REMOTE INTELLIGENT SENSOR - AREA MONITOR
This method eliminates the ‘zero drift’ that could otherwise occur with slight variation in the reflective value of the tape. It also ensures that system accuracy is not compromised when reusing the same tape spot. System accuracy is not compro- mised when reusing the same tape spot.
3.2DENSITY & TIME OPERATING MODES
Actual sampling takes place under control of one of two operating modes. The microprocessor automatically selects the appropriate mode in relation to the dynamic situation being monitored. The two modes are:
3.2.1DENSITY MODE. This operating mode is used when the sampled concentration is below a predetermined threshold. Under this condition, the gas sample period is a fixed four minutes.
During the four minutes, a sample is drawn into the system at a specified Flow Rate. The product of time and flow provides the measured volume used in the subsequent calculation of concentration.
The term ‘Density Mode’ is used because the sample volume is fixed and the variable used in the concentration calculation is stain density.
3.2.2TIME MODE. When the concentration level of the sampled gas exceeds the preset threshold value, the stain on the tape develops quicker. By measuring the time it takes to reach the threshold density, it is possible to calculate the sample concentration.
Using the ‘time mode’ provides real benefits; a reading and an attendant alarm are provided rapidly when concentrations are high, and the tape is never allowed to become saturated. Because the tape doesn’t saturate, the reaction is always in the linear region where the monitor’s accuracy is maximized.
An additional benefit is that the tape continues to act as a filter and prevents the sample getting past the optics block and contaminating the system beyond.
3.3CHANGE-OVER FROM DENSITY TO TIME MODE
With a 1,000 ppb instrument, the
3.4MINIMUM SAMPLE TIME
The minimum sample time in the time mode is approximately 16 seconds. This reponse occurs when the monitored concentration has reached, or ex- ceeded, the maximum range of the instrument.
3.5TAPE CASSETTE LIFE
The cassette has a nominal life of 30 days. However the rate at which the system uses tape, and the consequent cassette life is dependenty mainly on the gas concentration monitored.
If the level is zero, or very low, a tape step will occur every 16 minutes (4 x four minutes). This is because the two tracks are each used for two consecutive samples of four minutes as explained in Section 3.1.2.
If the monitored concentration is higher, the tape steps every two sample periods; also the sample period itself can reduce below 4 minutes (dependent upon the concentration reached). A tape step will therefore occur at least every 8 minutes (2 x 4 minute sample periods) and more quickly if levels are high.
Infrequent high gas concentrations will not have much effect on cassette life, but if monitored levels are continuously significant, cassette life will be noticeably reduced.
In most applications, since monitoring is being done to confirm an expected zero, or very low gas concentration, cassette life will be close to the
3.6DATA POINT STORAGE
At the end of each sample interval the calcu- lated data point is stored, as are the calculated TWA’s. The 15 minute TWA , and the 8 hour TWA are both available for immediate display at a touch of the appropriate front panel key switch. The stored data points can be printed out as a record when the optional Printer option is fitted.
3.7SYSTEM DISPLAY
The displayed information, including symbols, or icons, depends upon the operating conditions. These and related information are as described below.
3.7.1SYSTEM NORMAL, GAS CONCENTRA- TION ZERO OR LOW. This is the usual operating condition and three pieces of information are dis- played.
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