About Calibration

Every gas monitoring system needs to be calibrated. Each gas-monitoring channel is calibrated separately.

HOW OFTEN DOES A GAS MONITORING SYSTEM NEED TO BE CALIBRATED?

Although some companies calibrate their systems only once or twice a year, the recommendation is that the system calibration (for all sensors except infrared) be re-calibrated after 30-days and be checked at every 90 days thereafter. Checking it every 30 days would be considerably safer. Our system can detect many fail conditions, but not every fail condition. For instance, if a sensor has become poisoned or access to it has been blocked, it may lose some or all of its sensitivity.

(Poisons: Sensors can be quickly destroyed or poisoned by certain materials. Even low concentrations of poisoning substances can cause serious problems. The two most common phenomena are coating and etching.

Blocking: Certain conditions can cause a sensor not to function. When this happens, normal gas sensing is blocked until the conditions are removed. The most common block is lack of oxygen. Sensor flooding can cause Oxygen deprivation.)

Calibration is the only way to detect this condition. In addition, as sensor’s age, they begin to lose their calibration more quickly. Sensors older than one year need more frequent calibration to maintain their accuracy and to prevent false alarms. Calibration is also necessary whenever a sensor is replaced.

INITIAL CALIBRATION:

An initial calibration can be made within 30 minutes after system power is applied to the sensor. However, to achieve accuracy, a second calibration should be made no sooner than 24 hours after systems power is applied.

WHAT IS REQUIRED FOR CALIBRATION?

Calibration gases: Zero air and Span Gas
Calibration gas regulator & sensor connector assembly

TWO TYPES OF CALIBRATION:

FLOW and STATIC

A calibration system must be chosen for each gas to be monitored. Some gases can be calibrated by a flow method; others are considered too dangerous to use this method and a static system is provided for them. There is only one choice per gas.

FLOW CALIBRATION KITS

Combustible Gases includes everything (a sensor gas connector with tubing, a pressure and flow regulator assembly, a calibration adapter, and a calibration screwdriver), except the gas bottle, needed to calibrate a sensor and comes in a carrying case which can hold one or two bottles of cal. gas.
 
Calibration Gas Selection for Combustible Gas Detectors
It is not always desirable or necessary to calibrate with the specific gas which is most likely to be present.

STATIC CALIBRATION KITS

Toxic Cases Static calibration systems come in one form which includes a Calibration Chamber, a gas calibration gauge and valve assembly, a calibration screwdriver, and a calibration adapter. You must select the gas to be used separately. These gas canisters are 36 litres and provide approximately 400 calibrations.

If there is a background gas, a canister of zero air and a calibration kit should be selected so that a true zero calibration can occur. This is especially important for toxic gases such as hydrogen sulfide where a large background can exist without the calibrator’s knowledge.

ZERO CALIBRATION

ZERO calibration requires that the sensor be exposed to clean air. The best way to assure accurate ZERO calibration is to apply a source of clean or Zero Air to the sensor. Once this is done, calibration with the sense gas can be initiated.

Zero gas is clean air, and is an excellent way of insuring that a small release of gas is not near the sensor while zeroing the sensor signal during calibration.

SPAN CALIBRATION

Remove the clean air source and apply the span gas of the appropriate concentration to the sensor.

CATALYTIC BEAD SENSORS

The most popular sensing method for Combustible Hydrocarbon (CHC) gases. Not used for toxic gas monitoring. This is the best all around sensor for ambient CHC monitoring. It is simple, reliable, and inexpensive. The disadvantages apply mainly in atypical applications.

Principle of Operation. A coil of wire is coated with glass or ceramic material that is, in turn, coated with a catalyst. The coil is electrically heated to a temperature that will allow it to burn (catalyse) the CHC being monitored. When burned, all CHCs liberate heat proportional to the concentration of the CHC present. This heat increases the temperature of the wire coil, increasing its resistance. The increase in resistance is measured electrically and is the source of the signal.

Advantages of Catalytic Sensors: Sensors are small, can be made mechanically rugged, signal is generated directly by actually burning the gas which is the property of the gas being measured; specific - only measures combustible gases; inexpensive; long life - usually 2 to 4 years; simple to calibrate and maintain.

Disadvantages of Catalytic Sensors: Span loss is undetectable except during calibration, can be poisoned by trace amounts of silicone, halocarbons and metallo-organic substances unless special methods of prevention are used, small signal-to-noise ratio, signal can go back to zero in CHC concentrations above their stoichiometric mixtures, some sensors can be permanently damaged when exposed to high or continuous concentrations of gases being measured (Alphasense offers highly poison resistant catalytic sensors).