Gas cylinders are purchased at high concentrations, in a balance of air or nitrogen and then diluted to a lower concentration using a gas dilution calibrator such as the Aeroqual AirCal 1000 or Aeroqual AirCal 8000. Other gas dilution systems can also be used.
Span calibration gas must be delivered to the monitor in a balance of air.
Aeroqual doesn’t recommend calibrating from a cylinder of mixed gases. Only single gases should be used to calibrate Aeroqual gas modules.
Calibration directly from a gas cylinder without dilution isn’t recommended.
Recommended cylinders
Calibration gas should be purchased from a reputable calibration gas supplier. There’s a range of gas cylinder sizes available from small portable and disposable cylinders suitable for air freight through to large cylinders which must be correctly transported using appropriate safety protocols. Smaller cylinder sizes are generally more convenient, especially for field calibrations. Contact your gas supplier for advice, pricing and delivery.
Aeroqual recommends the following cylinders from Calgaz for portability:
- The 6D steel cylinder which holds 103 liters of gas.
- The 8AL aluminum cylinder which holds 58 liters of gas. It’s the same physical size as the 6D cylinder.
The following table lists the gases we recommend to span calibrate the most common gas modules. It shows concentrations for both the direct method and the dilution method.
Unfortunately, ozone is one of the few gases which can’t be stored in a cylinder due to its high reactivity. Ozone must be generated using a calibrated ozone generator. We recommend the Model 306 Ozone Calibration Source from 2B Technologies. You can buy this directly from Aeroqual (part number AQM O3CAL).
| Gas module | Calibration gas | Gas module measurement range | Direct method recommended concentration Min – max | Dilution method recommended concentration Min – max | Recommended Calgaz cylinder |
|---|---|---|---|---|---|
| O3 | O3 | 0 – 0.5 ppm | 0.1 ppm 0.08 – 0.15 | Ozone is delivered by an ozone generator | N/A |
| NO2 and Ox | NO2 | 0 – 0.2 ppm | 0.1 ppm 0.05 – 0.15 ppm | 20 ppm 10 – 100 ppm | 8AL 20 ppm NO2 Balance Air Part: A0446046 |
| NOx | NO2 or NO | 0 – 0.5 ppm | 0.1 ppm 0.05 – 0.2 ppm | 20 ppm 10 – 100 ppm | 8AL 20 ppm NO2 Balance Air Part: A0446046 |
| CO | CO | 0 – 25 ppm | 10 ppm 5 – 15 ppm | 1000 ppm 500 – 5000 ppm | 6D 1000 ppm CO Balance N2 Part: A0436734 |
| SO2 | SO2 | 0 – 10 ppm | 0.5 ppm 0.2 – 1.5 ppm | 20 ppm 10 – 100 ppm | 8AL 20 ppm SO2 Balance N2 Part: A0446071 |
| VOC | Isobutylene C4H8 | 0 – 20 ppm | 10 ppm 5 – 15 ppm | 1000 ppm 500 – 5000 ppm | 6D Isobutylene 1000 ppm Balance Air Part: A0436843 |
| H2S | H2S | 0 – '10 ppm | 0.5 ppm 0.2 – 1.5 ppm | 20 ppm 10 – 100 ppm | 8AL 20 ppm H2S Balance N2 Part: A0446004 |
| CO2 | CO2 | 0 – 2000 ppm (0.2 %) | 1000 ppm | 1000 ppm 500 – 1500 ppm | 6D 1000 ppm CO2 Balance Air Part: A0189932 (span) / A0194363 (zero) |
A single gas module calibration will use 1 to 2 liters per calibration if using Aeroqual calibration systems. This means that approximately 70 calibrations can be achieved with the 6D cylinder and 40 calibrations can be achieved with the 8AL cylinder.
Gas concentration
The above table shows the span gas concentrations recommended by Aeroqual, however, you can choose a concentration that best suits your calibration needs.
There are 2 approaches to choosing a span gas concentration. You can choose a concentration that:
- best represents the concentrations expected at the measuring site, or
- is 80 % of the full measurement range of the module. This is an approach specified by some manufacturers and is written into the standard operating procedures of some municipal monitoring networks based on reference monitoring equipment.
As an example, imagine the range of CO concentrations at your location are between 1 and 8 ppm with an average of 4 ppm. You might choose to calibrate the CO module at 5 ppm which in the middle of the expected measurement range and just above the average expected concentration. Or, you might just calibrate the CO module at 20 ppm, which is 80 % of the full range (0 to 25 ppm).
Cylinder size
Any gas cylinder size can be used with the portable AirCal 1000.
Any gas cylinder size can be used successfully with the integrated AirCal 8000, however the cylinder enclosure is only designed to hold the 6D and 8AL cylinders from Calgaz.
Larger cylinders can be used with the AirCal 8000 but they’ll need to be externally secured and protected.
Smaller-sized cylinders from Linde and Calgaz are easy to store and transport. Both brands have different outlet thread types so you must select a pressure regulator with the correct inlet connection type (thread) to match the cylinder connection.
Calgaz cylinders have a C10 (⅝ – 18 UNF) thread (C10 fitting) which easily fits the gas regulator supplied by Aeroqual.
Calibration gas suppliers
Certified calibration gas can be procured from international providers such as: