COMBUSTIBLE GAS METERS BASIC INFORMATION AND TUTORIALS

COMBUSTIBLE GAS METERS FOR SAFETY ENGINEERING
What Are Combustible Gas Meters?



These meters use elements which are made of various materials such as platinum or palladium as an oxidizing catalyst. The element is one leg of a Wheatstone bridge circuit. These meters measure gas concentration as a percentage of the lower explosive limit of the calibrated gas.

The oxygen meter displays the concentration of oxygen in percent by volume measured with a galvanic cell. Other electrochemical sensors are available to measure carbon monoxide, hydrogen sulfide, and other toxic gases. Some units have an audible alarm that warns of low oxygen levels or malfunction.

Calibration of Combustible Gas Meters 
Before using the monitor each day, calibrate the instrument to a known concentration of combustible gas (usually methane) equivalent to 25%-50% LEL full-scale concentration.

The monitor must be calibrated to the altitude at which it will be used. Changes in total atmospheric pressure from changes in altitude will influence the instrument's measurement of the air's oxygen content. The unit's instruction manual provides additional details on calibration of sensors.

Special Considerations.
• Silicone compound vapors, leaded gasoline, and sulfur compounds will cause desensitization of the combustible sensor and produce erroneous (low) readings.

• High relative humidity (90%-100%) causes hydroxylation, which reduces sensitivity and causes erratic behavior including inability to calibrate.

• Oxygen deficiency or enrichment such as in steam or inert atmospheres will cause erroneous readings for combustible gases.

• In drying ovens or unusually hot locations, solvent vapors with high boiling points may condense in the sampling lines and produce erroneous (low) readings.

• High concentrations of chlorinated hydrocarbons such as trichloroethylene or acid gases such as sulfur dioxide will depress the meter reading in the presence of a high concentration of combustible gas.

• High-molecular-weight alcohols can burn out the meters filaments.

• If the flash point is greater than the ambient temperature, an erroneous (low) concentration will be indicated.

If the closed vessel is then heated by welding or cutting, the vapors will increase and the atmosphere
may become explosive.

• For gases and vapors other than those for which a device was calibrated, users should consult the manufacturer's instructions and correction curves.

Maintenance of Combustible Gas Meters
The instrument requires no short-term maintenance other than regular calibration and recharging of batteries. Use a soft cloth to wipe dirt, oil, moisture, or foreign material from the instrument. Check the bridge sensors periodically, at least every six months, for proper functioning.

A thermal combustion-oxygen sensor uses electrochemical cells to measure combustible gases and
oxygen. It is not widely used in the area offices.

PORTABLE FIRE EXTINGUISHER SAFETY TIPS BASIC INFORMATION AND TUTORIALS

SAFETY TIPS ON THE USE OF PORTABLE FIRE EXTINGUISHERS



A portable fire extinguisher can save lives and property by putting out a small fire or containing it until the fire department arrives; but portable extinguishers have limitations. Because fire grows and spreads so rapidly, the number one priority for residents is to get out safely.

Safety tips
Use a portable fire extinguisher when the fire is confined to a small area, such as a wastebasket, and is not growing; everyone has exited the building; the fire department has been called or is being called; and the room is not filled with smoke.

To operate a fire extinguisher, remember the word PASS:
 - Pull the pin. Hold the extinguisher with the nozzle pointing away from you,  and release the locking mechanism.
 - Aim low. Point the extinguisher at the base of the fire.
 - Squeeze the lever slowly and evenly.
 - Sweep the nozzle from side-to-side.

For the home, select a multi-purpose extinguisher (can be used on all types of home fires) that is large enough to put out a small fire, but not so heavy as to be difficult to handle.

Choose a fire extinguisher that carries the label of an independent testing laboratory.

Read the instructions that come with the fire extinguisher and become familiar with its parts and operation before a fire breaks out.

Install fire extinguishers close to an exit and keep your back to a clear exit when you use the
device so you can make an easy escape if the fire cannot be controlled. If the room fills with smoke, leave immediately.

