Industrial Hygiene Air Sampling with Constant ... - ACS Publications

30 Industrial Hygiene Air Sampling with Constant Flow Pumps W. B. BAKER, D. G. CLARK, and W. J. LAUTENBERGER

Downloaded by UNIV OF ARIZONA on January 7, 2013 | http://pubs.acs.org Publication Date: April 2, 1981 | doi: 10.1021/bk-1981-0149.ch030

Applied Technology Center, Ε. I. du Pont de Nemours and Company, Incorporated, Wilmington, DE 19898

In air monitoring program accurate concentration determina tions require careful attention to the sample collection and subsequent analysis of the collected sample. The analysis is the most c r i t i c a l step, especially i f very low levels (ppb) of contaminants are being determined. However, in many cases, sampling may be the least accurate step since this job is performed in the f i e l d with portable equipment under conditions far less favorable than those that can be created and controlled in the lab(1). NIOSH states that acceptable a i r monitoring methods must come within ±25% of the true concentration for 95% of the samples taken. The error factor attributed to sampling pumps is a coefficient of variation of 5% (2). If there is no bias in the sampling pump, the accuracy i s : Accuracy = 2 CVp (Assumes no bias) =+10% where:

CV = 5% p

Errors in sampling (sample volume determination) are due to erroneous measurement of time or flow rate. Time can be measured so a c c u r a t e l y that f l o w r a t e e r r o r s make up the m a j o r i t y of the -10% v a r i a t i o n a t t r i b u t e d to the average sampling pump. Sampling E r r o r s f o r Flow V a r i a t i o n s Flow r a t e v a r i a t i o n s cause three types of sampling e r r o r s : 1) d i r e c t sample s i z e e r r o r ; 2) time-of-day b i a s ; and 3) r e s p i r a b l e sampling e r r o r s . A d i r e c t e r r o r occurs when the

0097-6156/81/0149-0491 $05.00/0 © 1981 American Chemical Society

In Chemical Hazards in the Workplace; Choudhary, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


492

C H E M I C A L HAZARDS IN T H E W O R K P L A C E

sampler i s c a l i b r a t e d to operate a t a given flow r a t e but changes f l o w r a t e . I f a sampler i s c a l i b r a t e d a t 1 l i t e r / m i n . but operated a t an average f l o w r a t e of 0.9 l i t e r s / m i n . , t h i s 10 percent f l o w r a t e e r r o r would cause a 10 percent e r r o r i n sample s i z e . D i r e c t e r r o r s can occur i n simple p o s i t i v e - d i s p l a c e m e n t pumps as the r e s u l t of b a t t e r y discharge and other e f f e c t s . F i g u r e 1 shows the b a t t e r y discharge e f f e c t on f l o w rate(_3) . The i n i t i a l f l o w r a t e decrease i s caused by the NiCd b a t t e r y discharge c h a r a c t e r i s t i c . D i r e c t e r r o r can a l s o r e s u l t from the e f f e c t of i n l e t pressure changes on flow as shown i n F i g u r e 2(3). Since the pump i s working on a compressable f l u i d , a i r , any i n c r e a s e i n the pressure drop reduces the flow r a t e . In the f i e l d , the c o l l e c t i o n device w i l l accumulate d i r t and dust i n a d d i t i o n to the d e s i r e d sample. This w i l l cause the pressure drop t o i n c r e a s e w i t h time. I f the sampling pump does not c o n t r o l the f l o w , i t w i l l decrease as pressure drop i n c r e a s e s . Some samplers i n c o r p o r a t e an accumulated volume i n d i c a t o r such as a stroke-counter t o permit c o r r e c t i o n f o r the above e f f e c t s . However, time-of-day b i a s can occur i f the exposure i s not constant. A h i g h contaminant l e v e l would be erroneously i n d i c a t e d i f a h i g h exposure occurred during the s t a r t of the sampling p e r i o d r a t h e r than a t the end. When performing r e s p i r a b l e sampling, the t o t a l e r r o r w i l l be g r e a t e r than the f l o w r a t e e r r o r . Cyclones used i n r e s p i r a b l e sampling a r e designed t o operate a t a s p e c i f i c f l o w r a t e . A d e v i a t i o n from t h i s s p e c i f i c f l o w r a t e w i l l cause g r e a t e r c o l l e c t i o n e f f i c i e n c y v a r i a t i o n s depending on the type of cyclone and f l o w r a t e ( 4 ) . I t becomes evident that accurate sampling r e q u i r e s a constant f l o w r a t e . Many methods have been used i n the a t t e p t to provide a constant flow sampler, i n c l u d i n g : rotometers, motor-voltage r e g u l a t o r s , r e s t r i c t i n g o r i f i c e s , pump s t r o k e counters and motor-current compensation. These methods attempt to provide compensation i n d i r e c t l y f o r the e r r o r e f f e c t s ( 5 , 6 ) . The f l o w c o n t r o l method described i n t h i s d i s c u s s i o n provides d i r e c t monitoring and c o n t r o l of sampler flow r a t e . Theory o f Operation A simple p o s i t i v e - d i s p l a c e m e n t sampler system i s shown i n F i g u r e 3. The b a s i c system contains a b a t t e r y and motor connected to a p o s i t i v e - d i s p l a c e m e n t pump mechanism and provides an e f f i c i e n t means f o r moving a i r through the sampler. I n order to provide feedback f l o w c o n t r o l the system must be expanded t o i n c l u d e the means t o monitor a i r f l o w through the pump t o compensate f o r f l o w v a r i a t i o n s . F i g u r e 4 shows a simple v e r s i o n of a feedback f l o w c o n t r o l sampler system. An o r i f i c e and pressure switch a r e used t o monitor a i r f l o w .


