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Evaluation of Optical Instruments for Real-Time Continuous Monitoring of Smoke Particulates
Results of the evaluation are broken down into the respective criteria as detailed in the Evaluation Criteria section. Table 3 summarizes suitability, ease of use, and reliability results. Other than the statistical results for accuracy and comparison of identical instruments to each other, evaluation results are subjective, based on the authors' operational experience with the instruments and their opinions.
| Price (dollars) |
Portability | Learning curve |
Setup | Data collection |
Data manipulation |
Calibration | Reliability | Customer service | |
|---|---|---|---|---|---|---|---|---|---|
| Met One GT-640 | 5,382 | 7.5 | 7.5 | 7 | 8.5 | 7.5 | ¹8 | 9 | 10 |
| MIE DataRam | 11,000 | 10 | 8.5 | 10 | 9 | 8.5 | 9.5 | 9 | 10 |
| Radiance Research | 4,795 | 9.5 | 7.5 | 9.5 | 8.5 | 8 | 7.5 | 8.5 | 8 |
| Optec NGN-3 | 12,900 | 7.5 | 7 | 9.5 | 7.5 | 8.5 | 8 | ²NA | 10 |
| Andersen aethalometer | 16,645 | 8 | 8 | 9.5 | 9.5 | 8.5 | ³NA | ²NA | 10 |
| ¹ Results based on model tested. Newest model has simplified calibration. | |||||||||
| ² Insufficient data. Did not operate long enough to allow a good judgment. | |||||||||
| ³ Received factory calibrated. Did not repeat factory calibration procedure. | |||||||||
Mass Concentration Range—All of the continuous, real-time instruments evaluated can estimate mass concentration levels from 1,000 µg/m³ and lower.
Portability—The MIE DataRam was the smallest and most portable of all the instruments, although none is really heavy or bulky. The DataRam comes with its own protective carrying case that measures 11 x 12 x 16 in. The Radiance Research nephelometer is lightweight, a little larger than the DataRam, and does not come with a carrying case. The other instruments are larger, heavier, and more cumbersome than the DataRam and Radiance nephelometer. One person can carry any one of the instruments without a problem.
Power Consumption—All the continuous real-time instruments are configured to run on standard 110- to 115-V ac power. Except for the Andersen aethalometer, all the instruments consume very little power and can be configured to run on 6- or 12-V batteries. The MIE DataRam is the only instrument that has an internal, 6-V, 6.5-Ah battery supplying enough power to run the instrument for at least 24 h on a full charge. The DataRam's inlet heater requires an additional battery. The DataRam can also be configured to operate on an external 6-V power source. The Radiance nephelometer can be configured to run on a 12-V battery. It consumes about 4 W of power. More power is required if the inlet heater is attached. The Met One GT-640 can be operated with a 12-V battery. It draws about 2 A. The Optec NGN-3 requires a 13.8-V dc power source providing a minimum of 5 A. A large battery recharged by a solar panel could power most of these instruments.
Weatherproof Enclosure—These instruments contain sensitive electronic components that must be protected from harsh environmental conditions, especially rain. The Met One GT-640 is the only instrument that comes standard in a weatherproof enclosure. An optional weatherproof enclosure is available for the Andersen aethalometer. The manufacturers do not supply weatherproof enclosures for the Radiance Research and Optec NGN-3 nephelometers or for the MIE DataRam.
Cost—The cost for each instrument varies depending on the accessories. The least-expensive instrument was the Radiance Research nephelometer, which costs $4,795. The Met One GT-640 costs $5,382, (including a tripod and PM2.5 cutoff device). The MIE DataRam costs $11,005, (including the inlet heater, PM2.5 cutoff device, and ambient air-sampler inlet). The Optec NGN-3 costs $12,900. The Andersen aethalometer costs $16,645. Consult the manufacturer for current prices.
Learning Curve—Each instrument has its own peculiarities, but none is extremely difficult to learn to operate. Changing monitoring parameters such as log interval and times is easiest on the MIE DataRam and Met One GT-640. All the parameters can be modified by using the front panel keypads and following simple directions. Parameters for the Andersen aethalometer can be changed using its own keypad. A computer is needed when changing some of the operational parameters on the Radiance and Optec nephelometers.
Setup—All of the real-time continuous monitors are relatively easy to transport and set up. Once the monitors have been calibrated, all that has to be done is to take them to the desired location and turn them on. The only difficulty encountered was placing the Met One GT-640 on its tripod due to the awkward mounting brackets, the unit's weight, and the tripod's height.
Data Collection—A laptop computer using an RS-232 port and Microsoft Windows standard HyperTerminal interface was used to download the data from all of the instruments except the Optec NGN-3 and the Andersen aethalometer. A dedicated laptop using a communication package called ProComm Plus was used to log data from the Optec NGN-3. The Andersen aethalometer internally logs its data to a floppy disk.
Data Manipulation—Microsoft Excel spreadsheets were used to manipulate the data. All instruments provide mass concentration (µg/m³) except the Radiance nephelometer, which provides Bscat. Bscat can readily be converted to mass concentration within Excel. The Met One outputs data with a header at 40-minute intervals. The header must be removed and data points consolidated for graphing or averaging. The DataRam outputs data every other line in the spreadsheet. When opening the file, blank lines must be eliminated before graphing or averaging. Both the Met One and the DataRam offer optional software packages to eliminate these operations.
Calibration—The DataRam and Met One use internal calibration methods. The DataRam performs its zero calibration and completes a span check when a knob on the back panel is rotated to insert the calibration rod. The LCD panel prompts the user to perform the operations. The DataRam can also be gravimetrically calibrated for a particular aerosol. The mass concentration on its 37-mm membrane filter is compared to the optically derived concentration by computing the Time Weighted Average (TWA) using the Estimated Time (ET) and flow-rate values. The Met One's internal zero and span calibration is performed by turning a knob on the instrument's side panel. Zero and span can be adjusted by using a small flathead screwdriver and turning two screws in the side panel (the latest model of the GT-640 internally performs zero and span adjustments).
The Radiance Research nephelometer requires particle-free air (zero air or a HEPA filter) to perform the zero calibration and an external refrigerant gas, such as Freon, SF2 or CO2, for the span constant. The constants on the panel display must be adjusted to match the zero and span readings. The Optec NGN-3 has an internal zero air filter. An external refrigerant gas (R-134) is used to obtain the measured scattered light value for the span calibration. Achieving a stabilized calibration for these instruments can take 20 minutes or longer. The Andersen aethalometer performs its calibration internally during each startup.
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