CA1077740A - Chemical dosimeter having a constant flow air sampling pump - Google Patents

Abstract

Abstract of the Disclosure An improved chemical dosimeter for individual use which collects on a filter particles or vapors present in an air stream which is being pumped through the dosimeter at a constant rate, the improvement is the use of a variable drive pump that is connected to the filter and that is driven by an electric motor and is controlled by a feed back circuit of an integrator and an amplifier and maintains a constant flow of air through the dosimeter;
the integrator receives a signal from a pressure switch that detects changes in the flow of the air stream through the dosimeter by a change in a pressure drop of the air which is being pumped through an orifice;
the dosimeter is worn by an individual and at the termination of a period of time, such as a work day, the filter is removed and the collected contents are analyzed by conventional techniques such as gas chromatography to deter-mine a level of exposure of the individual using the dosimeter.

Description

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BACXGROUND OF THE INVENTION
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This invention relates to a dosimeter and in particular to a chemlcal doslmeter designed for indlvidual use that has a constant air stream flowing through the dosimeter.
Chemical dosimeters are known and have been used by individuals in an effort to determine the level of ex-posure of an individual to foreign substances in air, for example, to chemical vapors or fumes, dust particles and the llkeO The dosimeter is worn by the individual and air is pumped through a filter which traps foreign sub-stances in the air. At the end of an individual's exposure period, the filter is removed and analyzed for any foreign substances. The problem has been with these chemical dosimeters that the air flow rate through the dosimeter has not been accurately controlled. For example, if the filter was partially blockcd so that intake o~ air was momentarily stopped or reduced for a period of time, it was not possible to ad~ust and increase the flow rate of air to compensate for the stoppage or reduction of air passing through the filter of the dosimeter. An~ reduction in the air flow rate reduces the amount of foreign substances collected by the filter thereby giving an inaccurate level of exposure of the individual.

SUMMARY OF THE INVENTION

An improved chemical dosimeter for individual use that collects on a filter means particles or vapors present in an air stream which is pumped through the dosimeter at a c~nstant flow rate; the improvement that is used therewith comprlses ~.

- 2 -a variable drive pump tubularly connected to the filter means and coupled to an electric motor draws the air stream through the filter means and pumps the air stream into an air reservo~r;
the air reservoir connected to the pump .
retains excess ai~ supplied by the pump to maintain a constant flow rate of the air stream through an orifice that is connected to the ) air reservoir;
the orifice ls positioned in a tube attached to the reservoir and to an exhaust port, the alr stre~m is pumped through the orifice and thereby creates an air pressure drop;
a differential pressure switch positioned in a tube connected to the exhaust port and in parallel with the orifice is activated by a change in the air pressure drop of the air stream and creates a low voltage electrical input signal that is fed to an integrator circuit;
the integrator circuit electrically connected to a power source and to the pressure switch uses the low voltage input ~ignal generated by the pressure switch and integrates thls signal into a signal that is fed ~nto an amplifier c~rcuit;
the amplifier circuit electrically connected to a power source and connected in series to the integrator circuit and to the electric motor amp~ifies the signal . generated by the integrator circuit and feeds an O amplified slgnal to the electric motor and thereby changes ~L~777~0 the speed of the motor drlving the pump to maintain the air stream at a constant flow rate.

BRI~F DESCRIPTION OF THE DRAWINGS
_ FIG. 1 - is a block diagram of the chemical dosimeter.
FIG. 2 - is a schematic circuit diagram for one embodiment of the dosimeter.
FIG. 3 - is a schematic circuit diagram for one preferred embodiment of the dosimeter which contains a low air flow detector circuit and a batter~ test circuit.

DETAILED DESCRIPTION OF THE INVENTION

The chemical dosimeter is designed for individual use and is compact in size and is about 1 1/2 inches x 2 11/16 inches x 5 3/16 inches and is light in weight about 14 ounces. The dosim ter can be carried comfortably by a worker, for example, in a pocket, or a belt, in a neck band and the like, without inconvenience or hindrance to work activities. The dosimeter is of reasonable cost and is rugged in design and is well suited for service in industrial environments.
The che~ical dosimeter with its constant flow feature improves the accuracy with which a wide variet~ of environmental hazards to individuals can be monitored.
Monitoring for vin~l chloride vapors in industrial work areas and monitoring for toxic radon gas and toxic related products of radon gas in mines are t~pical of important applications of the dosimeter.

