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A Calibration Program for Rotorod Samplers
A calibration program for Rotorod samplers David A Frenz* and Jay C Elander†
Background: Despite widespread use in the United States, the need for calibrating Rotorod Samplers on a regular basis has not been demonstrated. Objective: The purpose of this present investigation was to inspect Rotorods that had been operating in the field to determine whether they conformed to the manufacturer’s operating specifications. Methods:Allergists who own Rotorods voluntarily returned their devices to the manufacturer for calibration over the course of two successive winters. The speed of each device’s motor was measured with a stroboscopic tachometer. Results: Of the 89 devices examined, ten were found to be operating at the manufacturer’s exact speed specification (2400 RPM) and another 31 were within 5%. The remaining devices deviated by more than 5% from the prescribed speed or possessed motors that would not operate. Conclusions: The authors underscore the importance of calibrating Rotorod Samplers to maintain data quality. Ann Allergy Asthma Immunol 1996;76:245–246.
INTRODUCTION Despite the Rotorod Sampler’s widespread use by allergists during the past three decades, the subject of device calibration has received only scant attention.1,2 This matter is especially critical, however, since improper Rotorod performance may yield spurious data for aerobiologic research, drug studies, and use by the public. The Rotorod Sampler is a rotating arm impactor that collects particles on two rapidly moving plastic rods.3,4 The atmospheric concentration (C) of the particles (p) impinged on the collector rods is a function of the volume of the air (V) sampled by the rods. Specifically, p C⫽ (1) V where V ⫽ rod area ⫻ path diameter ⫻ ⫻ motor speed ⫻ time sampled
(2)
* Multidata, Inc.; 10801 Wayzata Boulevard, Suite 330; Minnetonka, Minnesota. † Sampling Technologies, Inc.; 10801 Wayzata Boulevard, Suite 340; Minnetonka, Minnesota. Received for publication July 19, 1995. Accepted for publication in revised form September 18, 1995.
VOLUME 76, MARCH, 1996
The first three terms in equation 2 are constants. The fourth term is also assumed to be constant; however, changes in the motor’s speed affect the volume of air sampled and ultimately C. For example, a speed that is actually slower than the value used in equation 2 causes equation 1 to underestimate the atmospheric concentration of particles. One notices from equation 2 that this effect is linear. The manufacturer certifies that the motors of all Rotorod Samplers are functioning at a factory-established value of 2400 RPM on the date of purchase. The purpose of this present investigation was to inspect Rotorod Samplers operated by allergists in the field. Motor speeds were measured to determine whether devices were performing within the manufacturer’s specifications. MATERIALS AND METHODS During the winters of 1993–94 and 1994 –95, all purchasers of Rotorod Samplers (Sampling Technologies, Inc., Minnetonka, MN) were encouraged by letter to calibrate their devices. Eighty-nine Rotorods were returned to the manufacturer for this procedure: 41 during the first winter and 48 during the second. Sixty-four of the machines were from the Model 85 series, a blue-
green colored device with an umbrella top and aluminum body manufactured between 1982 and 1991. Of the remaining 25 samplers, 18 were from the Model 95 series produced from 1991 to 1993 and seven from the most recent Model 40 series produced from 1993 to the present. Both the Model 95 and 40 are gray-colored and constructed with a fiberglass body. All of the devices were used outdoors by allergists to collect pollen grains and spores to determine a daily “pollen count.” The speed of each sampler’s motor was determined with a stroboscopic tachometer (Strobotach Model 1531AB, General Radio Company, Concord, MA). Motors that deviated from the prescribed speed of 2400 RPM were adjusted, if possible, or replaced. RESULTS Of the 89 samplers returned for inspection, the motors of 24 devices would not operate. Fifteen motor failures were found among the Model 85s, eight among the Model 95s and one among the Model 40s. Of the 65 samplers whose motors would function, ten devices were found to be operating at the prescribed speed of 2400 RPM (Table 1). Fiftyone of the remaining devices operated slower than 2400 RPM, while four were operating faster. As a group, Model 85s presented the greatest variability in motor speed (range: 1700 to 2725 RPM). In contrast, motors in Model 95s and 40s presented much narrower characteristics (range: 1950 to 2400 and 2260 to 2600 RPM, respectively). Of the 49 operable Model 85s, the motors of five devices deviated by more than 10% from 2400 RPM. Two Model 95s also varied by more than 10%, however, no Model 40s presented a variance this large. The mean speed of Model 85 and 95 motors was slower than 2400 RPM
245
