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An inexpensive sequential sampler for gas tracer studies
Armospheric Environmenr Vol. 13, pp. 341-342. Pergamon Press Ltd. 1979 Punted I” Great Britain.
TECHNICAL
NOTE
AN INEXPENSIVE SEQUENTIAL SAMPLER GAS TRACER STUDIES (First
received 4 May 1978 and
infinalform
8 September
FOR
1978)
Abstract - A simple, inexpensive automatic air sampler capable of taking five consecutive samples over an adjustable sampling period is described. The main application for the sampler is in the field of gas tracer studies. A feature of the unit is that the sampling period can be set precisely thus enabling concentration (rather than dosage) to be measured directly.
1. INTRODUCTION
2. DESCRIPTION
In conducting atmospheric diffusion and tracer experiments, it is generally necessary to obtain air samples from a large number of locations (twenty such locations may be a minimum). Furthermore, the relatively short period of release of tracer gas (typically less than one hour) and the possible uncertainty in the time of travel of tracer to a given location, make it desirable to take a number of consecutive samples at each location to ensure that the tracer is sampled. If this is not to involve excessive manpower requirements, automated samplers are necessary. However, with the number of sampiers required, the cost of each unit is a prime constraint. This note describes a simple, inexpensive, automatic sampler capable of taking five consecutive samples over an adjustable sampling period. The time of sampling can be preset within close limits thus allowing concentration to be measured directly. [In most diffusion experiments concentration is estimated from a measured dosage and assumed sampling period (Islitzer and Slade, 1968 ; Start et al., 1975)]. This particular sampler has been designed for use in SF, tracer experiments, but minor modifications (for example use of different sample bags) would make it suitable for sampling other gases. The total cost per unit is SASO.
The sampler includes a timer which switches on in turn five battery operated pumps each of which inflates a separate plastic bag. The timer is initiated by the alarm contacts on a small battery clock (Smiths Industries No. C7498WO34). Sampling can thus be delayed by up to 12 h after deployment of the samplers. A schematic of the unit is shown in Fig. 1. Completely remote initiation of sampling by, for example, radio control, is possible but expensive and potentially unreliable. 2.1 Timer The timer circuit uses two CMOS integrated circuits, an oscillator/divider (type 4060) used as a timer and a decade counter/divider (type 4017) used as a sequential switch. It operates as follows. When the clock alarm contacts close, the reset pins of the timer (pin 12) and sequential switch (pin 15) are grounded (see Fig. 1) thus initiating the cycle. The RC oscillator built into the timer and controlled by the component values connected to pins 9, 10, 11 of the timer, is activated and its frequency is divided by 16384 before passing to the sequential switch via pin 3. Each pump is activated in turn as the appropriate pin of the sequential switch is
Fig. 1. Schematic of sampler circuitry.
342
Technical
Note
switched high by the output from the timer. The diodes ensure that during pump operation the reset pins continue to be held low even if the alarm contacts reopen. Up to nine motors may be switched in turn, but in practice only five are connected. When the last motor is deactivated, sampling is terminated by one of two circumstances. If the alarm contacts are open the timer switches off. However, if the alarm contacts are still closed the “enable” pin of the sequential switch (pin 13) is held high by pin 5 thus deactivating the switch until the alarm contacts open or the timer is turned off manually. The sampling period is of course the same for each pump. It may be adjusted by varying the frequency of the astable oscillator in the CMOS timer, and can easily be set to better than OS”,. The clock alarm setting mechanism locates positively and reproducibly at approx.2.min intervals. By rotating thealarm hand relative to its shaft. the alarm settings can be synchronized for all clocks. Uncertainty in setting the time results in a variation of up to 15s in the start of the sampling cycle on a given alarm setting. Thus the largest discrepancy between the start times for corresponding samples of two separate samplers is I min for a 20 min sampling period. 2.2 Pumps Each sampling unit contains five diaphragm pump/motor sets. Inexpensive battery powered aquarium pumps are readily available and have proved to be suitable with minor modification. In order to accommodate different sampling periods it is necessary to make the air flow rate adjustable. This was done by removing the eccentric part of the shaft of the aquarium pump motor and replacing it by a pair of metal discs (see Fig. 2) one of which is firmly attached to the motor shaft, while the other supports a second shaft through its centre to drive the diaphragm. A screw clamps the two discs near their edges, thus allowing an adjustable offset between the motor shaft and the driving shaft. The flow rate can be adjusted from zero to about a litre per minute. However, adjustment of the flow-rate is not critical ; the requirement being to collect enough air for analysis without filling the sample bag. So long as the bag is not pumped tight, the load on the pump and hence the sampling rate, remain constant. When driven by a 1.5 V cell the pump motors draw about 0.5 A, so a single alkaline “D” cell can be used to sample for up to 4 h. For longer sampling times a sealed lead-acid “X” cell can be used. 2.3 Sampling hugs Each pump inflates a 5 I. “wine-cask” plastic bag. These bags are a triple laminate of polyethylene (inner), nylon (middle) and polyvinylidene chloride (outer). They are designed to resist diffusion of air and so they also retain SF, very well. SF, samples show no detectable change when stored for over a week in the bags. Laboratory tests have shown that SF, samples can be transferred from one bag to another without loss, thus indicating that adsorption is not a problem. Other types of bag can suffer from losses ofup to 10s; per day (Glossop and Hamilton, 1977).