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Re~errlng to the block dlagr~m of FIG. lt abasic arrangement o~ the chemlcal dosimeter is shown. Air is pumped ln at the lntake 1 at a constant flow rate and pas6ed through a ~ilter 2. The air intake and fllter are connected by a tube to a varlable drive air pump 3 driven b~ an electric D.C. motor 90 The air is pum~ped to the reservoir _ which moderakes the flow level of the air and eliminateæ surge~ of air created by strokes o~ the pump.
An orifice 5 such as an adjustable needle valve ls positioned in tube leading to the exhaust port 7a and causes an air pres~ure drop. A pres~ure switch 6 i6 positioned in parallel to the ori~ice and is activated by any change in the air pre~sure drop. The pre~sure switch 6 ls electrically connected to the integrator circuik 7, which u~iliæes the input from the pressure ~witch and generates an electrical signal. The signal generated by the integrator 7 is fed to the ampli~ier circuit 8 which amplifies the signal ~nd the sig~al control~ the speed of the electric motor 9 driving the air pump 3 to provide a constant ~low rate of air through the doæimeter. The integrator and the amplifier are electrically connnected to a D.C. power source 11 which usually is a battery. An on-o~f ~witch 10 is posit-lonad be~ween the power source 11 and the amplifier and lntegrator circuitæ.
Configurations other than the above for the dosime~er can be used. For example, the orlfice can be tubularly connected in series to the filter a~d the pu~p.
The pump drawq an air stream through the orifice a~d through the filter. A~ a~ove, a pressure ~itch is in parallel relat1on~hip to the orl~ice and measure~ any change in an air pressure drop. In another example, a filter~

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orifice and reservoir are tubularly connected in series to a pump and the pump draws the air through the filter, orifice and reservoir. A pressure switch is positioned in parallel to the orifice to measure any change in an air pressure drop.

The filter 2 of the dosimeter can be adapted to entrap almost any type of substance such as gases, liqulds or solids. If mechani~al filtration is only required, for example, to collect dust particles to which a worker is exposed, a filter is provided which will entrap particles of 0.01 microns or larger. If the filter is to entrap a gas such as sulfur dioxide, a chemlcal filter is used which will entrap this gas. If vapors are to be entrapped, then a filter such as a charcoal filter, is used which entraps vapors. At the end of tihe period which an individual is wearing the dosimeter, such as an 8 hour work day, the filter is removed and examined for the substance or sub-stances to which the individual was exposed. A simple count of particles under a microscope may be used or the filter can be anal~zed, for example,with a gas chromato~raph.
A variable drive air pump is used in the dosimeter.
GenerallyJ a diaphragm type pump is used that pumps from about 10 to 3000 cubic centimeters per minute~ Other pumps such as piston pumps9 rotary pumps and centrifugal pumps can also be used~
The pump is electrically connected to a conven-tional D.C. motor of about 0.0001-0.02 horsepower~ The motor is a variable speed motor and operates from about 1,000 to 20,000 revolutions per minute. Under some cir-.~rJ cumstances, a reducing ~ear can be used between the motor and the pump.

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The reservoir is usually an integral part ofany framewor~ on which the various components used in the dosimeter are mounted and is milled or cut into the framework with appropriate openings. Part of the reservoir may be enclosed with a thin sheet of an elastomer so that any pulsat~ons of the air stream created by the pump can be readily dampened by the elastomer absorbinp the pulsation.

The purpose of the reservoir is to smooth any pulsations of the air stream created by the strokes of the pump at least to some degree before the air stream passes through the orifice. Without the reservoir, a uniform flow ratebfthe air stream cannot be providedO me volume of the reservolr is as small as possible but of sufficient volume to reduce the pulsations of the air stream.
A typlcal reservoir used with a pump that pumps air at about 25 to 200 cubic centimeters per minute is about 1/8 inch by 1 1/2 inches x 3/4 inch and is covered with an elastomer about 3/4 inch x 1 1/2 inches.
An orifice such as an adJustable needle valve is positioned in a tube connecting the reservoir to the exhaust port. An orifice is used that creates a pressure drop of about 0.4-4.0 inches of water. Usually a pressure drop of 2,5-3.5 inches of water is used.
A dif~crcntial prcssurc switch of a relativcl~
high level of sensitivity is used and is sensitive to a pressure drop change in the air stream of about 0.1-0.5 inches of water.
The integrator circuit takes the on-off signal penerated b~ the pressure switch and formulates a slowly ~0 changin~ continuous signal therefrom which is fed into . . ~.

