extreme danger of attempting to "clean out” a lamp with a compressedair jet or attempting to light a lamp while holding it in front of a jet of compressed air, particularly when the working place is filled with explosive gas. Obviously, flame safety lamps, although tested in air velocities higher than those likely to be encountered in any underground ventilating current, will not stand being exposed to a compressed-air jet of extremely high velocity. In this instance the compressed air had a nozzle velocity of approximately 90,000 feet per minute. This is plainly a condition to which a flame safety lamp should not be exposed. 2. In 1953 an explosion occurred in a Pennsylvania anthracite mine when compressed air was impinging on a flame safety lamp inserted into an accumulation of methane. One man was fatally injured and two men seriously burned in the explosion. Eleven other men, working in the area, escaped without injury. In this case, an air-line hose had been inserted in the opening over the top of the collar of a timber set, which was 21 feet from the face, and the compressed air was turned on while the cut of coal was blasted. The miner waited 20 to 30 minutes after blasting, then went to a spot approximately 20 feet from the face and raised his lamp to test for gas when the explosion occurred. When the lamp was examined after the explosion, the two gauzes had discolorations on the sides near the tops, indicating intense heat had been applied there. Bureau of Mines tests have shown that the gauzes of a flame safety lamp when subjected to jets of compressed air will heat to a temperature above the ignition point of methane and will ignite accumulations of gas outside the lamp. ELECTRIC METHANE DETECTORS Various models of electric methane detectors have been designed and built. The earlier models, in general, were too cumbersome and intricate, and it was not until 1915 or later, when the electric cap lamp was developed, that reasonably efficient and safe detectors of this type were obtainable. Later, research and development resulted in highly efficient and easily operated types of electric methane detectors being placed on the market. Most of the detectors depend on the combustion of the gas sample; and, for this reason, the accuracy on high percentages of methane is impaired if the oxygen content of the air is less than 16 percent. In 1920, the Bureau of Mines issued Schedule 8A 84 as part of its effort to increase safety in mining and to establish a definite procedure for investigating the safety and efficiency of electric methane detectors. Under this schedule and revised Schedules 8B and 8C, the Bureau has approved 10 types of electric methane detectors. All are portable, and can be taken into a mine readily and used without too great a need for technical ability. A list of the devices in order of approval follows: 84 Bureau of Mines, Procedure for Establishing a List of Permissible Methane Indicators for Mines: Sched. SA (Sched. 8 not issued in printed form), Aug. 25, 1920: Sched, 8A revised and reissued as Sched. SB, Nov. 17, 1926; Sched. 8B revised and reissued as Sched. SC, Oct. 31, 1935. Instruments covered by approvals 806, 809, 810, and 811 are the only detectors marketed currently. The other units have been replaced by subsequent models or have been withdrawn from manufacture. Accordingly, this discussion is confined to the above four approvals. M. S. A. (MINE SAFETY APPLIANCES CO.) METHANE DETECTOR (W-8) This detector employs an electrical circuit, the basis of which is the Wheatstone bridge. The detector unit has a filament of specially activated platinum (as an integral part of the bridge circuit), which is heated by means of current from an Edison miner's cap-lamp battery. The temperature of the filament is high enough to burn methane-air mixtures. Particles of dust or other foreign matter, unless filtered from the air sample, will burn on the filament and may tend to poison the filament and destroy the accuracy of the device. After the bridge circuit has been balanced in fresh air for zero or "no gas," a sample of air containing methane is drawn over the hot filament. The gas burns on the hot surface of the filament, thereby raising its temperature and increasing its electrical resistance. The change of filament resistance unbalances the electrical circuit of the instrument; and the degree of unbalance, which is proportional to the percentage of methane present, is indicated on a meter graduated directly in percent methane. A compensating unit, which stabilizes the instrument against fluctuations that would otherwise be caused by changes in surrounding temperature, humidity, air pressure, applied voltage, etc., has a filament of deactivated platinum wire over which the gas sample passes without change. This filament is in the leg of the bridge circuit opposite the activated filament and tends to minimize the variables. Two scales on the indicating meter are provided as an aid to more accurate reading of the low percentages. One scale is graduated in 40 divisions (each division representing 0.05 percent) from 0 to 2.0 percent; the second is graduated in 30 divisions (each division representing 0.10 percent) from 