The Ring Test Facility as described by Dr. J. Kettenbach and Dr. S. Jacobi, HLfU 1997 is detailed below. The ring test-facility is laid out for a maximum number of 20 participants. The maximum sample flowrate is 250 l/min. To assure accurate mixing of gases as well as constant flowrates, the system is controlled by means of 8 mass flow controllers (MFC) of different ranges. Different sources (permeation tubes, highly concentrated gas standards from pressurized cylinders) can be used as primary reference gas standards.
To provide well known O3 concentrations, an O3 - generator along with a reaction
chamber for “Gas Phase Titration” (GPT) is included. To check the influence of water vapour to the measurements, it is possible to add different amounts of moisture to the gas flow. All gas flows are controlled by means of high precision MFC’s and are fed into a reaction or mixing chamber made out of glass. The outlet of this reaction chamber is distributed to all individual gas sampling exit ports via a glass tube of 27mm (ID). All parts which are in contact with the gas are made from glass or teflon ( ¼”, 1/8”) tubes.
(i) Carrier Gas
In general, different carrier gases for different trace gases (gas standards) are to be used. To achieve correct and clean NO-concentrations, N2 is used as carrier gas. For
all other gases, synthetic air should be used as carrier gas. MFC 1 and 2 are used to control the carrier gas supply. The range of each of those MFC’s is 100 l/min. Therefore a maximum carrier gas flowrate of 200 l/min can be achieved. The controlled carrier gas supply is directly connected to the mixing chamber. MFC 3 ( range:0-50 l/min) is used to control the amount of water vapour added to the total gas mixture, if necessary. The separate flow of carrier gas via MFC 3 is passed through an impinger system filled with water. It is assumed that the gas stream is saturated by water vapour (relative humidity about 100%) after having passed the water reservoir through a fritted glass filter. To assure complete water vapour saturation, the level of water in the glass vessel must be at least 5 cm above the inlet of the gas into the water.
(ii) Dilution System
The permeation system in combination with an O3 – generator is manufactured within
one 19 unit. The temperature of the permeation oven can be set alternatively to 40 or 50oC by using a switch on the front plate of the unit. The flowrate of diluent gas through the permeation system is controlled by MFC 4 ( range 0-5 l/min.) Synthetic air or purified and dried air from a compressor should be used as diluent gas for the permeation system. The exit of the permeation system is directly connected to the mixing chamber.
(iii) O3 – Generator
As mentioned above, the O3 generator is part of the 19” unit of the permeation system.
The generator in general consists of an UV- lamp ( length: 15” ) mounted into a glass- chamber. The voltage of the lamp and thus the intensity of radiation is controlled by a high precision potentiometer on the front plate. The flowrate through the O3 – generator
is controlled by MFC 5 (range: 0-5 l/min). Synthetic or purified and dried air from a compressor can be used.
The air must provide a constant fraction of O2 which is not lower than 20%. The exit of
the O3 generator is connected to an injector T. The injector is used to add NO gas to
facilitate the gas phase titration. The NO-gas is controlled by MFC 7 ( range: 0-50 ml/min). A teflon tube of 1 m length ( 6/4 mm OD/ID) is used as a reaction chamber where the GPT takes place. The exit of this GPT- reaction chamber again is connected to the main mixing chamber.
(iv) Generation of calibration gases using highly concentrated primary
gas standards in pressurized gas cylinders ( e.g. SO2 ,NO,CO).
MFC 6 to 8 (ranges: 0-500,0-50 and 0-5 ml/min) can be used to hook up pressurized gas cylinders of various components as primary source to generate test or calibration gases. Typical mixing ratios used range from 100 to 1000 ppm. To achieve practical concentrations (also in the emission range), a suitable MFC (6-8) can be chosen depending on the concentration of the primary gas standard and the total gas flow.