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the amplifier circuit. The integrator circuit is biased at about +C.6 volts and the signal from the switch increases to about 1.2 ~olts when the pressure switch is activated and decreases to about -~0.0 volts when the switch is deactivated. The integrator circuit produces a gradually decreasing output voltage which ~eeds i~to the amplifier when the pressure switch is closed and a gradually increasing voltage when the pressure switch is open. The circuit is constructed of conventional transistors, cap-acitors and resistors. Examples of the circult will be described hereinafter.
The amplifier circuit receives the signal generated by the integrator circuit and amplifies the signal so that the electric D.C. motor can be controlled at variousspeeds to insure a constant flow rate of the air stream through the dosimeter. The amplifier circuit ampli~
fies the signal from the integrator to a maximum of about 96~ of the total voltage of the power source. For example, for a 5 volt power sourceJ the signal will be ampli~ied to 4.8 volts. Generally, the amplifier has an impedance of greated than 10 ohms and up to l megohm. However, an amplifier with an impedance of less than lO ohms can be used, e.g., 0.01-lO ohms impedance~ The amplifier is constructed by conventional transistors, capacitors and resistors.
The power source usually is a battery of about 5-6 volts. Generally, a nickel cadmium batterY of 4 cells is used. A direct current power source of rectl~ied A.C.
current can also be used.

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One optional circuit that can be used in the dosimeter is a battery te~t c~rcuit. The circuit uses a precision voltage detector which can be ad~usted to the voltage sf each cell ~nd is set to be activated at ~he full charge voltage of the battery. A llght emltting diode which is activated by a ~witch i~ usu~lly used to indicate a full charge of the bat,tery.
Ano~her optional circuit that can be used in the dosimeter is a low air ilow detector circuit which is connected to the integrator circuit and 18 activated when the vsltage output of the integrator circuit is at hieher than normal operational levels caused by an interruption of the air ~tream being pumped through the dosimeter. The low flow ~etector circuit comprises a bistable multivi-brator circuit electrically connected to an indicator light such as a light emitting diode.
Reierring to the circuit diagram of FXG. 2, battery B 1 which æupplies powcr to the circuit has its negative (-) terminal connected to COMMO~ and its positive ~) terminal connected to power switch ~W 1~ The other side of SW 1 is connected to the positive (~ BUS.
A~plifier ~ 1 (which may be operational amplifier such as one of the ~our ~mpllfiers ln a type ~M 324 Quad Operational Amplifier) is connected ln an integrating configuration with a ~eedback capacltor C 1 (typically 10 microfarads). C 1 is connected ~rom the output to the invert~ng (-3 input of the a~plifier A 1. The input resistor R 3 (typically 1 megohm) is connected to the inverting i~put of A 1. me values of R 3 and C 1 deter~ine the integration rate and a~ect the response of the control circuit. The values are selected to give the best control with a particular pump and accumulator.

~' ~C~777~0 Resistor R 1 (typically 10 K ohms) ls connected from the ~ BUS to one anode of diode, CR 1 (typically type lN4148) and the cathode of CR 1 is connected to the anode of diode CR 2 (typically type lN 4148) ~hich has the cathode connected to COMMON. This provides bias voltages of approximately o.6 volt at the CR 2 anode and 1.2 volts at the CR 1 anode due to the forward voltage drops of the two diodes. The o.6 volt point is connected to the non inverting input (+) of the amplifier9 A 1, to bias the +
input at o .6 volts above COMMON, through a resistor R 4 (typically 1 megohm) which minimizes amplifier offset voltage effects. A resistor R 2 (typically 10 K ohm) is connected from the input resistor R 3 to COMMON or ground.
This provides 0.0 volts to the input reslstor when pressure switch SW 2 is open. SW 2 typically is a pressure switch that operates at 3.0 inches of water pressure. The inte-grator produces a gradually decreasing voltage at the amplifier output when SW 2 is closed and a gradually increasing voltage when SW 2 is open. The voltage at the amplifier A 1 output is a motor speed signal which when amplified by an amplifier (described hereinafter) determines the pump motor speed. Connectlon from the ~ BUS
and COMMON are made to Al to provide power. These connec-tions provide power for A 2 and A 3.
The motor speed signal is applied to amplifier A 2 (typically 1/4 of a type LM 324) through resistor R 5, (typically 2~2 K ohm) to the non-inverting (~) input of A 2. The amplified signal ~rom the output of A 2 is applied to the base of transitor Q 1 (typically an NPN
type 2N2926) through resistor R 8 (typically 10 K ohm).

