MODEL A-316 CARBON MONOXIDE (CO) MONITOR AND ALARM FOR COMPRESSED AIR TESTING

Transcription

1 MODEL A-316 CARBON MONOXIDE (CO) MONITOR AND ALARM FOR COMPRESSED AIR TESTING FOR OPERATION FROM 115 AC POWER Andersen Medical Gas 12 Place Lafitte Madisonville, LA

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3 A I. INTRODUCTION This Model A316 Carbon Monoxide (CO) Monitor and Alarm is a 115 V AC dust-resistant instrument that can be surface-mounted as close as possible to the air supply but outside any classified hazardous area. It continuously monitors a compressed air sample connected to its sample inlet and it gives an alarm when: -the CO in the sample exceeds a preset level (adjustable, initially set at 10 PPM). -there is a discontinuity in the detector circuit It indicates CO concentration on a digital display, and has a pilot light to verify that the instrument is on and operating properly. The instrument is assembled into a powder-coated sheet metal housing 4" x 6" x 3½ overall. A 1/8 FPT opening for introducing the sample gas is located in the right hand side of the flow block at the bottom of housing. Visible through windows in the front face are the LCD display showing ppm CO, plus pilot and alarm lights. The 3-wire power cord plugs into a RFfiltered socket on the upper right side, and the alarm buzzer extends through the top. II. DETAILED DESCRIPTION When the hinged and gasketed front door is opened after releasing the latch on the right hand side, the interior is visible and accessible. Each major component is described in this section. A. Main Circuit Board The electronic circuit board occupies the main part of the housing, toward the front. It is secured by two slotted bars, and it can be removed if necessary by taking out two screws holding the lower bar. Electrical connections are by cables which connect to sockets on the rear. On the front face are these components of interest to the user: 1. Display is a liquid crystal type which reads from 0 to 199 PPM of CO in steps of one digit. 2. ZERO potentiometer, the lower of the group of three multi-turn potentiometers directly to the right of the display. It is used to set reading to 00 while known CO-free air is passing through the system. 3. SPAN potentiometer, directly above the ZERO potentiometer, is used to set the reading to a correct value while a known calibrating

4 A sample is passing through the system. 4. ALARM potentiometer, directly to right of SPAN potentiometer, is used to set the level at which the alarm is activated. Generally this is initially set at 10 PPM ma potentiometer, directly below the SPAN potentiometer, is used to set the 100 ppm signal output to 20.0 ma. This is a factory adjustment and is intentionally not marked ma potentiometer, directly below the 20 ma potentiometer, is used to set the zero signal output to 4.0 ma. This is also a factory adjustment. 7. NORMAL light, green, shows that power is being provided to the circuit and that the system is in normal operation. 8. ALARM light, red, comes on and glows steadily when CO is detected above the alarm point level. It also indicates a sensor failure or disconnection by blinking on and off. In normal operation, a slight flicker can be seen in the red light, a little faster than once per second, as a confirmation of normal operation. B. Power Supply The instrument internally operates from 12 V DC. This is supplied by a modular 115 AC to 12 V DC power supply. The built-in power supply is supported on a rectangular printed circuit board mounted to standoffs at the rear of housing. When the AC power cord is plugged in, AC power flows to the AC end of the power supply, and 12 volts DC flows from the DC end of the power supply to the main board. AC power enters the housing through an EMI filter socket in the upper right hand corner of case. The AC power cord has a standard grounding-type plug for a normal 115 AC power outlet and the instrument end mates with the filter socket. C. Flow System The air to be analyzed passes through the instrument under its own pressure. The components it flows through, in sequence are: 1. Sample inlet, 1/8 FPT size, to accommodate any suitable tubing fitting having a 1/8 male pipe connection. 2. Flow block, a bored aluminum block with a cavity for receiving and sealing the CO sensor, is secured to the outer bottom of the case. A porous metal flow restrictor reduces the pressure of the compressed air to atmospheric pressure before it reaches the sensor. The CO sensor seals to the block by means of an O-ring seal against the bottom of the cavity.

