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.

TOXIC GAS METERS BASIC INFORMATION AND TUTORIALS

What Are Toxic Gas Meters?

Toxic Gas Meters

Description and Application. 

This analyzer uses an electrochemical voltametric sensor or polarographic cell to provide continuous analyses and electronic recording. In operation, sample gas is drawn through the sensor and absorbed on an electrocatalytic sensing electrode, after passing through a diffusion medium. 

An electrochemical reaction generates an electric current directly proportional to the gas concentration. The sample concentration is displayed directly in parts per million. 

Since the method of analysis is not absolute, prior calibration against a known standard is required. Exhaustive tests have shown the method to be linear; thus, calibration at a single concentration, along with checking the zero point, is sufficient.

Types: Sulfur dioxide, hydrogen cyanide, hydrogen chloride, hydrazine, carbon monoxide, hydrogen sulfide, nitrogen oxides, chlorine, and ethylene oxide. These can be combined with combustible gas and oxygen meters.

Calibration.

Calibrate the direct-reading gas monitor before and after each use in accordance with the manufacturers instructions and with the appropriate calibration gases.

Special Considerations.

• Interference from other gases can be a problem. See manufacturers literature.

• When calibrating under external pressure, the pump must be disconnected from the sensor to avoid sensor damage. If the span gas is directly fed into the instrument from a regulated pressurized cylinder, the flow rate should be set to match the normal sampling rate.

• Due to the high reaction rate of the gas in the sensor, substantially lower flow rates result in lower readings. This high reaction rate makes rapid fall time possible simply by shutting off the pump. Calibration from a sample bag connected to the instrument is the preferred method.

PHOTOIONIZATION METERS BASIC INFORMATION AND TUTORIALS

What Are Photoionization Meters?

Description and Applications.
Ionization is based upon making a gas conductive by the creation of electrically charged atoms, molecules, or electrons and the collection of these charged particles under the influence of an applied electric field.

The photoionization analyzer is a screening instrument used to measure a wide variety of organic and some inorganic compounds.

It is also useful as a leak detector. The limit of detection for most contaminants is approximately 0.1 ppm.

Calibration. The procedure for calibration involves applying the
calibration gas (typically 100 ppm isobutylene) to the instrument
and checking the reading.

Special Considerations.
The specificity of the instrument depends on the sensitivity of the detector to the substance being measured, the number of interfering compounds present, and the concentration of the substance being measured relative to any interference.

Many models now have built-in correction or correlation factors. After calibrating the unit on isobutylene, select the gas to be measured.

The instrument will automatically correct for the relative sensitivity of the gas selected. Some instruments are listed by an NRTL for hazardous locations.

Check the operating manual for specific conditions.

Maintenance.
Keeping these instruments in top operating shape means charging the battery, cleaning the ultraviolet lamp window, light source and replacing the dust filter. The exterior of the instrument can be wiped clean with a damp cloth and mild detergent if necessary.

Keep the cloth away from the sample inlet, however, and do not attempt to clean while the instrument is connected to line power.