30.

BAKER E T A L .

493

Constant Flow Pump Sampling

2.2 j -

2.03

Ε

·—•—· -

--

1.{

1.6

1.4

J

1.2 2

L

3

4

J 5

L

J

I

6


Time (hours)

Figure 1.

Flow rate vs. time, simple positive-displacement pump

2.0

Ε =•

1.5

1.0

10

Figure 2.

in,et

15 20 25 30 Inlet Pressure (in. of H2O)

35

40

Flow rate vs. inlet pressure, simple positive-displacement pump

vlypump-

Exhaust

Positive Displacement

Battery

Figure 3.

Simple positive-displacement sampler system


494

CHEMICAL

HAZARDS

IN

THE WORKPLACE

The pressure developed across an o r i f i c e v a r i e s w i t h flow as shown i n the equation(7): F2 Ρ = Constant χ


Where: F = flow r a t e Ρ = f l u i d density As f l o w i n c r e a s e s , pressure drop i n c r e a s e s . When the pressure exceeds the s w i t c h set p o i n t , the contacts w i l l c l o s e . The f l o w r a t e a t which t h i s occurs i s determined by the o r i f i c e s i z e and the pressure s w i t c h set p o i n t . A v a r i a b l e o r i f i c e i s used so that flow r a t e can be adjusted. The pressure s w i t c h i s connected to an i n t e g r a t o r c i r c u i t . The i n t e g r a t o r c i r c u i t i s c h a r a c t e r i z e d by an output v o l t a g e which changes s l o w l y . T h i s v o l t a g e increases when the pressure s w i t c h contacts are open i n d i c a t i n g that the f l o w r a t e i s below the set p o i n t . When the f l o w r a t e i s above the set p o i n t , the switch contacts a r e closed and i n t e g r a t o r output decreases. The output v o l t a g e of the i n t e g r a t o r i s a m p l i f i e d and a p p l i e d to the motor. In t h i s system the motor s t a r t s t o speed up when the f l o w r a t e i s low and slows down when the f l o w r a t e i s h i g h . Thus the c o n t r o l system w i l l always c o r r e c t i n the proper d i r e c t i o n f o r any f l o w r a t e e r r o r . F i g u r e 5 f u r t h e r i l l u s t r a t e s i n t e g r a t o r o p e r a t i o n and a s s o c i a t e d ramping e f f e c t s . Again i t can be seen that w i t h the pressure s w i t c h open, the i n t e g r a t o r ramps up w i t h time; and conversely when the switch i s c l o s e d , the i n t e g r a t o r ramps down. I n a pump w i t h p u l s i n g f l o w , the s w i t c h opens and c l o s e s w i t h each pump s t r o k e because i t responds to the momentary increases i n f l o w and pressure. During each r e v o l u t i o n the pump b r i e f l y speeds up and slows down f o r equal periods of time when operating a t constant c o n d i t i o n s . F i g u r e 6 shows the o p e r a t i o n of the c o n t r o l system when a f l o w i n c r e a s e i s r e q u i r e d . Some e x t e r n a l i n f l u e n c e reduce the f l o w r a t e below the set p o i n t . This caused the pressure switch t o stay open longer during each c y c l e and the output v o l t a g e g r a d u a l l y increased u n t i l the f l o w r a t e was r e s t o r e d to the set p o i n t . Thus i t i s shown that the feedback f l o w c o n t r o l system w i l l a c t t o c o r r e c t f o r any c o n d i t i o n that would cause f l o w variation. With a p u l s i n g f l o w , there i s an e r r o r e f f e c t which may cause overcompensation. The equation f o r t h i s e r r o r e f f e c t is(7): (Error) Where:

