Pure Aire Monitoring

Comparison Methyl Bromide Gas Detection Techniques

PureAire Air Check Advantage Continuous Monitor,
Dräger 81 03 391 and Kitagawa 157SB Colorimetric Sample Tubes

by

Al Carrino
PureAire Monitoring Systems, Inc

Introduction

The following report summarizes methyl bromide gas testing performed for the California EPA Department of Pesticide Regulation DPR. The current method for determining worker exposure in fumigation is through the use of colorimetric tubes. PureAire has introduced a continuous monitor and this report compares this technique to grab sampling methods.

Shipment of walnuts, almonds and perishable vegetables require fumigation to kill bugs and pests on commodities before distribution to customers. One of the most common used pesticides in California is methyl bromide. Methyl Bromide is a gaseous pesticide used to fumigate soil, crops, commodity warehouses, and commodity-shipping facilities. Up to 17 million pounds of methyl bromide is used annually in California to treat grapes, walnuts, strawberries, and other crops. In California, the Department of Pesticide Regulation (DPR) regulates the use of methyl bromide, which is responsible for establishing the permit conditions that govern the application of methyl bromide for pest control. _1.

This fumigation process exposes produce to high concentrations of methyl bromide in sealed fumigation chambers for specified time periods. Fumigation can take place at orchards and farms where produce is grown or at international airports and ocean seaports where commodities enter the US. Upon completion of the fumigation cycle, chambers are ventilated with fresh air, and a determination of air quality is performed before workers are permitted to transport to storage or shipping areas.

The OSHA and NIOSH permissible exposure level (PEL), for methyl bromide is 20ppm and expressed as a ceiling level. ACGIH uses a 1ppm TWA for 8 hours and the DPR has a adopted this level for storage areas. After produce has been removed from fumigation chambers, methyl bromide can off gas low concentrations of over 1ppm for many hours. When fumigated products are stored in closed warehouses, workers can be exposed to low-level concentrations that over time could result in exceeding the lower 8-hour limit.

One example of off gassing is fumigation of fresh flowers. It’s mandated that an extended ventilation period in clean air must be provided before loading into florists’ vans.

The methyl bromide monitoring regulation for accessing worker exposure in the State of California is defined in their reference manual titled, “Methyl Bromide Commodity Fumigation.” This procedure specifies the use of a grab sample method using colorimetric length of stain tubes. Four manufacturers are listed in their regulation and this paper will discuss testing of two tube manufacturers. Testing includes the Dräger, Model # 81 03 391 low range tube and the Kitagawa, Model #157SB methyl bromide sample tube.

The approved DPR reference method requires a worker to manually take air samples through a colorimetric detector tube using a hand sample pump. Testing air concentrations must be performed after every fumigation cycle. Depending on the manufacturers’ tube, sampling can last 8 minutes for full tube stain development.

PureAire has developed a real time continuous methyl bromide monitor for accessing worker exposure in both chamber clearing and storage areas. This paper will introduce the features of this system, as well as demonstrate the detection capability as compared to colorimetric stain tubes when challenged to various methyl bromide concentrations.

A review of published data has found that the use of colorimetric tubes have a few drawbacks.

1. They only provide a one-time grab sample of the environment, not capable of continuous monitoring.
2. The accuracy of colorimetric tubes will vary due to temperature and correction factors must be used to obtain a final reading.
3. The stain reading must be interpolated between the graduations printed on the tube. Reading the stain when it’s developed on an angle between graduations makes it difficult to determine the precise reading. Manufactures recommend the user to estimate the highest and lowest reading and split the difference.
4. Accuracy of the final reading can vary from 15% to 30%.

The two manufacturers compared in this report, Dräger and Kitagawa, express the accuracy of their methyl bromide detector tube as a relative standard deviation (RSD).