Table 1. Motor Performance Data for Operable Rotorod Samplers Returned for Calibration (n ⫽ 65) Rotorod Samplers (number) Motor Speed (RPM) Model
Mean ⴞ SD
<2400
2400
>2400
<5%
5%–10%
>10%
85 95 40
2302 ⫾ 144 2278 ⫾ 131 2417 ⫾ 132
40 9 2
7 1 2
2 0 2
32 6 3
12 2 3
5 2 0
(2302 and 2278, respectively) Model 40s, however, presented a mean speed slightly faster (2417 RPM). DISCUSSION The volume of air sampled by a Rotorod Sampler is a function of how fast its motor moves two collector rods through the atmosphere. From equation 2, one notices a linear relationship between V, the volume of air sampled, and the motor speed. Specifically, less air is sampled per unit of time as the motor speed decreases. Allergists assume that the motor speed is constant (2400 RPM) when they calculate pollen counts from particles recovered by their Rotorod Samplers. This present investigation tested that assumption by measuring the motor speeds of 89 Rotorod devices. On the basis of the samplers inspected by the authors, three important findings emerged. First, many of the devices returned for calibration were operating satisfactorily. Ten devices (11%), many of which were the older Model 85s, were found to be operating at the manufacturer’s specification for motor speed. Further, another 31 devices (35%) were operating within 5% of the prescribed speed. This suggests that these devices were functioning reliably in the field prior to inspection by the authors. Second, a significant number of devices had drifted from 2400 RPM.
246
Speed Deviated from 2400 RPM by
Seventeen devices (19%) were operating at speeds deviating by 5% to 10% from 2400 RPM, and seven devices (8%) had drifted by more than 10%. The most extreme cases, one Model 85 and one Model 95, deviated by more than 20% from 2400 RPM. This indicates that data collected with these devices prior to calibration should be approached cautiously since they likely underestimate the aeropollen concentration. Third, the motors in 24 devices (27%) had failed in the field prior to being returned for calibration. This raises the unfortunate possibility that these devices had performed poorly for some length of time prior to calibration. Data obtained with these devices prior to motor failure should be treated with the greatest caution. These findings underscore the need for regular calibration of Rotorod Samplers. Since even a 10% decrease in motor speed is difficult to detect without a stroboscopic tachometer, regular inspection appears extremely prudent. On the basis of the data presented, the authors recommend calibrating Rotorod Samplers annually. The winter months provide allergists in many parts of the country with a logical opportunity to perform this procedure. One can calibrate a Rotorod device with the aid of a stroboscopic tachometer. Many institutions own this equipment and often employ technicians who calibrate devices at an in-house
standards laboratory. Alternatively, Sampling Technologies also offers this service for a nominal charge. Regardless of where a calibration is performed, this common sense measure assures reliable data for research and communication of pollen count information to the public. ACKNOWLEDGMENTS The authors thank Ms. Jody Lindquist of Sampling Technologies, Inc., for coordinating the Rotorod Sampler calibration program. REFERENCES 1. Solomon WR, Burge HA, Boise JR, et al. Comparative particle recoveries by the retracting Rotorod, Rotoslide and Burkard Spore Trap sampling in a compact array. Int J Biometeor 1980; 24:107–116. 2. Frenz DA. Sampler calibration [letter]. Ann Allergy Asthma Immunol 1995; 74:276. 3. Grinnell SW, Perkins WA, Vaughan LM. Sampling apparatus and method. Patent no. 2,973,642. Washington: United States Patent Office, 1961. 4. Brown T, Frenz DA, Wimpsett TL et al. Operating Instructions for the Rotorod Sampler. Minnetonka, MN: Sampling Technologies, Inc., 1993. Requests for reprints should be addressed to: David A. Frenz Vice-President/Director of Research Multidata, Inc. 10801 Wayzata Blvd., Suite 330 Minnetonka, MN 55305-1533
ANNALS OF ALLERGY, ASTHMA, & IMMUNOLOGY
Background: Despite widespread use in the United States, the need for calibrating Rotorod Samplers on a regular basis has not been demonstrated. Objective: The purpose of this present investigation was to inspect Rotorods that had been operating in the field to determine whether they conformed to the manufacturer’s operating specifications. Methods:Allergists who own Rotorods voluntarily returned their devices to the manufacturer for calibration over the course of two successive winters. The speed of each device’s motor was measured with a stroboscopic tachometer. Results: Of the 89 devices examined, ten were found to be operating at the manufacturer’s exact speed specification (2400 RPM) and another 31 were within 5%. The remaining devices deviated by more than 5% from the prescribed speed or possessed motors that would not operate. Conclusions: The authors underscore the importance of calibrating Rotorod Samplers to maintain data quality. Ann Allergy Asthma Immunol 1996;76:245–246.
