COMBUSTIBLE GAS INDICATORS

The combustible gas indicator is an instrument used to detect various flammable gases and vapors. Combustible gas indicators are made by several manufacturers. Although they all operate on the same general principles, they vary in certain details. Each indicator is accompanied by a complete set of operating and maintenance instructions. In addition, a condensed operating routine is attached to the inside of the case cover. Before using any combustible gas indicator, read the instructions carefully.

A combustible gas indicator employs a heated platinum-wire filament associated with a Wheat-tone bridge or modified potentiometertype circuit. The combustible gas indicator can quickly, safely, and accurately detect all combustible gases or vapors associated with fuel oils, gasoline, alcohol, acetone vapors, illuminating gas, fuel gas, hydrogen, and acetylene in mixtures with air or oxygen. The indicator can detect small quantities of these gases or vapors up to the lower explosive limit (LEL). It gives a reliable indication of the mixture if it is in the upper explosive range or beyond the upper explosive limit. The higher and lower explosive limits define the range of a concentration of a material, expressed in the percent in the air, that will burn or explode if ignited. The LEL is the minimum percent by volume of a gas that, when mixed with air at normal temperature and pressure, will form a flammable mixture. The higher explosive limit (HEL) is the point at which an explosion is certain. The combustible gas indicator does not identify combustible gas or gases; it merely indicates that one or more combustible gases are present in a certain proportion.

The combustible gas indicator is provided with flame arresters to prevent flashbacks. The three combustible gas indicators you will be concerned with are the Navy type E, the Model GN type E, and the Model 40.

Navy Type E Combustible Gas Indicator

The Navy type E combustible gas indicator (fig. 6-34) has a scale of 0% to 100% LEL. The

Figure 6-34.-Combustible gas indicator, Navy type E.

scale is color-coded red above the 60% mark to readily indicate dangerous gas concentrations. Use the following procedures to operate this combustible gas indicator.

1. Open the cover to expose the rheostat knob and the indicator panel.

2. Connect the amount of sampling hose that you will need.

3. Lift the left end of the rheostat knob ON-OFF bar and turn the rheostat knob one-quarter of a turn clockwise.

4. Flush the combustible gas indicator with fresh air by squeezing the aspirator bulb five times. Squeeze the aspirator bulb another two times for each 10 feet of sampling hose that is connected to the indicator.

5. Adjust the rheostat knob until the meter pointer rests at the zero mark on the scale. If the needle stays to the left of the zero mark with the rheostat knob turned completely to the clockwise direction, the batteries are weak and need to be replaced. Six D-cell batteries (BA-30) are required to operate this indicator. If the needle moves completely to the right of the scale and cannot be adjusted to the zero mark, the detector filament is burned out and needs to be replaced. A spare detector filament is located under the indicator panel.

6. Place the end of the sampling hose inside the space to be tested.

7. Readjust the meter pointer to zero if necessary.

8. Squeeze the aspirator bulb five times for the indicator and two times for each 10 feet of sampling hose being used. Continue checking various locations within the space until the space is determined to be safe or a gas concentration is located. If a gas concentration is detected, continue checking and note where the highest reading is obtained.

9. Flush the unit in fresh air by squeezing the aspirator bulb five times for the indicator and two times for each 10 feet of sampling hose used.

10. To secure the indicator, turn the rheostat knob counterclockwise until the arrow on the knob points to the OFF position. The locking bar will drop into position in its slot. This indicates that the rheostat is in the OFF position.

11. Secure the cover.