5 A CO sensor, a three-electrode electrochemical device in a cylindrical plastic housing, responds to any CO that is present in the atmosphere passing over the lower face. CO diffuses through a permeable membrane and enters into an electrochemical reaction, producing a current directly and linearly proportional to the CO content. Sensor plugs into four sockets on the face of a small circuit board which connects by three wires to the main circuit board. This small circuit board is held down by two screws which provide pressure against the O-ring seal to prevent leakage around the sensor. D. External Connections Two terminal blocks are installed on the lower crossbar. The one on the left has three screw terminals, for connection to the internal alarm relay, and the one on the right has two screw terminals for a 4-20 ma signal circuit. Wires to each of these two external circuits can enter the housing through the ½" conduit opening in the left hand side of the case. Connections are as follows. 1. The internal relay has 10 amp SPDT contacts, which are brought to the marked terminals C, NO and NC. Notes for connection of internal relay: The internal relay is normally energized to enable a fail-safe external connection in accordance with NFPA 99C, Item , as an indication of loss of power. A separately powered external alarm or other signal device should be wired through the relay terminals C and NC, and will be activated whenever the instrument is not receiving power. The same external alarm action will occur when the CO reading rises to the alarm level and the internal alarm circuit is activated. Terminals C and NO will be connected in normal operation, with power on and CO reading between 0 and the alarm point (usually 10 ppm). Note: Whenever the system is intentionally turned off, some extra steps will be required to disable the external alarm, which otherwise will respond to the loss of power. 2. The 4-20 ma signal circuit is brought to the two terminals marked + and -. Output signal is 4.0 ma at 0 ppm, 20.0 ma at 100 ppm. E. External Components 1. Buzzer, a solid-state sounding device on the top of the case. 2. Flow block, described above, is secured to the outside bottom of the case, and an opening in the case allows the sensor to be

6 A inserted from inside the case. 3. Calibration gas inlet fitting, on left-hand end of flow block, is a hose barb with internal orifice that restricts flow of calibration gas as it enters the block during calibration. A second opening on the rear of the block provides for exhaust of the calibration gas to prevent pressure buildup during calibration. 4. Alarm Silence switch, a pushbutton on the right hand side, when pressed activates a time delay circuit that holds the buzzer silent for 4 minutes, then re-energizes it if the reading remains above the alarm level. III. OPERATION A. Placing in operation Note: This procedure presumes the availability of a calibration kit similar to the # K. If one is not already available it should be purchased at the time the system is activated. 1. Install the instrument in the desired location, close to the air supply and to a source of power, and where it will be reasonably protected but readily seen. Mount the housing by means of #10 screws through the flanges top and bottom. 2. Make external relay and/or 4-20 signal connections as desired, referring to II.E. above. 3. Connect the air supply to be monitored, to the 1/8 FPT opening in the outer face of flow block, using appropriate tubing and fittings. CAUTION: This Carbon Monoxide Monitor must be connected to a suitable filter unit providing clean air, free of oil, water, and dust. 4. Using the flowmeter and tube supplied with the calibration kit, attach the inlet end of tube to the calibration fitting on left side of flow block. Turn on air pressure. 5. While holding a finger over the small hole in rear of flow block, observe reading on flowmeter. It should read at least 0.3 while held in vertical position. This confirms that sample flow system is working. 6. Connect power cord to the nearest AC power source. 7. When connected, the green light should come on and the red light and buzzer may also come on momentarily. The green light should flash for about 15 seconds. Display should then show a reading of 00 or close to it. 8. Open case by pulling forward on latch. Turn ZERO potentiometer clockwise to bring display to the desired alarm setting, for example 10 ppm. If the alarm does not come on, or comes on too soon, adjust it

7 A using the ALARM ADJ potentiometer. Clockwise rotation decreases the alarm level. Then set reading back to Connect a source of nitrogen or CO-free air to calibration gas inlet, using the flowmeter supplied with calibration kit, and set flow to If reading is other than 00, then turn the ZERO potentiometer to obtain a zero reading; clockwise rotation increases reading. 11. Disconnect the air sample from calibration inlet, connect a known sample of 10 ppm of CO in air or nitrogen, and again set the flow to 1.0. Note: The alarm buzzer can be silenced by pressing the silence button on lower right hand side. This will lock the audible alarm out for 4 minutes. 12. Verify that the display reading increases and the alarm light and buzzer operate. 13. If reading does not reach the desired level corresponding to the known gas concentration, adjust it using the SPAN potentiometer. Clockwise rotation will increase reading. 14. Remove the CO sample and be sure the air line sample is connected to the sample inlet. 15. Instrument is now in operation, and will actuate an alarm if CO concentration rises above the preset alarm level. Note: The above steps are needed only for the first operation and periodically during the life of the instrument. B. Normal Operation 1. Instrument will analyze the sample and show CO content on the display, in parts per million (PPM). The green NORMAL light will glow continuously and the red ALARM light will flicker about once a second. 2. When the CO concentration exceeds the alarm point (initially set at 10 PPM) the red ALARM light will come on steady, the green NORMAL light will go out and the buzzer will sound a steady tone. The internal alarm relay will activate, and any connected external circuits will respond accordingly. 3. When the CO concentration drops below the alarm setting, the indicators will automatically return to normal. C. Abnormal Indications Detector open circuit: If there is a discontinuity in one of the detector leads, the indicator lights will show as follows:

