Bacharach 2772-0803 manual System Operation And Features, Complete Sampling Sequence, Tape

REMOTE INTELLIGENT SENSOR - AREA MONITOR

3SYSTEM OPERATION AND FEATURES

3.1COMPLETE SAMPLING SEQUENCE

The sequence starts with system and cassette being purged before gas sampling starts. In addition, a reference light level reading is taken from the fresh tape spot under the optics block.

Effective purging requires the enclosure to be properly sealed. Ensure that the cable glands and the door are air tight. Unused cable glands can be tightened onto a short piece of cable.

Purge inlet and exhaust ports can be piped to a remote location, where this is necessary.

3.1.2 TWIN TRACK TAPE SAMPLING. At

start-up the incoming sample is passed through the

PHOTO

DIODE

LIGHT

REFLECTED

OFF TAPE

SURFACE

INCOMING

SAMPLE

LIGHT

EMITTING

DIODE

OPTIC

BLOCK

lower tape half (track 1). When that sampling period

is complete, the microprocessor decides if a stain has

formed on the tape. If not, the next sample is again

passed through the same spot on track 1. Conversely

if a stain was formed, the second sample is switched

to pass through the upper half (track 2). The same

sequence occurs at the end of that sample period and

track 2 is reused if no stain is detected.

At this point the tape is stepped on and the next

sample passed through the next track 1 spot. Figure

TAPE

Figure 3-1. Sampling Sequence

During the sampling period the tape spot is scanned every two seconds. This frequent scanning detects the change in the reflected light value that occurs if a stain develops.

If no stain, or a low density stain, is detected, sampling continues for a fixed four minutes. At the end of this time calculated concentration for that cycle is displayed and the next cycle starts.

The development of a significant stain shortens the sampling cycle and the concentration value is displayed immediately, as described in more detail below.

When a concentration above alarm set point is detected, the gas alarm relay/s opens to initiate external alarm systems and warning devices. This is in addition to the visual warning display.

3.1.1AUTOMATIC PURGE CYCLE. A purge sequence occurs after every tape step. The pump runs at a higher Flow Rate for 10 seconds; air inside the tape cassette, and the enclosure, is exchanged for filtered air. The incoming air is filtered as it passes through a ‘scrubber’ filter in the cassette. This is shown in the diagram in Figure 2-8.

Purging prevents the build up of gas in the system, and in the cassette where it could pre- expose the tape. The ‘scrubber’ filter is automati- cally renewed at each cassette change.

3-2 shows the sequence where no stain develops and

the maximum of four sample periods occurs before

the tape is stepped.

3/4	7/8	TRACK 2
1/2	5/6	TAPE
		TRACK 1

STEP

4 SAMPLE PERIODS OF 4 MINUTES EACH = 16 MINUTES PER TAPE STEP

Figure 3-2. No Stain, Both Tracks Used Twice

Figure 3-3 shows that only two sample periods per tape step occur if significant stains develop on the tape.

2	4	TRACK 2
1	3	TAPE
		TRACK 1

STEP

2 SAMPLE PERIODS PER STEP. EACH SAMPLE PERIOD IS A MAX OF 4 MINUTES, OR LESS IF THE STAIN IS SIGNIFICANT = 8 MINUTES PER

TAPE STEP

Figure 3-3. Stain On Tape, Both Tracks Used Once

3.1.3TAPE REFERENCE MEASUREMENT. At the start of each sample period, the tape spot being used is scanned by the light pulse (generated by the optics system), see Figure 3-1. The value of the reflected light is measured and stored as the refer- ence against which the light value, during and at the end of the sample period, is assessed..

Instruction 2772-0803 Page 3-1