e = Constant χ *f χ Δ Ρ ~{~ i s frequency of p u l s a t i o n i s the pressure drop


30.

BAKER E T A L .

495


Orifice l n , ? t

Exhaust

V-L/Pump I I ' ό ) Motor >

Feedback Signal


Battery

Switch Position

Integrator/ Amplifier

Figure 4.

Feedbackflowcontrol sampler system

J—I

Γ

Integrator Output Voltage

Time-

Switch Position

Figure 5.

Integrator operation

u — L _ T

Integrator Output Voltage

Time-

Figure 6.

Integrator operation duringflowincrease


496

CHEMICAL

H A Z A R D S IN

THE WORKPLACE

As pump load i s i n c r e a s e d , t h i s e r r o r e f f e c t causes the f l o w r a t e t o i n c r e a s e more than necessary. This problem i s e l i m i n a t e d by u s i n g an accumulator to f i l t e r the f l o w through the o r i f i c e . F i g u r e 7 shows the completed feedback c o n t r o l system which i n c o r p o r a t e s an accumulator between the pump and o r i f i c e .


P r a c t i c a l Systems F i g u r e 8 shows the a i r passages through a t y p i c a l feedback c o n t r o l constant f l o w sampler. The a i r enters on the l e f t , proceeds through the v a l v e s and diaphragm of the pumping mechanism i n t o an accumulator. This accumulator i s a s m a l l space w i t h a l a r g e s u r f a c e covered by an elastomer sheet to provide flow-smoothing a c t i o n . A pressure switch i s connected to taps l a b e l e d P.S. High and P.S. Low. The pump mechanism i s operated by a s m a l l e l e c t r i c motor and gearhead. A d d i t i o n ôf the c o n t r o l c i r c u i t and b a t t e r y complete t h i s constant f l o w system. Other sampling pump a p p l i c a t i o n s r e q u i r e m o d i f i c a t i o n s of t h i s c o n t r o l system f o r proper performance. When the previous system was a p p l i e d t o a h i g h f l o w m u l t i - c y l i n d e r pump, the i n h e r e n t l y smooth f l o w e l i m i n a t e d the need f o r an accumulator. The pressure switch experienced r a p i d wear due to the high r a t e of o p e r a t i o n , about 100 c y c l e s per second (6000 per minute). This problem was e l i m i n a t e d by adding a pneumatic f i l t e r as shown i n F i g u r e 9. The pneumatic f i l t e r c o n s i s t s of a d i f f e r e n t i a l accumulator and f o u r o r i f i c e s and f i l t e r s the pressure s i g n a l before i t reaches the pressure s w i t c h . This f i l t e r had a time delay i n passing the s i g n a l t o the pressure s w i t c h , but by v a r y i n g the s i z e of the o r i f i c e s , i t was p o s s i b l e to a d j u s t the time delay so that the pump and c o n t r o l system c y c l e d a t a r a t e of 8 c y c l e s per second, extending switch l i f e f o r s e v e r a l years of normal o p e r a t i o n ; y e t , the c y c l i n g remained r a p i d enough t o e l i m i n a t e any n o t i c e a b l e v a r i a t i o n i n flow. Another v a r i a t i o n of the system was r e q u i r e d f o r a s i n g l e c y l i n d e r (diaphragm) pump that operated a t a high speed and produced p u l s a t i n g flow. As shown i n F i g u r e 10, t h i s pump uses a combination of the two c o n t r o l systems p r e v i o u s l y described. Because t h i s p a r t i c u l a r sampler was not r e q u i r e d to f i l l bags, the pneumatic f i l t e r was not designed w i t h d i f f e r e n t i a l accumulators. A l a r g e r accumulator i n the a i r f l o w passage preceding the o r i f i c e provided enough f l o w smoothing f o r proper o p e r a t i o n of the needle v a l v e . An accumulator w i t h two diaphragms was used to smooth the i n l e t f l o w of t h i s sampler. Thus, i t has been demonstrated that p r e c i s e f l o w c o n t r o l i s p o s s i b l e w i t h many d i f f e r e n t pump mechanisms.


