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How to Sample for Particles, Water, and Oil in Compressed Air

compressed air testing air purity

Many in the food manufacturing business are wondering exactly how to go about complying with the requirements for routine monitoring of compressed air. SQF, BRC, FSSC 22000 and PrimusGFS schemes require monitoring but do not establish purity limits, nor do they provide guidance on how to accomplish this requirement.

Air quality is mentioned throughout the SQF Code Edition 7.2, in which we are given the guideline: “compressed air used in the manufacturing process and in direct contact with food products, primary packaging, or food contact surfaces, is clean and presents no risk to food safety and that it shall be regularly monitored for purity.” In this case, purity can be understood as “the absence of contaminants that could cause a food safety hazard.” [1]


SQF also states that, “food processing facilities need to operate from a fundamental assumption that compressed air can be a source of chemical and microbiological contamination.” [2] Similarly, BRC Global Standard for Food Safety Issue 7, Clause 4.5.4 states that, “Air, other gases and steam used directly in contact with, or as an ingredient in products shall be monitored to ensure this does not represent a contamination risk. Compressed air used directly in contact with the product shall be filtered.” [3]


What are the potential contaminants in compressed air at your facility? Take a look at the first article in this series titled Compressed Air Contaminants where we identify particles, water, oil aerosol, oil vapor, and microorganisms to be potential risks. Now we will turn our attention on how to sample for particles, water, and oil contaminants. Microorganisms will be covered in a future article. You can also view a recorded webinar on food grade air entitled Navigating the Path to Food Grade Compressed Air Quality at IFSQN.com.


Where to Sample


SQF recommends (in their FAQs) using 0.1um (micron) particle filters or as needed based on a risk analysis. We recommend that you take air samples after point-of-use (POU) filters if you have any installed. Your service distributor or trained maintenance staff should prepare the sampling ports. Setting up a sampling port can be as simple as adapting the pipe or tubing with the correct fittings. However, in some cases, you may need to break into the line to establish the necessary connections. Fully purge the system, as tapping into the distribution line may loosen rusty particles that can break loose from within the pipe. This is also true of metal or plastic shavings from newly threaded pipe, or sealing material.


Establishing a Monitoring Plan


Ultimately, the purpose of any monitoring program is to verify that your compressed air quality is in a state of control and will not contaminate your product. As you probably know, the compressed air system pulls ambient air in through the intake and compresses it for process use. What you may not know is that typical ambient air contains millions of inert particles, 5-25 grams of water, 1-5 micrograms of oil, and tens to hundreds of bacteria per cubic meter. The odds of contamination entering your system are against you from the beginning.


No single monitoring plan exists for all manufacturers. We recommend performing a risk assessment to identify your specific areas of concern and to determine what to test for, frequency of testing, number of sampling points and ultimately, the acceptable purity limits.
Another method used to determine these factors is the performance of baseline analyses. We recommend testing for particles, water, and oil using ISO 8573-1:2010 purity limits at various points throughout your facility. The analytical laboratory classifies your compressed air quality according to the ISO 8573 purity classes. This allows you to verify that the quality of air matches the type of filtration installed at your facility. With sufficient data points, a trend analysis can be performed to make an educated decision on whether the air quality is acceptable for all product lines or if improvements are necessary to ensure food safety.


An excellent source of information is the Food and Beverage Grade Compressed Air Best Practice Guideline 102 [2] from the British Compressed Air Society (BCAS). It recommends testing at least semi-annually, unless otherwise determined by the HACCP process or manufacturers’ recommendations. If an alteration to the design or distribution piping is made, or maintenance is performed that could affect the quality of the air, then additional testing should be performed. For air that comes in direct contact with the product, they recommend ISO 8573-1:2010 [2:2:1], and for indirect contact, purity classes [2:4:2]. (Note the order of the purity classes are always [Particles:Water:Oil] or [P:W:O].)


When selecting a laboratory to provide your compressed air quality assessments make certain that you are getting what you pay for. According to the FDA(4) “Valid analytical results are essential to make informed decisions that impact public health. At its heart, laboratory accreditation is about laboratories’ consistently producing valid results by focusing on assuring 1) management requirements for the operation and effectiveness of the quality management system within the laboratory and 2) technical requirements that address the correctness and reliability of the tests and calibrations performed in laboratory.”


Particle Testing


For purity classes 1-5, particles are measured by their size and quantity. ISO 8573-1 establishes three particle size ranges: 0.1 to 0.5 um (microns), 0.5 to 1.0 um, and 1.0 to 5.0 um. Also, there can be no particles greater than 5 um present to qualify for a class 1-5 purity rating. Only purity classes 1 and 2 require all three size ranges. Typically, compressor or filter manufacturers recommend filters for particle and oil removal to meet class 1 or 2 for the food industry.