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The signal from the collector of Q 1 is applied to the base of transistor Q 2 (typically a PNP Type 2N5226) through resistor R 10 (typically 1 K ohm). The output signal from the collector of ~ 2 is connected to the pump motor M 1, a variable speed, direct current motor. The other side of M 1 is connected to COMMON.
The emitter of Q 1 is connected to COMMON through resistor R 11 ~typically 220 ohm). Capacitor C ~9 (typically 0.01 microfarad) is connected from base to collector Q 1 to reduce noise in the circuit. The emitter of Q 2 is connected to the + BUS and the base is connected to the + BUS through resistor R 9 (typically a 1 K ohm).
A feedback resistor R 7 (typically 47 K ohm) ls connected from the collector of Q 2 to the inverting (-) input of A 2 to provide negative feedback. The inverting input of A 2 is connected to COMMON through resistor R 6 (typically 2.2 K ohm).
Resistors R 6 and R 7 determine the overall voltage gain of the circuit from the output of A 1 to the 2~ voltage connected to the pump motor. R 7 may be ad~usted to provide the optimum balance between fast control response and stable operation in pumps of various characteristics. Capacitor C 2 (typically 0.01 microfarad) is connected from the output of A 2 to the inverting input of A 2 to reduce circuit noise. This connection of A 2, Q 1, Q 2 and their associated resistor and capacitors is one of many amplifier circuits suitable for amplifying the motor speed signal from A 7 but this circuit provides a wide voltage range to the motor, typically O to ~() 4.8 volts for a power suppl~ of 5.0 volt~, and provides a o constant voltage output preferred in some pump configurations such as where very low motor speed for low flow is required.
A battery check circuit is built based on a special light emitting diodeJ D 1, (typically type HP 5082-4~32 manufactured by the Hewlett-Packard Corporation) which lights at a specific level of applied voltage (typically 2.4 volts). Amplifier A 3 (typically 1/4 of a type LM 324) has its inverting input (-) connected to the output providing a 1 X gain ~or signals applied to non-inverting input (+). The output of A 3 is connected to the anode (or ~ input) of D 1 and the cathode of D 1 is connected to one side of switch SW 3. 'me other side of SW 3 is connected to COMMON. D 1 will light if SW 3 ls closed and the output o~ A 3 is greater than a trigger voltage (typically 2,4 volts). Resistor R 12 (typically 100 K ohm~ is connected from the + BUS to one side of variable resistor, R 1~
(typically a 50 K ohm potentiometer). Resistor R 14 (typically 100 K ohm) is connected from the other side of R 13 to COMMON. The wlper of R 13 is ad~ustable to present 2.4 volts to the noninverting input of A 3 at the desired ~attery voltage check level, typically 5.15 volts for a battery constructed by connecting four nickel-cadmium rechargeable cells in series.
FIG. 3 is anothcr vcrsion of thc prcvious circuit in which A 4 is like A 1, R 15 is like Rl, R16 is like R 2, R 17 is like R 3, CR 3 is like CR 1, CR 4 ls like CR 2, SW 4 is like SW 1, SW 5 is like SW 2, B 2 is like Bl and C 5 is like C 1. A 4 is connected as A 1 of the previous - ~2 -~777~Q

circuit, except that the resistor in the non-inverting input is eliminated since it is not required when the amplifier offset voltage is low enough to have no effect on the integrator and except that capacitor C 4 (t~pically 0.5 microfarad) has been added across or in parallel with R 17 to provide faster response and more stablle operation with certain pumps.
The output of A 4 is amplified by the transitor, Q 3, (typically a type 2N3053) whose base is connected to L0 the output of A 4 via resistor R 18 (typically 2.2 K ohms)~
whose emitter is connected to CO~MON and whose collector is connected to the pump motor, M 2. The positive (+) lead of M2 is connected to the + BUS. This power amplif:Ler is a less complex circuit than in FIG. 2 but provides the same 0-4.8;voltage range ~or the motor. The output signal o~ this circuit has a constant current characteristic whlch provides good operation with most pump configurations.

The output s gnal from A 4 varies from about 0 to 0.75 volts during normal control but can lncrease ' gradually on up to a saturation level of approximately 3 volts (for a power supply voltage of 4.0 volts) when the pump cannot maintain the airflow such as when the inlet tube is kinked and the airflow is blocked. To detect when the output of A 4 exceeds 2.5 volts, a low flow detector is provided. mus, amplifier A 5 (typically 1/4 of a LM 324) is connected at its inverting input to a trip voltag~ lcvcl. If a voltage of a grcater magnitud~
than the trip voltage level is applied to the noninvertlng (+) input of A 5, the output of A 5 will change from the normal level of zero to a high level of approximately 4 volts (with a 5 volt power supply).

