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Terminology

Here is a glossary of ACM terms.

1. Method: A method is a list of gasses in software. The method contains the information needed to identify a gas on the list. An ACM can have one or more methods loaded depending on the user's application.
2. Area: An area refers to the actual sample port through which a sample is drawn at the top of the ACM. A standard ACM can accommodate up to 40 areas.
3. Slot: A slot is a position in the scanning sequence programmed by the user. A slot is identified by a number referring to its placement in the sequence. A slot must be programmed to scan a par-ticular area using a single method. The scan sequence can be tasked to monitor more critical areas (i.e. breathing zones) more often than less critical areas (i.e. gas cabinets). If this is the case, some of the slots in the sequence would contain identical areas. Slots can be made inactive by the user and thus skipped in the monitoring sequence.
4. FTIR Bench: The device that measures infrared absorption of the sample gas. FTIR stands for Fourier Transform Infrared, a technique for producing a continuous spectrum throughout its measurement range (400 to 5000 cm-1). The bench used in the ACM is a Michelson type FTIR.
5. Spectrum (pl. Spectra): The pattern of absorption of infrared light produced by the FTIR bench.
6. target.exe (target): The ACM executable program invoked automatically on power up or reset of the instrument. This is the monitoring software that controls the instrument during operation.
7. config.dat: The editable (ASCII) data file containing hardware and software configuration in-formation for target.
8. *.slt: The slot sequence programs set up by the user. “default.slt” is used whenever target is invoked.
9. *.rly: The relay control programs set up by users with programmable relays. “default.rly” is used whenever target is invoked.

ACM Functional Sub-systems

The ACM can be broken down into 3 major sub-systems. These are the sample flow system, the FTIR bench, and the computer system. Additionally, there are remote devices and alarm relay panels that can be used with the system as facility interfaces.

1. Sample Flow System: The sample flow system is composed of a series of tubes and valves. These control the delivery of sample air from the area being monitored to the sample cell. The sample is locked in the cell and then analyzed. After analysis, the cell is evacuated and the sample is exhausted. A bypass valve allows air from the next programmed monitoring area to be drawn through its tube during the analysis period for the current area. In this way, a fresh sample is always available for moni-toring. Every two hours, a background spectrum is taken. This involves filling the sample cell with zero gas (typically dry nitrogen). The background spectrum is used as a reference. The background function can also be performed on demand, but this is rarely necessary.
2. FTIR Bench: The FTIR bench is the instrument that obtains the infrared spectrum of the sample. IR source light is manipulated using a Michelson type interferometer. The IR beam is then passed through a window in the bench. A series of mirrors reflects the beam into the sample cell, where specific frequencies of the IR are absorbed. The beam, after traveling a fixed distance through the cell (typically 10 meters), then exits the cell. A second series of mirrors focuses the beam on the de-tector element. The detector produces a voltage proportional to the signal strength of the IR beam. This signal, called an interferogram, is then digitized and sent to the computer.
3. Computer, Display, and Keyboard :The computer receives the interferogram and converts it into an IR sample spectrum using a Fou-rier transform routine. The sample spectrum is then mathematically compared to the background spectrum generating an IR absorbance spectrum. Absorbance lines reveal the presence of chemi-cal components in the sample. By analyzing the position and height of these absorbance lines, any chemicals in the chosen method (gas list) can be identified and quantified. The concentra-tion of a selected gases are then reported on the display and a status for the gas is determined ac-cording to warn and alarm levels set by the user. If a concentration exceeds these levels, appropriate relays are triggered. The computer can be configured to store the absorbance and background spectra to disk. In order to save disk space, sample spectra can be stored under vari-ous conditions, typically only when a warn or alarm level is detected. These stored spectra are useful for data archiving or for identifying unknown gasses (requires sending the disk to Te-loSense). A keyboard and display are provided with the unit to allow for local user interfacing.
4. Alarm Relays: Various alarm relay options are available with the ACM. Power On, Malfunction, General Warn Level, and General Alarm Level relays are standard, and user programmable relays are usu-ally added to the system. These relays can be wired to external lights and horns which serve to alert personnel in various parts of the facility that a gas leak has been detected.
5. Remote Terminal and Printers: A local or remote printer can be used for generating hard copy reports from the ACM. The printer can be configured to output scan reports under various conditions, typically only when warn or alarm conditions occur. Shift summary reports can also be printed if desired. A remote terminal can also be added to an ACM giving a user control of the equipment at a location away from the instrument. Other interface options include communications through an RS-232 port (for use with a PLC, for instance), and Local Area Networking (LAN) for interfacing with other AT based monitors and computers. The LAN option also allows the hook-up of a modem for off-site control and reporting.

ACM Software

ACM operation is controlled by an executable program called target (target.exe). All the necessary files for running the ACM including target.exe are contained in the DOS directory C:\ACMDATA (unless the directory has been renamed by the user). When the system is powered up or reset, the computer automatically invokes target and the unit begins its user programmed scan sequence. To enter the main menu of target from the normal scanning mode, hit the escape (Esc) key.

ACM Displays and Printouts

The ACM Displays slot information upon the completion of each scan. Similar displays are generated in the locked mode, in normal slot scan mode, or when performing a demand scan. Upon completion of a subsequent scan, the display will immediately update. In order to preserve the scan information, printouts can be requested. (Printouts can be induced under various conditions as described above). Additionally, shift summary reports can be generated giving information for each slot with overall results and time weighted averages, as well.