Development of Sampling System for Investigations on Pollutant Transport

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DEVELOPMENT OF SAMPLING SYSTEM FOR

INVESTIGATIONS ON POLLUTANT TRANSPORT

Mrton BalczBudapest University of Technology and Economics, Department. of Fluid Mechanics

Zoltn SzucsnBudapest University of Technology and Economics, Department. of Fluid Mechanics

Gbor KalmrBudapest University of Technology and Economics, Department. of Fluid Mechanics

Istvn GoricsnBudapest University of Technology and Economics, Department. of Fluid Mechanics

ABSTRACT

In case of wind tunnel modelling of pollutant transport in the atmospheric boundary

layer tracer gas is used as pollutant. Observance of modelling and similarity laws provide

that the tracer gas concentration obtained from wind tunnel modelling is proportional to the

real pollution concentration. Simultaneous sampling in the different points of the modelled

area and thereafter the storage of samples are necessary, while the samples pass the gasdetector serially and the concentration of each sample is determined. The sampling is called

representative on condition the local wind speed and the sampling velocity are equal

(isokinetic sampling). However the local wind speed is varied from point to point thereby the

sampling speed has to be varied simultaneously. At the Department of Fluid Mechanics a

multi-channel automatic sampling system has been developed which is appropriate for

simultaneous and isokinetic sampling.

This paper presents the key components, operation and the results of preliminary tests of the

system.

Gas samples are stored in pneumatic cylinders actuated by linear stepper motors. The way of

gas samples is controlled by valves. The concentration of samples is measured by gas

chromatograph equipped with a flame ionization detector (FID). The whole measurement

system is controlled by a Labview program running on PC.

KEYWORDS

Pollutant transport modelling, PC controlled sampling system, isokinetic sampling

1 INTRODUCTIONThe micro scale dispersion investigations have growing importance especially in

cities. The widely used mathematical modelling of pollutant transport requires validation.

Beside the field tests where some unpredictable factors - for example uncontrollable and

fluctuating wind and background concentration during the measurements, etc. - causeuncertainties, the physical modelling is the next possibility. In case of wind tunnel


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investigations controllable parameters can be used. The approaching wind, the background

concentration, the tracer release can be kept constant or can be measured. Naturally the

reliability of results is based on the quality of modelling. This latter is ensured by

conformance of modelling laws. A VDI Guideline on the wind tunnel modelling of

atmospheric flows [1] collects these requirements.

2 EXPERIMENTAL SETUP

Figure 1 shows a typical measurement arrangement. The tracer gas, usually methane

(CH4) or ethane (C2H6) which substitutes the real pollutant, is released by digital mass flow

controllers. After eventual mixing with air, the tracer is led to the sources. The distribution of

tracer gas over the modelled area has to be similar to the pollution distribution over the real

terrain, so special point, line or area sources are built in the model.

Figure 1. Experimental setup

Sampling of polluted air happens by suction through plastic tubes from the measurement

points and has to fulfil the following requirements:

a) Simultaneous samplingb) Isokinetic sampling

The first requirement means that samples from all measurement points have to be taken at the

same time. The second requirement states that the sampling velocity has to be equal to the

local wind velocity. Due to various local wind speeds the suction must be set to different flow

rates. The new mchannel sampler unit developed at the Department of Fluid Mechanics fulfils

both requirements. After sampling, the unit puts the samples serially into the detector unit.

Details of these units are presented in the next paragraphs.


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3 THE SAMPLING AND MEASURING SYSTEM

The multi channel sampler equipment has been built of pneumatic elements. Figure 2

shows the sketch of the system. Each sample channel consists of a 100ml pneumatic cylinder

actuated by a computer controlled linear stepper motor. The sampling velocity is determined

by the velocity of the piston. Since each cylinder possesses an own step motor and every step

motor is individually controlled, the velocity of the piston thereby the sampling velocity canbe varied. Two electromagnetic valves have been coupled up the cylinder. All channels are

connected to a mechanical multiplexer, an earlier 48-port pressure scanning valve. Depending

on the valve positions each piston can:

a) Suck sample from the wind tunnel model into the cylinder

b) Put sample from the cylinder off the unit (exhaust)

c) Put sample from the cylinder through the mechanical multiplexer towards the

detector unit

Figure 2. The sampling system

The sampling process is the following (* - position does not matter):

Channel Action Purpose Position

valve 1 valve 2 multiplexer

Cleaning steps

all pull out piston from the

cylinder (each channel withthe calculated isokinetic

velocity)

move false sample from the

connection tube between modell andsampling unit into the cylinder and

suck correct sample into the tubes

left * *

all push piston in remove false sample from thecylinder and vent

right left *

Sampling step

all pull out piston from thecylinder (each channel with

the calculated isokineticvelocity)

fill cylinder with sample left * *

Measurement steps

1 push piston in (constant

velocity)

push sample of channel 1 through the

multiplexer towards the detector

right right 1

2 push piston in (constant

velocity)

push sample of channel 1 through the

multiplexer towards the detector

right right 2

... Repeat measurement steps until samples from all channels have been forwarded to the detector


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The detector unit contains a gas chromatograph equipped with a flame ionization

detector (FID) and a 6-port, two-position sampling valve (see Figure 3). A FID can detect

hydrocarbons in a hydrogen flame. The carbon ions in the flame are exposed to an electric

field between two electrodes and cause an electric current between the electrodes that is

proportional to the amount of carbon in the flame. Detailed description of the FID is to find in

the literature e.g. in [2].The sampling valves main function is

a) to collect a known volume of sample gas ( 1 st position)

b) to put that known volume of sample into the FID. ( 2nd position)

In the 1st (standby) position the sample comes through and fills a 50l sample loop and

is exhausted. Through the other ports, air (also called carrier gas), goes to the FID detector.