To measure particles as small as 0.1 um, a calibrated laser particle counter (LPC) is required. For many food manufacturers, this may prove to be impractical due to the availability and expense. While LPCs can be costly if only a few samples are to be taken on an infrequent basis, an LPC is extremely helpful when a particle contamination problem exists. The laser particle counter can be used to rapidly sample multiple locations with on-site test results, and is therefore ideal for troubleshooting. The use of an LPC has played a crucial role in identifying the source of particle contamination from a variety of sources at some of our customers’ facilities. Contamination has been detected from O-rings in valves and filter housings, flexible tubing, distribution piping, and plastic or metal fittings.


On the other hand, particles sized at 0.5 um and larger can be sampled easily and at a significantly lower cost. This is accomplished by using a filter membrane inside a filter housing and passing a known volume of the compressed air over the membrane. Samples are lightweight and easily transportable worldwide, and analyzed using an optical microscope.


For purity classes 6, 7 and X, particles are measured by weight only. These classes are most often used for industrial tools and pneumatically operated machines with filtration by general-purpose filters. Commonly, class 6 limit of 5 mg/m3 is the maximum allowable limit for breathing air used by sport and commercial divers in the U.S. No particle size or quantity is included in these classes. Instead, results are reported by mass weight in mg/m3. Analyses are performed using a calibrated micro-balance. Classes 6, 7, and X are not typically considered food grade air quality by filter or compressor manufacturers.


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Trace Analytics offers the LPC Rental Program for our US customers. [5]
Photo credit: Trace Analytics, LLC


Common Sources of Particle Failure


A common error identified through a monitoring program is the installation of inappropriate tubing or piping after a point-of-use filter. Even a short, brand new, 2-foot length of galvanized piping can be the source of particle contamination.


When the more stringent class 1 particle limit is required (more common in pharmaceutical or electronics) it becomes critical to limit or eliminate the use of quick disconnect fittings, valves, gauges or anything with O-rings because they can lead to sporadic particulate contamination. When a sample is taken from an outlet, it is important to ensure that the sampling process itself isn’t contributing to contamination. The connection between the point of use and the sampling equipment should be short, straight, and preferably made of stainless steel. There should be no elbows, tees, valves or dead ends.


In some cases, where stainless steel piping cannot be used, flexible tubing with low particle shedding properties must be used. Recommended tubing material types, in order of preference, include: stainless steel, conductive polymer, polyester, vinyl (if plasticizer does not interfere), polyethylene, copper, Teflon, and aluminum. [5]


Water Vapor Testing


There are several ways to measure dew point in compressed air. There are in-line instruments or sensors that provide constant measurements, can transmit data, and/or alarm when results are outside of the required parameters. There are also portable devices that can be used to confirm that the water vapor is in the proper range at use points.


It is important to follow the manufacturer’s recommendations and calibrate the equipment/sensors to assure consistency of results. Another quick and inexpensive method for water vapor determination are length-of-stain (detector) tubes. They are a portable method for determining water vapor for both refrigerated and desiccant dryers. Typically, these require a known quantity of compressed air to flow through the tube at a specific flow rate. A chemical reaction occurs between the water vapor in the sample and the chemicals in the tube. This will be represented by a length of color stain on the tube. Sampling times vary between 2.5 and 12.5 minutes—depending on a variety of factors.



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AirCheck Kit K8573NB sampling for water as shown.

Photo Credit: Trace Analytics, LLC


Water Sampling Tips


To prevent the interference of ambient moisture permeating into the compressed air sample stream, select impermeable materials, such as polished stainless steel or PTFE (best known brand is Teflon) . Avoid using hygroscopic materials like rubber, as these materials can allow ambient moisture to permeate into the tubing and affect the results. The use of polished or electro-polished stainless steel is important to prevent any water from collecting on the inner surface of the sampling apparatus.


Any type of connection between the sampling apparatus and the sampling outlet should be short, straight and without dead ends. Avoid the potential for leaks by limiting elbows, tees, and valves.


Total Oil Testing


The terminology used to describe oil can be quite confusing. It is important to understand which form of oil will be tested. Some common terms for oil include: condensed hydrocarbons, oil mist, oil aerosol (these are all aerosols), oil vapor, total gaseous hydrocarbons, and total volatile hydrocarbons (these are all gaseous). Aerosols are usually reported in milligrams per cubic meter (mg/m3) while oil vapors are often noted in parts per million (ppm). ISO 8573 specifies sampling for oil aerosol and oil vapor separately, then combining test results to comply with Total Oil limits for class 1 and 2. Results are reported in mg/m3. Critical control points should be monitored for both oil aerosol and oil vapor for better quality control. This is one of many reasons why testing to ISO 8573 purity classes makes sense for the food manufacturer.