(v) Gas Phase Titration (GPT)
As a prerequisite for GPT, a constant flow of a well defined NO- concentration is needed. Turning on the UV-lamp and choosing an appropriate setting for lamp intensity and air flow, will give a sufficient source of O3. After mixing these two gas streams by
means of the injector system, NO will be oxidized by O3 quantitatively within the reaction
chamber (teflon tube). Assuming a yield of 100% (quantitative reaction), the amount of NO2 formed is identical to the original amount of O3.
The mixing ratio of O3 (ppb) must not exceed 80% of the mixing ratio of NO (ppb). Only
if these conditions are met, the reaction NO+O3 ---> NO2 + O2
will consume the ozone completely.
(vi) Basic Calculations
¾ General equation to calculate gas mixtures, using high concentrated calibration gases from pressurized cylinders as initial source:
Vx
C = Cv * .____________________________
(V1 + V2 + V3 +V4 +V5 +V6 +V7 + V8 )
¾ General equation to calculate gas mixtures, using a permeation tube system as initial source
PT
C = _____________________________ (V1 + V2 + V3 +V4 +V5 +V6 +V7 + V8) .F
¾ General equation to calculate gas mixtures of O3, using an O3 – generator as
initial source:
Co3
C = ______________________________ (V1 + V2 + V3 +V4 +V5 +V6 +V7 + V8)
¾ General equation to calculate the relative humidity, in case the humidifier ( a simple gas bubbler or wash bottle) is used:
V3
RH = 100 * _______________________________ (V1 + V2 + V3 +V4 +V5 +V6 +V7 + V8) Legend
C: concentration of the corresponding gas after dilution (ppm, mixing ratio) Cv : concentration of the , “initial source”
(ppm, mixing ratio of the calibration gas in pressurized gas cylinders)
CO3 : Source strength of the O3 – generator ( ppm/l)
RH : Relative humidity (%) PT : Permeation rate ( mg/min)
F : factor to convert mg/m3 ( mass concentration) to ppm ( mixing ratio) under standard conditions ( T = 0o C; P = 1013 hPa)
V1 : V8: flow rate controlled by mass flow controller no. 1 to 8
Vx : Flow rate controlled by mass flow controller no. x (x= 6,7 or 8,
corresponding to the supply of the initial calibration gas)
5.2 Reasons for Poor Quality of Data
The reasons for poor quality of data are as follows:
(i) Irregular Calibration of Equipments
Monitoring instruments are prone to drift and may show variations in measured parameters. Calibration if not done regularly can decrease the accuracy of readings. Calibration of respirable dust sampler in terms of flow and time if not done regularly can result in errors in flow and time and hence errors in the concentrations.
(ii) Improper Sample Collection, Preservation, Transportation and Analysis
Loss of samples can occur if they are not stored in ice box while transporting from field to laboratory and also if ice is not kept in ice tray while sampling. Data may not be comparable if analytical methods recommended by CPCB are not followed.
(iii) Lack of Trained Manpower
Manpower if not trained properly may not follow correct methods of sampling and analysis resulting in error in measurements.
(iv) Improper Location of Monitoring Station
If location of monitoring station is not representative of the area then data may not be useful for drawing any interpretation. If the location of the instrument is such that it does not satisfy the physical requirements of monitoring site such as height above ground level, distance from nearby sources etc. then data generated may not be of much use in determining status and trends.
(v) Lack of Infrastructure
Infrastructure in terms of proper shed for the protection of instrument during rain, coveroff during off time if not provided may result in corrosion of instrument and error in data generated.
(vi) Lack of Dedicated Manpower
If due to shortage of manpower, personnel involved in ambient air quality monitoring are also involved in other activities and the monitoring data is not generated for adequate number of days, then the annual average computed may not represent the true annual average.
(vii) Non-availability of Continuous Power Supply
If due to non-availability of continuous power supply, monitoring is not carried out for 24 hours in a day, then the daily average computed may not represent true daily average.