30.

BAKER E T A L .


/"—\

l n l e t

Collection Device

Accumulator " — , w

497 Adjustable Orifice Exhaust

Vj^/Pump

Motor (

ô

Pressure Switch Feedback Signal

Battery

Feedback Control


Figure 7. Feedback flow control system for low-speed pump

Adjustable Orifice

Inlet

Collection Device

V

Motor!

y

Battery

Figure 9.

!

yVariable Speed P u m p Differential Accumulator

Exhaust

^Orifice (Typ.) Feedback Signal

J Pressure Switch

Feedback Control

Feedback flow control system for high-speed pump with smooth flow


498

CHEMICAL

HAZARDS

IN

THE

WORKPLACE


Performance The f o l l o w i n g data i l l u s t r a t e s the performance of feedback flow c o n t r o l samplers under t e s t c o n d i t i o n s which represent f i e l d c o n d i t i o n s . F i g u r e 11 shows the f l o w r a t e s t a b i l i t y versus time f o r a sampler operated on i t s b a t t e r y . The flow c o n t r o l system maintained a constant f l o w r a t e even though the b a t t e r y was d i s c h a r g i n g ( 3 ) . F i g u r e 12 shows the f l o w r a t e s t a b i l i t y versus l o a d (input pressure drop). A v a r i a b l e o r i f i c e connected to the sampler was used to a d j u s t the pressure drop over a range of 0-16 inches water column pressure drop. Flow r a t e was monitored by a rotameter. The flow r a t e was constant to w i t h i n * 2 percent Q ) . F i g u r e 13 shows the f l o w r a t e s t a b i l i t y versus ambient a i r temperature. The samplers were operated i n an environmental chamber over a temperature range of 0°C to 70°C. A heat exchanger was connected to the sampler i n l e t to ensure that the temperature of the i n l e t a i r was the same as the ambient a i r temperature. The sampler f l o w r a t e was monitored w i t h a bubble tube o u t s i d e the chamber. F i g u r e 14 shows the a l t i t u d e e f f e c t on the sampler f l o w r a t e . T h i s t e s t was performed i n a vacuum chamber. The f l o w r a t e was monitored w i t h a bubble tube which was mounted i n the chamber and operated by remote c o n t r o l . Each data p o i n t i s an average of 18 p i e c e s of data: three samplers and three f l o w r a t e s which were monitored w h i l e both i n c r e a s i n g and decreasing the vacuum. I f sampling must be performed under p r e s s u r e , temperature and a l t i t u d e c o n d i t i o n s which d i f f e r from the c a l i b r a t i o n c o n d i t i o n s , the above e f f e c t s must be known and used as c o r r e c t i o n s i n sample volume c a l c u l a t i o n s . Calibration Any sampler, even one which w i l l m a i n t a i n constant f l o w , must be c a l i b r a t e d to ensure accuracy. D a i l y c a l i b r a t i o n ensures r e p e a t a b i l i t y . D i f f e r e n t methods used f o r c a l i b r a t i n g samplers i n c l u d e : rotometers, w e t - t e s t meters, pressure gauges across f i x e d o r i f i c e s , mass f l o w meters, hot w i r e f l o w meters and bubble tubes. Each of these c a l i b r a t i o n devices r e q u i r e s an a p p r o p r i a t e c o r r e c t i o n f a c t o r . Some of the devices measure mass f l o w r a t h e r than v o l u m e t r i c flow. Sampling r e q u i r e s volumetric flow c a l i b r a t i o n . Most of the c a l i b r a t i o n devices r e q u i r e o r i g i n a l c a l i b r a t i o n by a primary standard. A bubble tube i s the only d e v i c e mentioned that i s a primary standard(8). I t can be checked w i t h simple l a b o r a t o r y t o o l s .


BAKER E T A L .

499

Constant Flow Pump Sampling Main Accumulator

Inlet Collection Inlet Device Accumulator

Exhaust

Pump

, ·

Motor(o)

Orifice ( T y p . ) — •

Feedback Signal Battery

Kl Ha-

I Pressure Switch

Feedback flow control system for high-speed pump with pulsating flow


Figure 10.