8 A Working Electrode (WE) line (red wire) - - red and green light blink on and off, accompanied by buzzer. Display goes to SC. 2. Counter Electrode (CE) line (black wire) - - red and green light blink on and off, accompanied by buzzer. Display goes to SC. 3. Reference Electrode (RE) line (blue wire) - - CO reading goes off scale positive (1----), red lamp comes on steady, green lamp off and buzzer sounds continuously. 4. Entire detector disconnected, - - same as 1. or 2. above. IV. MAINTENANCE A. Routine Maintenance 1. Each day or period of operation, verify reading close to 00. Adjust if needed. 2. Every 3 months, check calibration. 3. Replace sensor whenever reading cannot be set high enough, during calibration B. Sensor Replacement 1. Unplug or disconnect power cable. 2. Open instrument cover. 3. Loosen and remove the two screws extending up from the bottom of flow block, into the threaded nuts of the sensor circuit board. This will allow the sensor with board to be lifted up and out of the block. Unplug sensor from board. 4. Install new sensor in the same position, after making sure the O-ring is in place at the bottom of the cavity. Insert and tighten the screws, which will pull the sensor down into contact with the O-ring seal. 5. Reconnect power and allow new sensor to stabilize. 6. After two hours of operation, recheck zero and span adjustments as explained in III. A. 7. Discard old detector cell, keeping in mind that it contains a small amount of sulfuric acid. C. Main Circuit Board The principal electronic components are all installed on the main printed circuit board, which is retained within the housing by two slotted bars. To remove the board, take out the two screws holding the lower bar, after which the board can be pulled out and the wires unplugged at their connectors.

9 A Keep track of the proper position of each connector. D. Buzzer Once the circuit board is removed, the buzzer can easily be taken out by unscrewing the locking ring at the top of case. E. Power Supply Once the circuit board is removed, this gives access to the power supply on its small subpanel board. First take out the EMI filter block by removing two screws, unplugging the white and black wires, and disconnecting the green wire at the screw post. Then take out the two screws holding the board to the case, pull out the board and power supply assembly. F. Flow Restrictor If sample flow rate as tested under III. A.4 declines below 0.25, the flow restrictor may have become partially plugged. Replacement is not a field operation, and the instrument should be returned to the factory for correction. V. CALIBRATION KIT A field calibration kit is offered for use in accurate adjustment of the instrument. It consists of one compressed gas cylinder containing 100 ppm CO in air, and one containing 100% nitrogen, all in a carrying case complete with flow adjustment valve, tubing, a flowmeter and a small screwdriver. To adjust the reading accurately: 1. Shut off compressed air supply, while instrument is in operation. 2. Connect the tubing to the flow adjustment valve and to the flowmeter. Connect the flowmeter outlet tube to the flow block hose barb. 3. Install flow adjustment valve on nitrogen cylinder. This will produce a flow which can be set to 0.6 scfh using the valve. 4. Watch display, and after reading stabilizes set reading to 00 using ZERO potentiometer. 5. After adjustment, connect 100 ppm CO cylinder in the same way. 6. Set flow to 0.6 and adjust SPAN to give reading corresponding to the value marked on the cylinder, about 100 ppm. 7. Restore all connections to original state. 8. Accuracy of instrument when calibrated as above, after careful zero adjustment, is within ± 1 ppm over the range 0-15 ppm, within ± 2

10 A ppm from VI. PARTS LIST The following is a list of items that may need replacement during the life of the instrument. Part No. DESCRIPTION Tubing, ¼ O-ring K Cal gas inlet K Flow block complete with flow restrictor K Sensor cable with circuit board Buzzer, replacement 57-A316 Circuit board, main A Cell, CO detector K Calibration Kit, consisting of: K Cylinder 100 ppm CO K Cylinder 100% N K Valve K Tube with flowmeter K Carrying case K Screwdriver

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