Calibration of Gas Sensors

gas-sensor

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According to the International Bureau of Weights and Measures , calibration is defined as:

“Operation that, under specified conditions, in a first step, establishes a relation between the quantity values with measurement uncertainties provided by measurement standards and corresponding indications with associated measurement uncertainties (of the calibrated instrument or secondary standard) and, in a second step, uses this information to establish a relation for obtaining a measurement result from an indication.”

Basically, it is setting up an instrument to read certain values when exposed to an environment with known or standard parameters.
Instrument accuracy is obtained only through frequent calibration through out the life time of the device in question. While gas sensors are not strictly analytical devices leaving a leeway for the sensor accuracy, it needs to be calibrated to ensure proper working conditions. Sensors fitted in a plant do not need the same level of precision as in the ones used in labs hence a rough calibration, done by any personnel, can have it working efficiently. Sensors are calibrated with a fixed schedule according to the time specified by the manufacturers, though portable sensors need frequent calibration than fixed, for safe environmental monitoring. Any instrument that needs to be calibrated is often compared to a highly accurate device with known set of parameters, normalizing the readings.

Most manufacturers recommend to perform a bump-test of the sensor at least monthly, a field or factory calibration or sensor check every 6-months, and a factory calibration or sensor replacement every 12-months.

During the initial usage of the sensor, it is good practice to check the sensor and re calibrate it after 30 days. The point being that the factors that were missed during the design of the sensor will surface by then at which point calibrating to the respective environment is necessary for effective monitoring.

Field Calibration

Performed at the place of installation with calibration gasses or devices that produce such gases. Most convenient when the sensors cant be removed to be send for calibration.

Zero calibration

Instruments are set to the zero reading through this calibration which is mostly very simple. The sensor is exposed to Zero air

Zero error

While analytical conditions use pure nitrogen or pure synthetic air as the zero point. But the practical approach is using the air surrounding the sensor, that is because atmosphere is a mixture of gases with a certain amount of water vapor. This is important because the absence of water can alter the zero point of the sensor post standard calibration causing drift especially for sensors like PID that require trace amount of moisture in the sample.
image Zero air

Calibration methods

  1. Surrounding air is normally clean
    If the instrument is indicating a close to zero reading, the first step is skipped. To double check, “clean air” in the facility collected via a plastic bag is exposed to the sensor for a couple of minutes. This is simple procedure to eliminate false alarm.
  2. Not so clean condition
    Use the compressed air that is available in plenty at the plant. Though there is a catch, such compressed air contains traces of gases that the sensor is designed to detect along with low humidity. To circumvent these the following procedures are followed.
    • activated charcoal filters most of the unwanted gases .
    • water vapor can be added into the air using a humidifier (usually a wet tissue) to the sampling system.
    • soda ash filter fixed in line with the system removes carbon dioxide.
    • Removal of Carbon monoxide CO is difficult and it has to be determined to be in same concentration in the sampling system and the ambient air.

Note: Sampling air is the air in the immediate proximity of the sensor under normal conditions. Another method to introduce water vapor to the sample is using Nafion tube.

Simple set up for calibration

Span Calibration

Calibrating the actual measurement range of the instrument is called span calibration. Done by sensing a known quantity of the calibration gas.

Span Error

The best calibration gas is a mixture of the target gas balanced in the background environmental air. While manually preparing that needs an expert, few calibration gases are commercially available.

  1. Premixed Calibration Gas
    Premixed gas mixtures are compressed and stored under pressure in a gas bottle. These small portable bottles can be
    • low-pressure- thin-walled, lightweight disposable bottles.
    • high-pressure- thick-walled aluminium pure hazardous chemicals with service pressure of 2000 psi.

A regulator assembly consisting of a pressure regulator, a pressure gauge, and an orifice flow restrictor is used To get this highly pressurized gas out of the bottle . The flow restrictor with a hairline hole for a constant air flow at a given pressure difference.
a constant air flow is achieved by reducing the high pressure from the bottle to only a few psi. Flow rates between 600-1000 cc/ min are most common. Flow rate is adjusted by the pressure regulator.gas sensor

  1. Permeation Devices
    They are sealed container that contains chemicals in liquid and vapor phase equilibrium, where in the chemical potential of the contents in stable over a period of time. The contents as gas molecules permeate through the container wall or the end cap made of permeable material. The material permeability and temperature determine the rate of the permeability which is constant over a period of time. At these constant conditions, a calibration gas is formed by mixing the air at constant flow rate with the gas molecules.
    Disadvantage : Continuous emission of chemicals is a safety issue and may not be compatible for diverse application hence they find use in laboratory analysis equipment calibration.

Bump Testing

Performed every month to ensure that the sensors are in working condition providing the appropriate signals. A known quantity of gas above the allowable range is sensed by the probe to determine its working condition, intensionally triggering the visual and audio alarms. The gas cylinder is quite small and portable as opposed to the one used in proper calibration. This is not precise hence can not be used as an alternative to calibration
Automatic bump tests are done using a docking station which are programmed to detect anomalies in the sensor.
Manual bump test utilizes a small portable cylinder of gas, a regulator, tubing, a calibration cup (if using a diffusion instrument), and a gas detector


Manual Bump Test

Steps for bump test :

1 - switch on the Gas detector
2 - Insert the tubing into the nozzle of gas cylinder
3 - Connect the tubing to the calibration cup
4 - fix the cup onto the gas detector
5 - Pinch the tube into a bend
6 - Pump air, the balloon inflates
7 - Alarm and other indicators are set off in the detector.