Feedback Control

π Signal d Accumulator U(Typ.)

140 Γ

120 -

100

80

60 40 3

4

5

Time (Hours)

Figure 11.

I

200

r

150

-

100

-

Flow rate vs. time, pump model P-125

50

6 Δ Ρ (in. of

Figure 12.

8 H 0)

10

14

16

2

Flow rate vs. load (pressure differential), pump model P-125


500

CHEMICAL

10

20

30

40

HAZARDS

60

50

IN T H E W O R K P L A C E

70


Temperature (°C)

Figure 13.

Temperature effect, pump model P-2500: (O) Pump #1, Ο #2

Pump

20 ι -

Ο

1

2

3

4

5

6

7

8

Altitude (χ 1000 Ft.)

Figure 14.

Altitude effect, pump model P-2500

Flow

LABORATORY

SAMPLER

Bubble Solution C u p

FlowBubble Solution Bulb

Figure 15.

Bubble tubes


30.

BAKER E T A L .


501

I f the bubble tube i s connected t o the i n l e t s i d e o f the pump, no c o r r e c t i o n s are r e q u i r e d . T h i s r e q u i r e s the sampler bubble tube shown i n F i g u r e 15. I f a standard l a b o r a t o r y type bubble tube i s connected to the exhaust s i d e of the pump, the volume of a i r which has gone through the sampler pump becomes a d d i t i o n a l l y h u m i d i f i e d by the soap s o l u t i o n i n the bubble tube causing an i n c r e a s e i n the volume. Connecting a bubble tube t o the exhaust s i d e of the pump can cause e r r o r s i n the range of 1 t o 3% i f the proper c o r r e c t i o n i s o m i t t e d ( 9 ) .


Conclusion A sampling method i s c u r r e n t l y a v a i l a b l e i n the f i e l d w i t h accuracy that exceeds NIOSH c r i t e r i a . T h i s accuracy r e s u l t s from a sampler which maintains true constant f l o w throughout the sampling p e r i o d . Acknowledgements The authors wish t o express t h e i r a p p r e c i a t i o n t o the many persons who have provided i n v a l u a b l e a s s i s t a n c e i n c l u d i n g : D. 0. Conn, I I I , f o r e a r l y guidance; V. W. Keedy, D. N. Mount and C. J . Lundgren f o r recent help i n design and t e s t i n g ; and A. S. P o l l a c k f o r e d i t o r i a l a s s i s t a n c e . Literature Cited 1. 2.

3.

4.

5. 6.

7.

The Industrial Environment - Its Evaluation and Control, NIOSH, 1973, p. 101. Taylor, D. G . ; Kupel, R. E.; Bryant, J. M. "Documentation of the NIOSH Validation Tests," NIOSH, Cincinnati, Publication No. 77-185, A p r i l 1977. Parker, C. D.; Lee, M. B . ; Sharpe, J. C. "An Evaluation of Personal Sampling Pumps in Sub-Zero Temperatures;" Research Triangle Institute Report for NIOSH. Contract No. 210-76-0124, September 1977. "Air Sampling Instruments for Evaluation of Atmospheric Contaminants;" American Conference of Governmental Industrial Hygienists, F i f t h E d . , Cincinnati, 1978. Moore, G . ; Steinle, S.; LeFebre, H. Am. Ind. Hyg. Assoc. J., 1977, 38, 195. Almich, B. P . ; Rubenzahl, Μ. Α . ; Carson, G. A. "Electronic Refinements for Improved Operation of Portable Industrial Hygiene for A i r Sampling Systems," NIOSH, Cincinnati, Publication No. 75-169; May 1975. Stearns, R. F.; Johnson, R. R.; Jackson, R. M . ; Larson, C. A. "Flow Measurements with Orifice Meters," D. Van Nostrand Company, Inc.; New York, 1951.


502

CHEMICAL HAZARDS IN THE WORKPLACE

8.

Nelson, G. O. "Controlled Test Atmospheres - Principles and Techniques," Ann Arbor Science Publishers, Inc.; Ann Arbor,

9.

Baker, W. C.; Pouchot, J. F. Flowmeter Survey for Measuring the Flow of Air and Other Gases; Teledyne Hastings - Raydist; Hampton, Va., March 15, 1979, 24-25.

1976.


RECEIVED October 17, 1980.


Industrial Hygiene Air Sampling with Constant ... - ACS Publications

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