Direct Monitoring

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Direct/Real

Time Monitoring

Ankur Sharma

MIHS, ISTARhttp://www.linkedin.com/in/r220206001
http://www.linkedin.com/in/r220206001http://www.linkedin.com/in/r220206001


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INTRODUCTION

The airborne concentration of many gases,

vapors, and particulates can be measured in the

field using direct-reading devices, thereby

eliminating the time delay and effort of sendingfield samples to a laboratory for analysis.

Most direct-reading devices are electronic

instruments, although there are some whereconcentration is indicated by color change

MIHS/IH675/12IH401/Direct/Real TimeMonitoring

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Real-Time methods and sample

collection device (SCD) methods arethe two major categories for

exposure monitoring.

SCD methods require laboratory

analysisof the sampling medium.

In contrast, real-time methods givethe results right in the field as the

monitoring is performedMIHS/IH675/12IH401/Direct/Real Time

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Direct or Real Time Monitoring

Direct monitoring or real time

measurements allow the

concentration of a contaminant to bedetermined on-the-spot

also known as Grab sampling, as the

sample is quickly taken and analyzed

immediately


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Sample is collected in the breathing

zone of the worker over a shortperiod and the concentration of

contaminant read instantaneously

Real-time methods are used for both

Occupational and Environmental

monitoring


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For Occupational Exposures:-

there are personal devices that aresmall and light enough to be worn by

the worker, larger portable instruments that can

be carried by the sampling

practitioner, and fixed systems for area monitoring


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For Environmental Exposure

Monitoring :-

Equipment is generally left in a singlelocation for the whole sample duration& so its size and weight is less important

Even when sampling equipment iscompact, the electrical powerrequirements for an extended samplingperiod usually require line connectionor a heavy battery pack


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Advantages of Direct-Reading

Instrument

It is a quick, simple and versatile technique

Ability to measure concentrations of

contaminants on-site with almost

Instantaneousresults

Direct monitoring is particularly useful where

there is a need to have immediate readings of

contaminant levels, for example in the case of

fast acting chemicals


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Continuous fixed station monitoring to alertpersonnel to emergency situations via audible

and visual alarms It is also useful for identifying periods of peak

concentration during the work cycle or workshift so that a control strategy can be

developed Identification sources of problems, such as

leaks, and the opportunity to evaluate the

effectiveness of control measures as soon asthey are in effect

It is a relatively economical method ofmeasurement


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Disadvantages of Direct-reading

Instrument

Good for quantifying exposures to knowncontaminants, they are very limited foridentifying of unknown substances

Cross-sensitivity may be a problem since otherchemicals will some- times interfere with a staintube reaction. For example, the presence ofxylene will interfere with stain tubes calibratedfor toluene

These devices are susceptible to malfunctions likeany sensitive instrument, and they often receiverough treatment in the field.


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Use of an instrument for the wrong application,and then the data are incorrectly interpreted. Forexample, a direct-reading photo-ionizationdetector (PID) device will not respond well tobenzene vapors, and so its use for this compoundcan yield erroneous information.

Inability to calibrate the instrument adequately.All direct-reading instruments require periodiccalibration checks using the gas or vapor ofinterest.

Variations in contaminant levels throughout thework period or work cycle are difficult to monitorby this technique


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Selection Criteria for Real-time Device Need for portability

Need for high sensitivity (low detection levels) and accuracy Conditions of useheat, humidity, use of a cart versus a shoulder strap

Appropriateness for the application

Purpose for sampling

Instrument availability and complexity of use

Presence and classification of hazardous (explosive) locations Specificity

Measurement period required

Minimum detectable limit

Required dynamic range

Time constant Type of data acquisition and display required

Interferences

Personnel for operation and maintenance

Personal choice based on past experience


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1. Colorimetric Systems

These indicate concentration or

exposure level by a color change in

the sampling medium often the length ofstain in a tube

where the length is proportional to

the airborne concentration of

contaminant.


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Colorimetric

The principle of colorimetric monitoring liesin the chemical reaction between a knowncontaminant and a detecting system.

most common techniques are either tubes orbadges. Because of the specificity ofcolorimetry, it is vital that the exactsubstance is known before sampling begins.