ISO 8573 has a few definitions that help clarify which hydrocarbons are to be tested:


- Oil: A mixture of hydrocarbons composed of six or more carbon atoms (C6+)
- Oil Aerosol: A mixture of liquid oil suspended in a gaseous medium having negligible fall velocity/settling velocity
- Organic Solvent: A mixture of or a combination of the following identified groups: alcohols, halogenic hydrocarbons, esters, esters/ether alcohols, ketones, and aromatic/aliphatic hydrocarbons
- Wall Flow: The proportion of liquid contamination no longer suspended within the air flow of the pipe


Oil Aerosol Testing


A common method for collecting oil aerosols for laboratory analysis is by passing a known volume of air across a pre-weighed filter membrane. The sample can then be analyzed by either infrared spectrometry or gravimetrically.


The path from the point of use to the collection method should be kept short, without bends or drops to avoid the loss of aerosols. The use of cylinders (without a filter membrane) to collect oil aerosols is not recommended because if oil aerosols are present they will adhere to the sides of the cylinder wall.


Oil Vapor and Organic Solvent Content Testing


Oil vapor analysis is suggested for the food industry and required by ISO 8573 for classes 1 and 2. To accomplish analysis for oil vapor consisting of hydrocarbons with six or more carbon atoms (C6+), a charcoal tube must be used to collect the sample. A known volume of air is passed through the collection tube. The sample is lightweight and can be easily shipped to a laboratory for analysis by gas chromatography.


Lighter hydrocarbons composed of five or less carbon atoms are not included in total oil purity classes. These lighter hydrocarbons—as well as other gases like carbon monoxide, carbon dioxide, sulfur dioxide and nitrogen dioxide—are addressed in section 8573-6 Gaseous Contaminant Content. There are no established ISO 8573 purity classes or limits for these other gases. Trace Analytics offers methods to collect the C5- hydrocarbons and other gases upon request.


Common Sources of Oil Failure


Clean, oil-free fittings are critical for a true reading of contamination. Oil aerosol and vapor are detected at very low levels. A slight amount of hydrocarbon contamination in a fitting is enough to produce unacceptably high levels of oil vapor (OV).


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The filter cassette holds three layers of membranes for oil aerosol collection, and the charcoal tube collects oil vapors.
Photo Credit: Trace Analytics, LLC


Solvents remain trapped in O-rings and fittings for a long time. Because of this, only solvents that are not C6+ hydrocarbons should be used. Many common cleaning agents are high in C6+ hydrocarbons. Research the solvents you use in your manufacturing area. Ensure that the air compressor inlet is not situated near a source of C6+ materials. This would include cleaning baths, solvent waste cans, process solvents, or other ambient sources of hydrocarbons such as forklift exhaust.


If contamination is expected in the ambient or process air, have the laboratory perform oil vapor (OV) analyses using gas chromatography/mass spectrometry (GC-MS), a technique that readily discerns between OV and other compounds. These other compounds can be reported separately, thus not impacting the OV level as might occur with gas chromatography with a non-specific detector, such as flame ionization detection (FID).


ISO 8573-1:2010 purity classes serve as the foundation for compressed air monitoring to meet certification requirements. As an internationally accepted standard, ISO 8573-1 is a common language available to the food manufacturer, compressed air system supplier, and testing laboratory.




[1] SQF Code Edition 7.2, http://www.sqfi.com/...code/downloads/
[2] SQF FAQs, http://www.sqfi.com/...f/faq/sqf-code/
[3] BRC Global Standard for Food Safety Issue 7, http://www.brcbooksh...378/food-safety
[4] U.S. Food and Drug Administration, Frequently Asked Questions on FSMA, http://www.fda.gov/F...b_Accreditation
[5] Particle Measuring Systems, Inc., Basic Guide to Particle Counters and Particle Counting, http://www.pmeasuring.com
[6] ISO 8573 specifications referenced above are copyrighted and are available for purchase online at http://webstore.ansi.org/.


Author Biography


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Ruby Ochoa, President and Co-Owner at Trace Analytics LLC


Ruby has over 30 years of experience in compressed air and gas quality testing. Demand from customers influenced Ruby and co-owner Richard A. Smith into finding a solution for manufacturers needing affordable ISO 8573 testing. Trace Analytics developed the AirCheck Kit™ Model K8573NB specifically to address the needs of today’s manufacturer. The kit captures samples for particles (0.5-5 microns), water, oil aerosol and oil vapor. All samples must be submitted to Trace’s A2LA accredited laboratory for analysis. Trace’s lab accreditation meets ISO/IEC 17025 criteria as established by FSMA. For more information, contact Ruby Ochoa, tel: (512) 263-0000 ext. 211, email: Ruby@AirCheckLab.com, or visit AirCheckLab.com.


See the interview where Ruby answers your questions about major contaminants on “Ask the Expert"

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