Dräger Model # 81 03 391 Methyl Bromide tube with a range of 0.2 to 2ppm, and 2 to 8ppm 3/a range, has a standard deviation of +/- 15% to 20%. ₍₂₎

Kitagawa Model #157SB, SEI certified (Safety Equipment Institute), methyl bromide tubes with a range 0.4-80ppm, has an RSD-low of 15%; an RDS-mid is 10% and an RSD- high is 10%. ₍₃₎

The variance in the accuracy of colorimetric tubes, as well as how accuracy is actually determined by various manufacturers, caused NIOSH to develop a test and certification program between 1973 and 1983. The accuracy specification of readings must be +/- 35% at 0.5 times the TLV, and 25% at 1.0 to 5.0 times of the gas or vapor. ₍₄₎

In addition to the lower accuracy of colorimetric tubes, the cost per tube can be very expensive. With one tube used per test sample, and 5 to 7 tubes per shift, the average annual cost for an orchard to test for methyl bromide using colorimetric tubes is $16,875 per year.

An alternative to grab sampling is through the use of a continuous monitor. PureAire uses a proprietary renewable electrochemical sensor cell, combined with a novel heated filament and sample pump. Methyl bromide is sampled, pyrolized and converted to bromine, through this formula: 2CH₃Br -> Br₂ + 2CH₃ + 2e-

The following test data will demonstrate that PureAire’s continuous methyl bromide monitor can offer fumigators a continuous, more accurate and cost effective method to monitor methyl bromide exposure at both high and low sub TLV concentrations.

Testing Procedure

Side-by-side gas challenges were performed between: PureAire’s Air Check Advantage Methyl bromide Monitor low range of 0-10ppm and high range 0-50ppm, the Model 81 03 391 Dräger low range tube with a range of 0.2 to 2ppm, and 2 to 8ppm, and the Model 157SB SEI certified, Kitagawa methyl bromide tube with a range of 0.4-10, 1-25, and 2-80ppm. The number of pump strokes determines dynamic range for both manufacturers’ tubes. Dräger requires 5 strokes, (100 cc/stroke) for their lowest range. They also require 1 preliminary activation stroke before sampling to activate their chemistry. Kitagawa requires 4 strokes, (100 cc/stroke) for their lowest range, and does not require a preliminary activation stroke.

A 105-liter methyl bromide (bromomethane), span gas cylinder from Scott Specialty Gases was used as the gas source. The gas concentration was certified at 200-ppm methyl bromide blended with nitrogen, and has an accuracy of +/- 10%, with a lot number 629101C and expiration date of October 17, 2008. A calibrated 500cc/min gas regulator was used to dispense the methyl bromide span gas from the Scott cylinder into a Tedlar bag. Methyl bromide span gas was diluted with room air using a Bendix II high flow sample pump at a flow rate of 500 cc/min. Concentrations of 0.5ppm, 1.0ppm, 5.0 ppm, 9.5ppm, and 33 ppm were generated. All testing was performed in PureAire’s laboratory in summer 2007.

The sample time required for full stain development for the Dräger #81 03 391 tube at 0.5ppm and 1ppm levels is approximately 8 minutes with 5ppm level being approximately 4 minutes. Dräger sample tubes from batch number XH-0101 with an expiration date of Aug 09 were used.

The sample time required for full stain development for the Kitagawa #157SB tube at for 0.5ppm and 1ppm levels is 6 minutes with 5ppm level at 1.5 minutes. Kitagawa sample tubes from lot number 504116, expiration Aug 09 and lot number 509027, expiration date of Feb 09 were also used for this testing.

PureAire’s Air Check Advantage requires a 1-minute gas exposure at a flow rate of 450 cc/min to determine a final gas concentration reading at 1ppm and higher concentrations. A 2-minute gas exposure is required for 0.5ppm and lower concentrations. A freshly recharged methyl bromide sensor cell was used and the pyrolizer voltage was adjusted for converting methyl bromide.

Test Results

The following data was obtained from our testing.