It is also wise to be sure of other potentialcontaminants, in case these interfere withthe operation of the detection system.


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The most widely used of colorimetric systems

would have to be the pump and tube assembly.

With this system, air is actively sampled through

the tube by the drawing action of the pump.

The glass tube is tightly packed with a chemically

reactive substance.

As contaminant-laden air passes through the

tube, it specifically reacts with the chemical

inside the tube. A resulting color change or stain will develop

along the length of the tube.


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Monitoring tools using colorimetric

technologies

Detector Tubes

Color Badges

Paper Tape Monitors

Colorimetric Wipes

Colorimetric Swabs


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Detector tubes

Classical measurement technique with first

patent in 1919

Glass tube containing a chemical media that

reacts with the contaminant of interest by

changing color

Concentration is read directly from a printed

scale on the tube


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Detector tubes

Used with a hand-operated pump tocollect on-the-spotmeasurements

Measurements typicallytake a few minutes tocomplete

Pump and tube arecalibrated by themanufacturer as a unit


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Reading Detector Tubes

For these tubes the calibration chart gives

concentration reading based on sample volume.

Long-term tubes report concentration in units of

ppm-hours. To calculate the average airborne concentration,

divide the ppm-hours reading by the sampling time

(in hours).


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Accuracy of Detector Tubes

Detector tube measurements are generally

reasonably accurate. Most have an error of +/- 20%.

However, the error by the reader can be significant.

Since occupational hygiene sampling already has itsshare of random and systematic error, an error such

as this may not necessarily be unacceptable.

The factors that may affect its accuracy include the

volume of air drawn through the pump,completeness of reaction, reading the stain and the

type ofcolorimetric tube system


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Application of Detector Tubes


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Qualitative Assessments

Leak Detection

Chemical Migration

Confined Space Entry(using extensionhose)

UnknownIdentification (usingHazmat kits)

Semi-quantitative

Spot checks of area or

breathing zone samples

Maximum levels during

various processes or

worker activities

Analysis of sample bagscontaining air emissions


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Sources of Error with Detector tubes

Accuracy is limited

Interfering compounds cause similar reactions

and readings on tube

Affected by temperature and humidity

Subjective differences in eyeballing the color

change to determine result


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Advantages of colorimetric stain tubes

They can be used to identify high-level, short-

term exposure,

The requirement for long and expensive

laboratory analysis is removed,

They can provide preliminary findings of

exposure, including alarm levels for

evacuation or remedial controls, and

They can be used to check controls.


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Color Badges

Immediate Visual Indicators

Badges contain a chemically coated filterpaper or indicator layer.

Target chemicals in air react with the chemical

coating used in the badge and produce a colorchange.

This change is a visual indicator to the worker

that an exposure has occurred. Color comparison charts allow for exposure

estimates.


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Applications ofColor Badges

Visual indication toevacuate work area inpresence of dangerouschemicals!

Semi-quantitativeexposure levelscreening

Leak Detection


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www.kandmenvironmental.com


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Three major classifications:

Instruments designed with one or more sensors

that respond to specific chemicals.

Units that respond to broader categories of

airborne chemicals such as combustible gases

or hydrocarbon vapors.

Devices that contain a sensing system that can

be tuned to respond to many different

chemicals. Infrared (IR) and gas

chromatography (GC) units typify this group


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A major plus of electronic direct reading

instruments is the ability to use a data logger

(either built into the device or external) to recordconcentration values and other parameters over

the sampling period.

This permits later computer analysis and plottingof the data, including calculation of TWA, short-

term, and ceiling exposures. The data, along with

field notes, may also help to identify the

circumstances that caused any high exposures

and the effectiveness of exposure controls.


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Portable Infrared Gas Analyzer


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Principle

In infrared spectrometry the basic principle utilized isthat many gases and vapors will absorb infrared

radiationand under standard conditions the amount of

absorption is directly proportional to the concentration

of the chemical contaminant. This instrument takes a sample of air, detects the

extent of interruption, i.e. absorption, of an infrared

beam and gives a display of the contaminant

concentration.

Because so many substances absorb infrared radiation

this is an extremely versatile instrument


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Instruments for Specific Gases

and Vapors

Electrical and Electrochemical

MIHS/IH675/12IH401/Direct/Real Time 36

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