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Resistor R 20 ~typically 47 K ohm) is conne^ted from the ~ BUS to resistor R 21 (typically 22 X ohm). The other side of R 21 is connected to COMMON. Tlle ~unction between R 20 and R 21 is connected to the inverting(-) input of A 5.
Resistor R 23 (typically 10 K ohm) and diode CR 6 (typically a type IN 4148 are connected in series and feed the voltage from the A 5 output to the noninverting input to keep the A 5 output high even if the original voltage signal ic removed. Resistor R 24 (typically 270 ohm);
light emitting diode, D 2 (typically a HP 5082-4484); and a momentary test switch SW 6 are series connected from the output of A 5 to COMMON. When SW 6 is closed with the output of A 5 high, D 2 will light. Amplifier A 5 may be reset to the low output condition by opening switch SW 4 to remove power from the circuit. Resistor R 22 (typically 1 megohm) i9 connected from the noninverting input of A 5 to COMMON to assure that A 5 does not inadvertantly go to the high output condition when power is first applied to cC the circuit. The anode of diode CR 5 (t~pically a type IN ~148) is connccted from thc output of A 4 to resistor R 19 (typically 100 K ohm) which is in turn connected to the non-inverting input of A 5 coupling the signal from A 4 into the low flow detector circuit. The forward voltage drop of CR 5 helps prevent spurious signals from falsely tripping the low flow detector. In this configuration, the circuit normally requires 45 seconds after flow is interrupted until the circuit trips. This tlme can be decreased by increasing the ratio of R 20 to R 21.
~,) The battery check circuit o~ FIG. 3 incorporates - 14 _ ~L~777~

a network of NPN silicone transistors, Q 4 - Q ~ to provide a bias voltage that is stable with temperature changes. Resistor R 25 is connected from the ~ BUS to point A. Resistor R 26 is connected from point A to the ~unction of the base of Q 4, the collector of Q 4, and the base of Q 5. The emitters of Q 4 and Q 6 are connected to COMMON and the emitter of Q 5 is connected to COMMON through resistor R 28. Resistor R 27 is connected from point A to the Junction of the collector of Q 5 and the base of Q 6. The collector of Q 6 is connected to point A. The resistors R 25 through R 28 are selected to give the best temperature stability o~
the voltage at point A. The voltage at point A typically is 1.5 volts. Resistor R 29 is co~nnected from -~ BUS to the inverting input of A 6. Resistor R 30 is connected ~rom the non-inverting input of A 6 to COMMON. The values of R 29 and R 30 are chosen to suit the desired battery test voltage. Resistor R 31 is connected from the output of A 6 to light emitting diode D 3. The other side of D 3 is connected to SW 6 similar to D 2. The other side of SW 6 is connected to COMMON.
In practical operation of the chemical dosimeter~
a worker is given ~he dosimeter to wear for an 8 hour work shif~, At the cnd of th~ shif~, the circuit l.s tested to determine if th~ intake waE blocked during the period by observing the light emitting diode (D 2 of FIG. 2) while pressing the momentary switch ~SW ~ of FIG 3). If the diode lights, blockage has taken place during the shift.
The filter is then removed from the dosimeter and sent to a laboratory for analysis and the results are recorded in ~ 77~0 .
- the workers' files. If there is excessive exposure, the worker can be withdrawn from the particular area and given ; another ~ob.
It is practical to maintain a chemical dosimeter bank from which each worker draws his own dosimeter at the beginning of his work shift and is returned at the end of the shift.
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It may be preferred to monitor only one worker of a given group and assume that the entire group has received the same exposure. If de~ired, individual dosimeters can be statically mounted in specific work areas and individual exposure can be approxlmated according to the time spent by the worker in a particular area.
The constant flow pump can also be used to fill .
a sample collection bag connected to the exhaust of the pump. This would provide a sample representative of the average gas present during the sampling period.
; Under some circumstances, it may be practical to encapsulate the entire electrical circuit used in the dosimeter into a compact module using conventional compounds.
This would provide a long service life under demanding en-vironmental conditions and increase reliability in monitoring . with a large number of-dosimeters.
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Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An improved dosimeter for individual use that has an electric motor, a power source, an exhaust port and a filter means in which particles or vapors present in an air stream being pumped through the dosimeter are collected on the filter means; the improvement in use therewith comprises a variable drive pump tubularly connected to the filter means and coupled to the electric motor draws the air stream through the filter means;
an air reservoir connected to the pump into which the air stream is being pumped retains excess air sup-plied by the pump and maintains a constant flow rate of the air stream;
an orifice comprising an adjustable needle valve being positioned in a tube attached to the air reservoir and to the exhaust port, wherein the air stream is pumped through the orifice and thereby creates an air pressure drop;
a differential pressure switch positioned in a tube connected to the exhaust port and in parallel to the orifice is activated by a change in the air pressure drop of the air stream and creates a low voltage elec-trical input signal;
an integrator circuit electrically connected to a power source and to the pressure switch uses the low voltage input signal generated by the pressure switch and integrates this signal;
an amplifier circuit electrically connected to the power source and connected in series to the integrator circuit and to the electric motor which amplifies the signal generated by the integrator circuit and feeds the amplified signal to the electric motor, thereby controlling the speed of the motor driving the pump in relationship to the signal generated by the pressure switch to maintain the air stream at a constant flow rate.

