This post explains how we calibrate high end sensors and we will also look at the pros and cons of using high end calibration on low cost sensors
In our previous blog post, we explained major challenges faced by low cost AQ sensors. We have used the words sensor/monitor/instrument interchangeably.
Calibrating high end sensors
Normally, for high-end (expensive) sensors, the calibration lasts for little over a 1 year. Assuming they are not suffering from any mechanical damage e.g. due to vibration or tilt. Periodical calibration is recommended if you depend upon high accuracy and precision.
Example: NO2 sensor calibration for high end sensors
Here are two methods we use to calibrate an NO2 sensor.
Method 1: Zero and Span Check
In this method, we calibrate the instruments with the primary standard (certified gas cylinders). Main components needed are:
Dilution system with certified Mass Flow Controls (MFC).
NO2 Gas cylinder (1000 ppb) with the national and/or international standards such as Bundesanstalt für Materialforschung und -prüfung/BAM, National Institute of Standards and Technology/NIST, Nederlands Meetinstituut/NMi, NPL.
“Zero air” Gas cylinder with the above mentioned standard.
Sampling manifold.
A dilution system, fed by the gas cylinders generates different NO2 mixing ratios. This is then fed to the instrument together with Zero Air. This is typically called a "zero and span check". A calibration curve (x-y plot) is fitted to get a linear curve (y=ax+b). The parameters of this curve can then be used to scale the latter instrument output.
Method 2: Intercomparision with certified instruments
This typically requires the following steps.
Use a professional (typically very expensive) calibrated instrument, with certified standards.
Put your instrument(s) inlet side-by-side with the calibrated instrument for a while to measure the ambient NO2.
Plot and fit the linear curve with both outputs. Be careful about the conclusions you make under these conditions. e.g. You may want to use your sensor to measure 100ppb NO2, but your environment may never reach that level. It is like trying to measure 100m with a 5m long ruler.
Calibrating low cost sensors
For most of us, a low-cost device is more accessible compared to the high-end instruments. Unfortunately, today there are no standards for such sensors, only vague guidelines. This has led to a flood of low quality monitors and confusion for the consumer.
Additionally, even if some manufacturers claim they are tested and calibrated, not all manufacturers offer genuinely good quality. The calibration may only last for a weeks/ months due to different calibration methods and different instrumentation quality.
Can we calibrate low cost sensors in the same way as the expensive sensors?
It is possible to apply the 2 calibration methods used for high end sensors, however there are pros and cons that must be considered.
Method 1. Zero and span check
We already know that the low-cost sensors are highly affected by environmental parameters. The gas generated from gas cylinders is normally 25 degree Celsius, 0% RH, which is ideal for calibration but not for a real world application. So you have to manually change the environmental parameters for a wider range calibration.
Pros
We can easily control the parameters such as Temperature, relative humidity, gas mixing ratio.
Cons
The low-cost sensors have a feature called “cross-interference”. e.g., the NO2 sensor may response to NO gas and O3 gas. This can make things unpredictable.
2. Intercomparision with certified instruments
Pros
It is possible to test and calibrate the sensor under real world conditions e.g. high traffic.
Cons
We cannot simulate every ambient condition e.g. extreme weather. So tuning the sensor can take some time while we can get enough samples from the required conditions
We often use different calibration contexts e.g. field calibration using high and low cost instruments. Here are some pictures from field calibration...
Whats next ...
We have been continuously improving the methods and algorithms required for Calibration. Moreover an automatic calibration method is also undergoing field trials. Stay tuned.
Sheng Ye, Research Scientist, Signify GmbH München
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