After switching to 2nd (operate) position the carrier gas input is connected to the sample loop

thereby the known volume of sample determined by the volume of sample loop is pushed by

the carrier gas into the FID detector. At this valve position the sample coming from the

sampler unit is exhausted directly into the environment. After a certain time the valve is

switched again into standby position and the cycle is finished.

(a)

(b)

Figure 3. The FID and the sampling valve in standby (a) and operate (b) position.

After the known volume tracer gas and air mixture has been injected to the FID

detector, the output voltage of the electrometer connected to the FID detector fast increases.

The area under this peak is proportional to the concentration of tracer gas and air mixture (see

Figure 4). The mean concentration has been determined as an average of at least three or

optimal five peaks. This means that for the accurate results the sample loop has to fill and

inject three or five times from the same sample (cylinder). This requirements and the dead


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volume of the valves and tubing has been determined the required volume of the sample

cylinder (100ml).

(a) (b)Figure 4. Concentration peaks on the signal checking monitor (a) calibration principle (b)

Before each wind tunnel measurement, a calibration of the FID is necessary. For this

purpose, the FID unit is connected to a calibration gas source with known concentration and

this known concentration is correlated to the area of the measured electrometer voltage peak.

A second point is given by the zero concentration of the carrier gas and the zero peak area

belonging to it, so the linear calibration equation can be determined.

4 CONTROL SYSTEM

The whole measurement process can be controlled by computer (Figure 5). Digital

mass flow controllers are connected via RS-485 cables and HART protocol (RS-232 can onlyhandle 1 unit, RS-485 up to 32 units).

Figure 5. Control and data acquisition system

The multi channel sampler is controlled by an USB digital I/O device, of which digitallines can set the valves and the multiplexer. Each linear stepper has an own microcontroller


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chip which can start and stop, change direction and speed of the motor, and also manages the

two limit switches. Both valves and steppers have amplifier circuits to amplify TTL digital

I/O signals to the voltage and current required by the specific device. A separate USB device

reads the analogue output voltage of the FID and can also steer the electromagnetic actuator

of the 6-port sampling valve.

All devices can be controlled from a program written in National Instrument Labview.The program is planned to run a measurement fully automatically.

5 PROJECT STATUS

After the discussion of several sampling methods, the present sampler configuration

was accepted. In mid 2005 a 2-channel test system was built. A similar system (without

isokinetic sampling) in Karlsruhe, Germany has been visited [3]. Repeatability and linearity

of the FID unit were checked using calibration gas of different concentrations and were found

good. After first tests of the sampler, reduction of dead volumes (thinner tubing and valve

connectors) was proposed. Instead of joining the tubes coming from the channels towards the

FID in an aluminium block, the use of a mechanical multiplexer was recommended. At the

moment further tests are carried out concerning measurement accuracy of the whole system.

6 CONCLUSIONS

As a result of development work at Department of Fluid Mechanics an automatic, PC

controlled sampling and measuring system was born. The system can contain up to 48

individually controlled channels. Thereby in case of pollutant transport investigation in wind

tunnel simultaneously 48 sampling points can be appointed. Using this new device the

accuracy, the repeatability and the reliability of sampling-based concentration measurements

can be increased.

REFERENCES1. VDI 3783 Part 12. Environmental meteorology, Physical modelling of flow and

dispersion processes in the atmospheric boundary layer, Application of wind tunnels,

Beuth Verlag GmbH, 2000

2. H.R.Trnkler, E. Obermeier: Sensortechnik. Handbuch fr Praxis und Wissenschaft

Springer, 1998

3. Christof Gromke, Ph.D. student, oral communication, Laboratory of Building- and

Environmental Aerodynamics, Institute for Hydromechanics, University of Karlsruhe,

Germany, 11.24.2005

AFFILIATIONS

Corresponding author: Mrton Balcz, Ph.D. student, Department of Fluid Mechanics,Budapest University of Technology and Economics, H-1111 Budapest, Bertalan L. u. 4-6.,

tel./fax: +36 1 4631560, +36 1 4633464,[email protected]

Authors:

Zoltn Szucsn, research engineer, Department of Fluid Mechanics, Budapest University of

Technology and Economics, [email protected]

Gbor Kalmr, research engineer, Department of Fluid Mechanics, Budapest University of


Istvn Goricsn, assistant professor, Department of Fluid Mechanics, Budapest University of


Acknowledgements:

Authors acknowledge the support of Hungarian projects OTKA T037730 and NKFP

3A/088/2004.

Development of Sampling System for Investigations on Pollutant Transport

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