2 to 5 percent. The instrument is normally set for reading on the 5-percent scale; but, if the reading is less than 2 percent, the 2-percent scale can be used by means of the range switch on the control panel. The complete detector (fig. 12) consists of an instrument case, the cover of which automatically controls the off-on switch: an aspirator bulb; a control panel for balancing the electrical circuit; and an Edison cap-lamp battery, which supplies the energy for operating the unit. Housed within the case are the electrical circuits, flow orifices, flashback arresters, and the combustion chamber. The detector is so designed that the battery voltage and zero setting of the meter can be checked at any time, either in fresh air or in air containing methane. When checking is done in methane-air mixtures, enough time, 2 or 3 minutes, must elapse after the last sampling to permit complete burning of any methane that may be present in the combustion chamber. The device may be used on either 3- or 4-cell Edison cap-lamp batteries. Special dropping resisters are supplied in the battery connectors to limit the current flow from the 4-cell battery. The flow diagram (fig. 13) shows the path of a gas sample through the detector, which, briefly, is as follows: By squeezing the rubber aspirator bulb, the gas sample is first drawn through the inlet coupling, either directly or with a hose extension for out-of-reach places. The sample then passes through the inlet flashback arrester into the combustion chambers and leaves through the outlet flashback arrester. The sample flow is controlled by orifices. OPERATING INSTRUCTIONS Directions for operating the detector are as follows: 85 1. Case, Control Panel, and Switches. (a) When the top of the instrument case is opened, the control panel is exposed and the electrical circuit is turned on automatically. The push-button switch at the left of the panel is normally up and connects the meter in the "Read" position of the circuit. When this switch is depressed and held, the meter is connected in the voltage-checking position. (b) The left or "B" knob, equipped with a special locking device, operates the balancing rheostat. The right-hand or "A" knob operates the rheostat used for adjusting the check setting. The scale switch at the right of the panel changes the meter connection from the 5-percent full-scale range to the 2-percent range. This switch is arranged so that it is automatically thrown to the 5-percent range side each time the case is closed. 2. Setting the Mechanical Zero.-(a) If the mechanical zero setting of the meter is disturbed during transportation, remove the screws from the cover protecting the face of the meter, turn the small bakelite screw on the meter slowly, and set pointer to zero. The case must be closed, so that the electrical circuit is turned off when making this adjustment. 3. Connecting the Battery. (a) Before connecting the battery to the instru ment, be sure that knob "A" is turned to its extreme counterclockwise position. This is important. (b) The above precaution is necessary since the instrument will be turned ON as soon as the case is opened, and the combustion units are exposed to the possibility of an excessive voltage before the check setting can be adjusted to its proper value. (c) The detector is used with an Edison cap-lamp battery. When the battery cover is attached to a battery, open detector top, and depress and hold the check switch while twisting the battery cover. If there is any movement of the meter needle it is an indication that the battery contacts are corroded. Mine Safety Appliances Company, Information and Instructions for Model W-8 Methane Detector: 4 pp. FIGURE 13.-Diagram of Mine Safety Appliances Co. Type W-8 Make this Clean contacts by rubbing with fine emery cloth, and repeat test. test every time the battery cover is attached to a battery and also every time the instrument is to be used after standing for a period with the battery cover in place on the battery. This check need be made only on model "P" batteries. 4. Initial Adjustment in Fresh Air.-(a) Open the indicator case: If the battery has been attached, this operation will automatically start the instrument. (b) Adjust check setting: Depress and hold the check push button at the left of the panel, and turn knob "A" until the meter pointer is at the proper check setting. This check setting is marked on the etched portion of the cover glass. Release the check button. This initial adjustment of the check setting need be only approximate, since it serves only to protect the combustion unit against any possibility of an excessive voltage while the next operation is being performed. If the proper check setting cannot be reached, even though the "A" knob is turned to its extreme clockwise position, the battery is discharged and should be replaced. (c) Aspirate fresh air through the detector for 30 seconds by squeezing the bulb. Stop aspirating, and allow the instrument to stand for at least 2 minutes before performing the next operation. (d) Adjust the electrical zero: Throw the scale switch at the right of the panel to the "2" or most sensitive position. If the instrument is adjusted |