2. The improved dosimeter of claim 1 in which the variable drive pump is a diaphragm pump.

3. The improved dosimeter of claim 1 in which the pressure switch is activated by an air pressure drop of 3 inches of water and an air pressure drop change of 0.1 to 0.5 inch of water.

4. The improved dosimeter of claim 1 in which the integrator circuit is biased at about +0.6 volts and the sig-nal from the integrator gradually increases to about +1.2 volts when the pressure switch is activated and gradually decreases to +0.6 volts when the switch is deactivated.

5. The improved dosimeter of claim 1 in which the amplifier circuit amplifies the signal from the integrator circuit to a maximum of about 96% of the total voltage of the power source and has an impedance of greater than 10 ohms.

6. The improved dosimeter of claim 1 in which the amplifier circuit amplifies the signal from the integrator circuit to a maximum of about 96% of the total voltage of the power source and has an impedance of less than 10 ohms.

7. The improved dosimeter of claim 1 which has electrically attached thereto a low air flow detector circuit comprising a bistable multivibrator circuit electrically con-nected to an indicator light.

8. The improved dosimeter of claim 1 which has electrically attached thereto a battery test circuit compris-ing a precision voltage detector adjusted to the voltage of each cell of the battery.

9. The improved dosimeter of claim 1 in which the variable drive pump is a diaphragm pump;
the orifice is an adjustable needle valve;
the air pressure switch is activated by an air pressure drop of about 3 inches of water and by an air pressure drop change of about 0.1 to 0.5 inch of water;
the integrator circuit is biased about +0.6 volts and the signal from the circuit gradually increases to about +1.2 volts when the pressure witch is activated and gradually decreases to +0.6 volts when the pressure switch is deactivated;
the amplifier circuit amplifies the signal from the integrator circuit to a maximum of about 96% of the total voltage of the power source and has impedance less than 10 ohms;
the power source is a battery that has a maximum of 5.5 volts and has nickel cadmium cells; and has electrically connected thereto a low flow air detector circuit comprising a bi-stable multivibrator circuit electrically connected to an indicator light; and a battery test indicator circuit comprising a pre-cision voltage detector adjusted to 5.2 volts.

10. A constant air flow sampling mechanism having an air inlet, an electric motor, a power source and an exhaust port which comprises a variable drive pump tubularly connected to the air inlet and coupled to an electric motor draws an air stream in through the inlet;
an air reservoir connected to the pump into which an air stream is being pumped retains excess air supplied by the pump and maintains constant flow rate of the air stream;

an orifice comprising an adjustable needle valve being positioned in a tube attached to the air reservoir and the exhaust port, wherein the air stream is pumped through the orifice and thereby creates an air pressure drop;
a differential pressure switch positioned in a tube connected to the exhaust port and in parallel to orifice is activated by a change in the air pressure drop of the air stream and creates a low voltage electrical input signal;
an integrator circuit electrically connected to a power source and to the pressure switch uses the low voltage input signal generated by the pressure switch and integrates this signal;
an amplifier circuit electrically connected to the power source and connected in series to the integrator circuit and to the electric motor which amplifies the signal generated by the integrator circuit and feeds an amplified signal to the electric motor, thereby control-ling the speed of the motor driving the pump in relation-ship to the signal generated by the pressure switch to maintain the air stream at a constant flow rate.