Gas module calibration

There are two types of calibration of the gas module:

1. Factory calibration pre-shipment. All gas modules are calibrated and must meet rigorous performance standards before being shipped. This is captured by the factory calibration certificate supplied with the monitor. Experience shows that Aeroqual’s factory calibration transfers well to the field, meaning that field calibration is not required during initial set up.
2. Field calibration. The monitor is designed so that the gas module can be calibrated in the monitor in the field. The field calibration set up and process is very similar to calibration of US / EU EPA approved ‘reference’ analyzers.
   Field calibration is only recommended when:
   • The gas module is clearly operating out of specification and the appropriate troubleshooting steps have been taken, or
   • Periodical validation of the data is required (recommended for advanced users only).

Field calibration is an activity that requires attention to detail and accuracy. You should read this entire calibration section and understand it before starting, even if you are an experienced practitioner.

The monitor is calibrated using a two-point calibration procedure, first by delivering zero air to the monitor inlet and adjusting the offset, and then delivering span gas to the monitor inlet and adjusting the gain.

Aeroqual provides all the equipment needed to perform field calibration. Aeroqual offers a portable calibrator called the AirCal 1000 and an integrated calibrator.

Aeroqual cannot supply calibration gas; this must be acquired locally.

Offset and gain adjustments are performed using Aeroqual Connect or Aeroqual Cloud.

If the AirCal 1000 is being used, there is a separate desktop application which can be downloaded from the website.

• Gas module calibration is achieved using certified calibration gas and a gas dilution calibrator.
• For ozone, an ozone calibrator must be used (ozone can’t be purchased in a cylinder). Aeroqual recommends and sells the Ozone Calibration Source by 2B Technologies (part number AQM O3CAL).
• When calibrating CO₂, gas is delivered directly to the gas inlet without the use of a calibrator.
• Aeroqual does not recommend calibration when first commissioning a new monitor. We recommend first calibrating 1-3 months after commissioning.
• The quality of the calibration gas makes a big difference to the success of a calibration. Only purchase calibration gas from a reputable calibration gas supplier.
• The monitor must be in a stable condition before calibration can be attempted. The monitor must have been running for at least 6 hours before calibration, ideally 24 hours.
• The zero calibration step takes approximately 30 minutes total across all modules. The span calibration takes approximately 30 minutes per module. Therefore, a monitor with 2 gas modules will take about 1.5 hours to calibrate (30 + (2*30)).

Typically, only advanced users will field calibrate their monitor. All other users can depend upon 12 monthly factory calibration of the gas module.

For some monitoring applications such as compliance monitoring, the calibration frequency may be regulated. Some monitoring applications may require periodical validation of the data and field calibration is the best way to achieve this.

Field calibration frequency is a balance between cost and data quality objectives. Field calibration will incur cost of time and materials. Therefore, more frequent calibration costs more because it requires more time and materials.

At the high-cost end of the spectrum are national ambient monitoring networks. The US EPA Quality Assurance Handbook is a good example of how prescriptive calibration requirements for such applications can be.

Aeroqual makes the following recommendations:

• Follow all local regulatory calibration requirements if these are defined for the monitoring activity.
• Plan routine calibrations at an interval that is consistent with the data quality objectives for your monitoring purpose.
• Field calibration may be necessary if the gas module is operating out of specification and all other troubleshooting steps have been taken.

This standard operating procedure assumes the monitor and calibration equipment are in good working condition and the monitor is ‘ready for calibration’.

• The monitor has been running for at least 6 hours.
• The gas lines and inlet filter are clean.
• You are familiar with the operation of all the calibration equipment.
• All calibration equipment, such as calibrators and gas bottles are working and within calibration where necessary.

Before you start, you’ll need to know:

• The total flow rate measured last time the monitor was serviced.
• The individual gas module flow rates last time the monitor was serviced.
• The current gain and offset for each module.

Record keeping is a crucial part of calibration. Information arising from the calibration should be entered into the tables in this form:

Calibration results form

You should also use the journal in Aeroqual Connect and Aeroqual Cloud to record calibration results and observations.

If I think the gas module isn’t responding correctly during calibration, first ensure you have followed the leak check and calibration instructions exactly. Should the expected response not occur, there may be a problem with the gas module.

Aeroqual gas modules have a finite lifetime and an unexpected response may be due to the sensor reaching the end of its life. If you think a gas module has failed or you’re unsure, email support@aeroqual.com.