Theoretical CH3Br concentration (PPM)	Air Check Advantage (PPM)	Dräger #81 03 391 (PPM)	Kitagawa 157SB (PPM)
0.5	0.4	0.7	1.2
0.5	0.5	0.7	1.5
0.5	0.5	0.6	1.1
0.5	0.6	-	-
0.5	0.5	-	-
1.0	0.8	1.1	1.6
1.0	0.8	1.1	1.8
1.0	0.9	1.2	1.3
1.0	0.9	-	-
1.0	0.9	-	-
5.0	4.4	5	5
5.0	5.0	4.5	4
5.0	5.0	4.2	4.5
5.0	5.1	-	-
5.0	4.7	-	-
9.5	10.1	No	11
9.5	10.5	testing	14
9.5	10.3	was	12
33	33	performed	34
33	33	using	30
33	33.2	Dräger at	37
33	33.7	9.5 and 30ppm	28

Conclusion

The above test data compared three different methods for detecting or monitoring methyl bromide during fumigation. From this information, the Air Check Advantage exhibited better accuracy and linearity for detecting methyl bromide at low, middle, and high concentrations. At lower and higher gas exposures, PureAire’s correlation coefficient, R-squared value was better than both Dräger’s low level methyl bromide tube and the Kitagawa standard range tube.

The Air Check is a continuous monitor with built in alarms and can provide instantaneous, real-time monitoring to alert workers of an unsafe condition.

The monitor has a built-in sample pump, for remote sampling up to 100 feet. With the addition of a multiport valve, it can provide measuring at multiple areas in the fumigation chamber during a clearing. Designed to provide remote, unattended monitoring, a continuous mA data stream is also provided to data loggers or remote controllers.

The use of a continuous monitor during the fumigation process can help to immediately detect leaking valves, leaks in process tubing, and chamber door seals during fumigation. Additionally continuous monitoring eliminates the need to subject workers to a possible risk site while manually take air samples with a hand pump.

By locating the monitor in the area between or near chambers and charging canisters, PureAire’s continuous monitor will provide 24-7 protection against methyl bromide leaks to quickly alert workers of unsafe conditions when it occurs. In a test during fumigation at one international airport, PureAire’s methyl bromide monitor uncovered a leaking regulator from a methyl bromide canister as the chamber was being charged. Although the area was well ventilated with a fully open garage door, methyl bromide concentrations of 6ppm to 9ppm were recorded approximately 7 feet away from the leaking cylinder.

The Air Check’s capability to monitor methyl bromide at sub TLV, 0.5ppm levels can offer a means to document worker exposure to methyl bromide off gassing from fumigated produce in storage areas.

Finally, the Air Check Advantage is a renewable technology. The annual cost of maintaining the monitor is approximately $75 for replacement materials. Kitagawa and Drager tubes are disposable and can cost a fumigator from $16,000 to $19,000 per year in consumable materials.

PureAire Monitoring Systems and its affiliate, Bionics Instruments Company Ltd, have been supplying methyl bromide monitors to the industry for over 30 years. Used for protecting workers against the risk of exposure, these monitors have been used to protect workers in chemical and petrochemical plants.

The Air Check has a built-in gas concentration display, dual level user selectable alarm relays, and 4-20mA output to remote controllers and data loggers. The Air Check electronics and sensor are housed in a Nema 4X IP 65 water resistant outdoor polycarbonate enclosure. This enclosure is suitable for non-hazardous environments. The Air Check Advantage Ex electronics and sensor are housed in a Killark Aluminum explosion proof enclosure for Class 1, Div 1, Groups B, C, and D hazardous environments. This enclosure is suitable for outdoor use and is FM, UL, and CSA rated. Standard gas ranges for PureAire’s methyl bromide monitors are 0-10ppm, 0-30ppm, and 0-50ppm.

Cited Sources

1. Methyl Bromide Characterization in California, National Academy Press, Washington, DC
2. Dr äger 90 22 860 2^nd edition February 2005 instruction sheet for # 81 03 391 methyl bromide tube
3. Kitagawa gas detector Tube System Handbook, OCT.01/1(G)
4. SEI Update Volume 18, Issue 1, March 2004

Al Carrino is the President and founder of PureAire Monitoring Systems. He has been involved in the manufacture and sales of gas monitoring systems since 1978 and has worked for MDA Scientific and MST Measurement Systems in executive management positions. At PureAire, he has been instrumental in new product development for methyl bromide, methyl iodide, nitrogen trifluoride, Acrylonitrile, and oxygen monitoring systems.