INTRODUCTION Despite the Rotorod Sampler’s widespread use by allergists during the past three decades, the subject of device calibration has received only scant attention.1,2 This matter is especially critical, however, since improper Rotorod performance may yield spurious data for aerobiologic research, drug studies, and use by the public. The Rotorod Sampler is a rotating arm impactor that collects particles on two rapidly moving plastic rods.3,4 The atmospheric concentration (C) of the particles (p) impinged on the collector rods is a function of the volume of the air (V) sampled by the rods. Specifically, p C⫽ (1) V where V ⫽ rod area ⫻ path diameter ⫻ ⫻ motor speed ⫻ time sampled
(2)
* Multidata, Inc.; 10801 Wayzata Boulevard, Suite 330; Minnetonka, Minnesota. † Sampling Technologies, Inc.; 10801 Wayzata Boulevard, Suite 340; Minnetonka, Minnesota. Received for publication July 19, 1995. Accepted for publication in revised form September 18, 1995.
VOLUME 76, MARCH, 1996
The first three terms in equation 2 are constants. The fourth term is also assumed to be constant; however, changes in the motor’s speed affect the volume of air sampled and ultimately C. For example, a speed that is actually slower than the value used in equation 2 causes equation 1 to underestimate the atmospheric concentration of particles. One notices from equation 2 that this effect is linear. The manufacturer certifies that the motors of all Rotorod Samplers are functioning at a factory-established value of 2400 RPM on the date of purchase. The purpose of this present investigation was to inspect Rotorod Samplers operated by allergists in the field. Motor speeds were measured to determine whether devices were performing within the manufacturer’s specifications. MATERIALS AND METHODS During the winters of 1993–94 and 1994 –95, all purchasers of Rotorod Samplers (Sampling Technologies, Inc., Minnetonka, MN) were encouraged by letter to calibrate their devices. Eighty-nine Rotorods were returned to the manufacturer for this procedure: 41 during the first winter and 48 during the second. Sixty-four of the machines were from the Model 85 series, a blue-
green colored device with an umbrella top and aluminum body manufactured between 1982 and 1991. Of the remaining 25 samplers, 18 were from the Model 95 series produced from 1991 to 1993 and seven from the most recent Model 40 series produced from 1993 to the present. Both the Model 95 and 40 are gray-colored and constructed with a fiberglass body. All of the devices were used outdoors by allergists to collect pollen grains and spores to determine a daily “pollen count.” The speed of each sampler’s motor was determined with a stroboscopic tachometer (Strobotach Model 1531AB, General Radio Company, Concord, MA). Motors that deviated from the prescribed speed of 2400 RPM were adjusted, if possible, or replaced. RESULTS Of the 89 samplers returned for inspection, the motors of 24 devices would not operate. Fifteen motor failures were found among the Model 85s, eight among the Model 95s and one among the Model 40s. Of the 65 samplers whose motors would function, ten devices were found to be operating at the prescribed speed of 2400 RPM (Table 1). Fiftyone of the remaining devices operated slower than 2400 RPM, while four were operating faster. As a group, Model 85s presented the greatest variability in motor speed (range: 1700 to 2725 RPM). In contrast, motors in Model 95s and 40s presented much narrower characteristics (range: 1950 to 2400 and 2260 to 2600 RPM, respectively). Of the 49 operable Model 85s, the motors of five devices deviated by more than 10% from 2400 RPM. Two Model 95s also varied by more than 10%, however, no Model 40s presented a variance this large. The mean speed of Model 85 and 95 motors was slower than 2400 RPM
245
Table 1. Motor Performance Data for Operable Rotorod Samplers Returned for Calibration (n ⫽ 65) Rotorod Samplers (number) Motor Speed (RPM) Model
