DC MUSEUM

M.S.A. EXLOSIMETER MODEL 2 OPERATING AND SAFETY PRECAUTIONS

The M.S.A. EXOPLOSIMETER is set in its proper operating condition by the adjustment of a single control. This control is a rheostat regulating the current to the EXPLOSIMETER measuring circuit. The rheostat knob is held in the “OFF” position by a locking bar. This bar must be lifted before the knob can be turned from the “OFF” position.

To test for combustible gases or vapors in an atmosphere operate the EXPLOSIMETER as follows:

1. Lift the left end of the rheostat knob “ON-OFF” bar and turn the rheostat knob one-quarter turn clockwise.
    
   This operation closes the battery circuit. Because of unequal heating on the various circuits elements, the meter pointer will move rapidly up the scale and then return slowly to some point below zero.
    
   
2. Flush fresh air through the EXPLOSIMETER.
    
   The circuit of the EXPLOSIMETER must be balanced with air free of combustible gases or vapors surrounding the detector filament. Five squeezes of the aspirator bulb are sufficient to flush the combustion chamber. If a sampling line is used an additional two squeezes will be required for each ten feet.
    
   
3. Adjust rheostat knob until meter pointer rests at zero.
    
   Clockwise rotation of the rheostat knob causes the meter pointer to move up scale. A clockwise rotation sufficient to move the meter pointer considerably above zero should be avoided as this subjects the detector filament to an excessive current and may shorten its life.
    
   
4. Place end of sampling line at, or transport the EXPLOSIMEMTER to the point where the sample is to be taken.
    
   
5. Readjust meter pointer to zero if necessary by turning rheostat knob.
    
   
6. Aspirate sample through EXPLOSIMETER until highest reading is obtained.
    
   Approximately five (5) squeezes of the bulb are sufficient to give maximum deflection if a sampling line is used; add two (2) squeezes for each ten (10) feet of line.
    
   This reading indicates the concentration of combustible gases or vapors in the sample.
    
   The graduations on the scale of the indicating meter are in percent of the lower explosive limit. Thus, a deflection of the meter pointer between zero and 100% shows how closely the atmosphere being tested approaches the minimum concentration required for explosion. When a test is made with the EXPLOSIMETER and the meter pointer is deflected to the extreme right side of the scale and remains there, then the atmosphere under test is explosive.
    
   If the meter pointer moves rapidly across the scale and on continued aspiration quickly returns to a position within the scale range or below zero, it is an indication that the concentration of flammable gases or vapors may be above the upper explosive limit. To verify this, immediately aspirate fresh air through the sampling line or directly into the EXPLOSIMETER. Then, if the meter pointer moves first to the right and then to the left of the scale, it is an indication that the concentration of flammable gas or vapor in the sample is above the upper explosive limit.
    
   When it is necessary to estimate or compare concentration of combustible gases above the lower explosive limit a Dilution Valve may be employed.
    
   The meter scale is red above 60% to indicate that gas concentration within that range is very nearly explosive. Such air-gas mixtures are considered unsafe where men must work.
    
   
7. To turn instrument off: Rotate rheostat knob counterclockwise until arrow on knob points to “OFF”. The locking bar will drop into position in its slot indicating that the rheostat is in the “OFF” position.