Know when to go.
Fire extinguishers are one element of a fire response plan, but the primary element is safe escape. Every household should have a home fire escape plan and working smoke alarms.

WORKING ON OR NEAR DE-ENERGIZED EQUIPMENT AS SUGGESTED BY IEEE STD 902-1998

WHY WE SHOULD WORK ON NEAR OR DE-ENERGIZED EQUIPMENT?

Working on or near de-energized equipment.

The definition of the term de-energized can be found in IEEE Std 100-1996 and in several other documents. It is defined as "free from any electrical connection to a source of potential difference and from electric charge; not having a potential different from that of the earth".

At first thought, some people might think that they are safe if the electrical equipment on which they are going to work is de-energized. However, things are not always as they appear.

The unexpected happens. A person should think, "What if...?." What if the wrong disconnect switch was opened? Or, since you can't watch the switch and work at the same time, what if someone turns the switch back on while you are busy working?

What if a source of voltage from another circuit somehow gets accidentally connected onto the conductors on which you are going to work? What if a very large induced voltage is present? The point is that there are several things to consider to ensure a person's safety while working. De energizing is only one part of creating an electrically safe work condition.

Establishing an electrically safe work condition
In the past, the methods that electrical personnel followed to protect themselves were lumped into a term called clearance procedures. In some cases, clearance simply meant permission to work on a particular system, whether it was energized or not.

In other cases, clearance meant taking measures to ensure that equipment is de-energized, and to reinforce those measures with formal safeguards against altering that de-energized status for as long as clearance is required. The latter use of the word clearance is closer to the hazardous energy control requirements in place today.

The term clearance is falling out of use in modern electrical safety terminology because it does not mean safety. Clearance (for work) is defined in 29 CFR 1910.269 as "authorization to perform specified work or permission to enter a restricted area."

Today, for safety purposes, the phrase "establish an electrically safe work condition" is preferred. An electrically safe work condition is defined in Part II of NFPA 70E- 1995. Section 2-3.1.3 of that document states

An electrically safe work condition shall be achieved and verified by the following process:

a) Determine all possible sources of electrical supply to the specifc equipment. Check applicable up-to-date drawings, diagrams, and identifcation tags.

b) After properly interrupting the load current, open the disconnecting device(s) for each source.

c) Where it is possible, visually verify that all blades of the disconnecting devices are fully open, or that drawout type circuit breakers are withdrawn to the fully disconnected position.

d) Apply lockout/tagout devices in accordance with a documented and established  policy.

e) Use an adequately rated voltage detector to test each phase conductor or circuit part to verify that it is de-energized. Before and after each test, determine that the voltage detector is operating satisfactorily.

f) Where the possibility of induced voltages or stored electrical energy exists, ground the phase conductors or circuit parts before touching them. Where it could be reasonably anticipated that the conductors or circuit parts being de-energized could contact other exposed energized conductors or circuit parts, apply ground connecting devices rated for the available fault duty.

When nondrawout, molded-case circuit breakers are being used as the disconnecting device mentioned in item b), visual verification of an open circuit, as suggested in item c), cannot be made.

One technique that could be used to verify true opening is to have a voltmeter, or other voltage indicating device, safely applied somewhere away from the breaker enclosure itself on the load side of the breaker before the breaker is opened.

Always try to place the voltmeter at a point where exposure to energized conductors is minimized. Then, have someone watch the meter as the breaker is being opened. Simultaneous opening of the breaker and disappearance of voltage is generally a good indicator of disconnection.

If that can't be done, the next best way is to measure load-side voltage (using safe practices and appropriate protective and test equipment), remove the meter, open the breaker, and measure again immediately. With multiple pole systems, all load-side poles should be verified to have voltage prior to disconnection.

Again, apply a voltmeter to one of the poles. After the breaker is opened and the first pole is verified, move the meter, as safely and quickly as possible, to verify deenergization of the other poles.