, the flow rate is 1 liter per minute, since it took the bubble one minute to displace 1 liter of air. If the bubble passes through the 1,000 milliliter burette in 30 seconds, or a half minute, the flow rate is 2 liters per minutes since 1 liter was displaced in the first 30 seconds. Two liters are displaced in the second 30 seconds, making up one minute. If the bubble displaces 1 liter in 15 seconds, the flow rate is 4 liters per minute, since an additional liter is displaced in each of the three other 15 second increments making up one full minute.
Any intermediate flow rate is calculated simply by dividing 60 by the time in seconds it takes the bubble to travel through the 1,000 milliliters of the burette volume. For example, if the bubble takes 30.8 seconds, the flow rate is 60/30.8 or 1.95 liters/minute. If the soap bubble takes 63 seconds, the flow is calculated as 60/63 or .95 liters per minute or 950 milliliters per minute.
Typically, three separate measurements are made and the average of these three reading is used as the flow rate at the specific setting to which the rotometer is adjusted. For example, the results for the individual runs may have indicated flow rates of 1.1 lpm, 1.2 lpm and 1.3 lpm. The average of these three measurements is 1.2 lpm, which indicates the actual flow rate for a rotometer setting of 1 cob.
Although glass burette soap bubble flow meters are still used today and provide a high degree of accuracy, pump calibrators have evolved, making the process simpler and more efficient. Today, electronic calibrators exist that do away with the need to manually time a bubble with a stop watch. The moving soap bubble principle is still used because that techniques produces very accurate flow rate results. The bulky, delicate glass burette has been replaced by a smaller, more rugged, clear plastic cylinder. Light emitting diodes are placed opposite photo sensors mounted on the outside of the cylinder at its top and bottom. The volume between these two points is known very accurately.
The air inlet of the pump is attached to the calibrator with a piece of plastic tubing. The pump is turned on and the flow control is adjusted to obtain a desired setting on the rotometer. When a plunger on the calibrator is depressed, a soap bubble forms at the bottom of the cylinder and is drawn through the cylinder by the suction provided by the pump. When the soap bubble passes through the light beam created between the first light-emitting diode and its companion photo detector, it starts the calibrator's internal clock. When the bubble passes the second diode and its companion photo cell, the internal clock is stopped.
Using the volume of the tube and the time from the internal stopwatch, the calibrator automatically calculates the flow rate and displays it on a digital readout. Additional trials that are run are averaged and displayed until the unit is reset. The calibrator also has a data output connection that allows the calibration results to be fed to a small data printer or logged onto a computer. Some calibrators have eliminated the soap bubble entirely and instead, use a circular float that is pushed up the calibration tube.
Contributing Editor John F. Rekus, PE, CIH, CSP, is an independent consultant. He has more than 20 years of regulatory experience and may be reached at (410) 583-7954.