Mean ⴞ SD
<2400
2400
>2400
<5%
5%–10%
>10%
85 95 40
2302 ⫾ 144 2278 ⫾ 131 2417 ⫾ 132
40 9 2
7 1 2
2 0 2
32 6 3
12 2 3
5 2 0
(2302 and 2278, respectively) Model 40s, however, presented a mean speed slightly faster (2417 RPM). DISCUSSION The volume of air sampled by a Rotorod Sampler is a function of how fast its motor moves two collector rods through the atmosphere. From equation 2, one notices a linear relationship between V, the volume of air sampled, and the motor speed. Specifically, less air is sampled per unit of time as the motor speed decreases. Allergists assume that the motor speed is constant (2400 RPM) when they calculate pollen counts from particles recovered by their Rotorod Samplers. This present investigation tested that assumption by measuring the motor speeds of 89 Rotorod devices. On the basis of the samplers inspected by the authors, three important findings emerged. First, many of the devices returned for calibration were operating satisfactorily. Ten devices (11%), many of which were the older Model 85s, were found to be operating at the manufacturer’s specification for motor speed. Further, another 31 devices (35%) were operating within 5% of the prescribed speed. This suggests that these devices were functioning reliably in the field prior to inspection by the authors. Second, a significant number of devices had drifted from 2400 RPM.
246
Speed Deviated from 2400 RPM by
Seventeen devices (19%) were operating at speeds deviating by 5% to 10% from 2400 RPM, and seven devices (8%) had drifted by more than 10%. The most extreme cases, one Model 85 and one Model 95, deviated by more than 20% from 2400 RPM. This indicates that data collected with these devices prior to calibration should be approached cautiously since they likely underestimate the aeropollen concentration. Third, the motors in 24 devices (27%) had failed in the field prior to being returned for calibration. This raises the unfortunate possibility that these devices had performed poorly for some length of time prior to calibration. Data obtained with these devices prior to motor failure should be treated with the greatest caution. These findings underscore the need for regular calibration of Rotorod Samplers. Since even a 10% decrease in motor speed is difficult to detect without a stroboscopic tachometer, regular inspection appears extremely prudent. On the basis of the data presented, the authors recommend calibrating Rotorod Samplers annually. The winter months provide allergists in many parts of the country with a logical opportunity to perform this procedure. One can calibrate a Rotorod device with the aid of a stroboscopic tachometer. Many institutions own this equipment and often employ technicians who calibrate devices at an in-house
standards laboratory. Alternatively, Sampling Technologies also offers this service for a nominal charge. Regardless of where a calibration is performed, this common sense measure assures reliable data for research and communication of pollen count information to the public. ACKNOWLEDGMENTS The authors thank Ms. Jody Lindquist of Sampling Technologies, Inc., for coordinating the Rotorod Sampler calibration program. REFERENCES 1. Solomon WR, Burge HA, Boise JR, et al. Comparative particle recoveries by the retracting Rotorod, Rotoslide and Burkard Spore Trap sampling in a compact array. Int J Biometeor 1980; 24:107–116. 2. Frenz DA. Sampler calibration [letter]. Ann Allergy Asthma Immunol 1995; 74:276. 3. Grinnell SW, Perkins WA, Vaughan LM. Sampling apparatus and method. Patent no. 2,973,642. Washington: United States Patent Office, 1961. 4. Brown T, Frenz DA, Wimpsett TL et al. Operating Instructions for the Rotorod Sampler. Minnetonka, MN: Sampling Technologies, Inc., 1993. Requests for reprints should be addressed to: David A. Frenz Vice-President/Director of Research Multidata, Inc. 10801 Wayzata Blvd., Suite 330 Minnetonka, MN 55305-1533
ANNALS OF ALLERGY, ASTHMA, & IMMUNOLOGY