Fig. 3. Crosswind distribution of SF, 1 km downwind release point. 0 measured points; --- fitted Gaussian.
of
2.4 Physical dimensions Total weight of the timer unit (including protective case, batteries and alarm clock) is 1.5 kg. Overall dimensions and weight of the sampler units depend on the size of the sampling bags. The unit described here is housed in a weather-proof box 60 x 50 x 45cm. Total weight is 13 kg.
3. FIELD TRIALS
Twenty sampler units have been used successfully in initial SF, tracer experiments. They are easily transported and deployed, and the only adjustment required in the field is the setting of the alarm clock. Figure 3 shows a typical concentration profile obtained from sampler units deployed across an SF, plume 1 km away from the release point. Acknowledgements - Useful discussions with, and detailed electronics design by, Mr. R. H. Hill, and the assistance of Mr. C. Elsum have been central to the success of this work. CSIRO Division of Atmospheric Physics, Station Street, Aspendale 3195, .4ustmlia.
B. L. SAWFORD P. C. MAYINS
diaphragm shaft
drw motor
Fig. 2. Modifications
to aquarium
shaft
REFERENCES
shaft
pump drive shaft.
Glossop L. G. and Hamilton B. H. (1977) Unpublished report of the Western Australian Department of Conservation and Environment. Islitzer N. F. and Slade D. H. (1968) Meteorology and Atomic Energy (Edited by D. H. Slade). Chap. 4, U.S. Atomic Energy Commission TID-24190. Start G. E., Dickson C. R. and Wendell L. L. (1975), J. appi. Met. 14, 333-346.
TECHNICAL
NOTE
AN INEXPENSIVE SEQUENTIAL SAMPLER GAS TRACER STUDIES (First
received 4 May 1978 and
infinalform
8 September
FOR
1978)
Abstract - A simple, inexpensive automatic air sampler capable of taking five consecutive samples over an adjustable sampling period is described. The main application for the sampler is in the field of gas tracer studies. A feature of the unit is that the sampling period can be set precisely thus enabling concentration (rather than dosage) to be measured directly.
1. INTRODUCTION
2. DESCRIPTION
In conducting atmospheric diffusion and tracer experiments, it is generally necessary to obtain air samples from a large number of locations (twenty such locations may be a minimum). Furthermore, the relatively short period of release of tracer gas (typically less than one hour) and the possible uncertainty in the time of travel of tracer to a given location, make it desirable to take a number of consecutive samples at each location to ensure that the tracer is sampled. If this is not to involve excessive manpower requirements, automated samplers are necessary. However, with the number of sampiers required, the cost of each unit is a prime constraint. This note describes a simple, inexpensive, automatic sampler capable of taking five consecutive samples over an adjustable sampling period. The time of sampling can be preset within close limits thus allowing concentration to be measured directly. [In most diffusion experiments concentration is estimated from a measured dosage and assumed sampling period (Islitzer and Slade, 1968 ; Start et al., 1975)]. This particular sampler has been designed for use in SF, tracer experiments, but minor modifications (for example use of different sample bags) would make it suitable for sampling other gases. The total cost per unit is SASO.
The sampler includes a timer which switches on in turn five battery operated pumps each of which inflates a separate plastic bag. The timer is initiated by the alarm contacts on a small battery clock (Smiths Industries No. C7498WO34). Sampling can thus be delayed by up to 12 h after deployment of the samplers. A schematic of the unit is shown in Fig. 1. Completely remote initiation of sampling by, for example, radio control, is possible but expensive and potentially unreliable. 2.1 Timer The timer circuit uses two CMOS integrated circuits, an oscillator/divider (type 4060) used as a timer and a decade counter/divider (type 4017) used as a sequential switch. It operates as follows. When the clock alarm contacts close, the reset pins of the timer (pin 12) and sequential switch (pin 15) are grounded (see Fig. 1) thus initiating the cycle. The RC oscillator built into the timer and controlled by the component values connected to pins 9, 10, 11 of the timer, is activated and its frequency is divided by 16384 before passing to the sequential switch via pin 3. Each pump is activated in turn as the appropriate pin of the sequential switch is
Fig. 1. Schematic of sampler circuitry.
342
Technical
Note
switched high by the output from the timer. The diodes ensure that during pump operation the reset pins continue to be held low even if the alarm contacts reopen. Up to nine motors may be switched in turn, but in practice only five are connected. When the last motor is deactivated, sampling is terminated by one of two circumstances. If the alarm contacts are open the timer switches off. However, if the alarm contacts are still closed the “enable” pin of the sequential switch (pin 13) is held high by pin 5 thus deactivating the switch until the alarm contacts open or the timer is turned off manually. The sampling period is of course the same for each pump. It may be adjusted by varying the frequency of the astable oscillator in the CMOS timer, and can easily be set to better than OS”,. The clock alarm setting mechanism locates positively and reproducibly at approx.2.min intervals. By rotating thealarm hand relative to its shaft. the alarm settings can be synchronized for all clocks. Uncertainty in setting the time results in a variation of up to 15s in the start of the sampling cycle on a given alarm setting. Thus the largest discrepancy between the start times for corresponding samples of two separate samplers is I min for a 20 min sampling period. 2.2 Pumps Each sampling unit contains five diaphragm pump/motor sets. Inexpensive battery powered aquarium pumps are readily available and have proved to be suitable with minor modification. In order to accommodate different sampling periods it is necessary to make the air flow rate adjustable. This was done by removing the eccentric part of the shaft of the aquarium pump motor and replacing it by a pair of metal discs (see Fig. 2) one of which is firmly attached to the motor shaft, while the other supports a second shaft through its centre to drive the diaphragm. A screw clamps the two discs near their edges, thus allowing an adjustable offset between the motor shaft and the driving shaft. The flow rate can be adjusted from zero to about a litre per minute. However, adjustment of the flow-rate is not critical ; the requirement being to collect enough air for analysis without filling the sample bag. So long as the bag is not pumped tight, the load on the pump and hence the sampling rate, remain constant. When driven by a 1.5 V cell the pump motors draw about 0.5 A, so a single alkaline “D” cell can be used to sample for up to 4 h. For longer sampling times a sealed lead-acid “X” cell can be used. 2.3 Sampling hugs Each pump inflates a 5 I. “wine-cask” plastic bag. These bags are a triple laminate of polyethylene (inner), nylon (middle) and polyvinylidene chloride (outer). They are designed to resist diffusion of air and so they also retain SF, very well. SF, samples show no detectable change when stored for over a week in the bags. Laboratory tests have shown that SF, samples can be transferred from one bag to another without loss, thus indicating that adsorption is not a problem. Other types of bag can suffer from losses ofup to 10s; per day (Glossop and Hamilton, 1977).
Fig. 3. Crosswind distribution of SF, 1 km downwind release point. 0 measured points; --- fitted Gaussian.
of
2.4 Physical dimensions Total weight of the timer unit (including protective case, batteries and alarm clock) is 1.5 kg. Overall dimensions and weight of the sampler units depend on the size of the sampling bags. The unit described here is housed in a weather-proof box 60 x 50 x 45cm. Total weight is 13 kg.
3. FIELD TRIALS
Twenty sampler units have been used successfully in initial SF, tracer experiments. They are easily transported and deployed, and the only adjustment required in the field is the setting of the alarm clock. Figure 3 shows a typical concentration profile obtained from sampler units deployed across an SF, plume 1 km away from the release point. Acknowledgements - Useful discussions with, and detailed electronics design by, Mr. R. H. Hill, and the assistance of Mr. C. Elsum have been central to the success of this work. CSIRO Division of Atmospheric Physics, Station Street, Aspendale 3195, .4ustmlia.
B. L. SAWFORD P. C. MAYINS
diaphragm shaft
drw motor
Fig. 2. Modifications
to aquarium
shaft
REFERENCES
shaft
pump drive shaft.
Glossop L. G. and Hamilton B. H. (1977) Unpublished report of the Western Australian Department of Conservation and Environment. Islitzer N. F. and Slade D. H. (1968) Meteorology and Atomic Energy (Edited by D. H. Slade). Chap. 4, U.S. Atomic Energy Commission TID-24190. Start G. E., Dickson C. R. and Wendell L. L. (1975), J. appi. Met. 14, 333-346.