Now that carbon dioxide (CO2) monitors are legislation in most countries and laws are in place in all states in the US since a while back, it is important to know how to live with them and how to make a ‘calibration’ or check-up of the functionality of the sensors.
It is important to know that a calibration adjustment doesn’t have to be made to a sensor that shows the right value – you don’t change the time on your watch if it is showing the right time.
This is a requirement in most places, but we find that it is neglected and not done according to the authorities’ directives. We believe that just as with elevators and such equipment, there should be a sticker on the sensor that indicates that it has been done and the time when it should be done again.
IR gas sensing principle
To understand the IR gas sensing principle, you have to know a bit of background. The CO2 measurement is based on the well-known principle of infrared (IR) absorption of radiation called the non-dispersive infrared (NDIR) technique.
This technique relies on the fact that molecules absorb light (electromagnetic energy) at spectral regions where the radiated wavelength coincides with internal molecular energy levels. In accordance to well-known quantum mechanical theory in physical chemistry, such energy resonances exists in the mid-infrared spectral region due to inter-atomic vibrations. Since different molecules are formed by different atoms (with different masses), the vibrational resonance frequencies (and wavelengths) are different for every substance. This fact is the basis for gas sensing through spectral analysis. By detecting the amount of light absorbed, within just a small spectral region that coincides with the resonance wavelength molecule, one gets a measure of the number of molecules of this particular substance. Well known properties of NDIR gas detection are:
The Lambert-Beer law describes the relation between resonant absorption A and gas concentration c:
Id = Io e-cds where A = (Io-Id )/Io.
Io is the incident light intensity; Id the transmitted light intensity; d the optical path length; and s the transition strength (a gas specific quantum mechanical constant).
In a typical NDIR gas sensor, an active IR light source is used to assure a high level of incident IR light flux Id onto a photo detector. For a given geometry, d is fixed and only two parameters (Io and s) remain to establish before this formula can be used to experimentally determine c. In practice, this is done using a two-step calibration procedure, where Io is determined first.
This first step is called the zero calibration, since it is performed by filling the optical path by a ‘zero-gas’ where c=0. Vacuum may be used here, but for practical reasons nitrogen is more commonly used as a buffer gas (nitrogen has no IR absorption).
The second calibration step, required to solve the remaining unknown parameter (s), is called the span calibration and involves the exposure of the optical path to a gas mixture with a known concentration, c.
Thereafter, Lambert-Beer law may theoretically be applied to measure c at any value. It is worth noting, that the span constant is closely related to the physical constants found in the exponent of formula (1), and hence is not expected to change with time for one and the same sensor.
This is unfortunately, not the case for the zero-calibration constant. We set Tzero and Tspan to compensate for deviants in temperature for each sensor.
Real life situations pose some limitations that must be considered in a practical device, such as component ageing and drift, system noise, and model imperfections. In order to solve these obstacles, using low cost approaches, LogiCO2’s gas sensor technology involves proprietary state-of-the-art solutions (see Figure 1) with innovative optics and microprocessor intelligence.
Component ageing & drift
Since the gas concentration is based on an absorption measurement where the absorption is detected on top of a large zero signal (Io), any small change of Io with time would be erroneously interpreted as a change of gas concentration (c).
Therefore, it is of vital importance to stabilise everything that affects Io, such as the active IR emission source, the optical transmission system, the IR detector and amplifier. The proprietary optical system is sealed from dust particles and has a mechanically rugged design. The detector is illuminated by a strong and stable optical signal, which minimises the need for further electrical amplifications and so reduces any associated amplifier problems.
Moreover, LogiCO2’s solution involves a careful study and selection of components, together with computer supervised manufacturing and sensor burn-in of each individual unit, plus a microprocessor-based intelligence that automatically detects and corrects any changes found during long term operation – the automatic baseline correction (ABC) algorithm.
In LogiCO2’s products each gas sensor measurement channel (x) is assigned to one 16-point calibration table, which contains a sensor specific linearisation curve for the raw data.
Calibration of individual channels, allowing for unit-to-unit variations compared to a fixed reference linearisation-curve, is made by the two parameters ZEROx and SPANx.
In all LogiCO2 IR gas sensors, both the ZEROx and SPANx constants are multiplicative – acting on the measurement data before linearisation takes place. ZEROx normalises the raw data to the counts value 61,440 (= system reference level) when no absorbing gas is present. This is done with a zero-gas exposed into the sensor cell. Alternatively, the raw data normalisation can be made at background level absorption. However, this is only done as a temporary solution in a last resort situation, since this assumes non-contaminated fresh air (410 ppm/sea level) to be present in the sensor cell, and so might give a less accurate result compared to when a zero gas is used. But even here you must take into consideration the altitude.
The SPANx constant is given from the factory calibration and does not vary with age.
In fact, untrained users attempting to span calibrate in the field without a proper climate chamber will most likely destroy the sensor’s functionality instead of improving it. This is especially the case at altitude, where the span test gas will not give a correct reading. The people attempting to do this must be informed that they are taking over the responsibility for the correct function of the sensor. It is a great responsibility.
The only alternative to this is to have a sensor made for ‘zero gas’ calibration, where the ‘zero-gas’ is used to read the sensor’s calibration at 0.00%. The factory span calibration is made in evacuated gas chambers under a very rigid control, avoiding the mistakes that are easily made by getting pressure in the tubes and not understanding how the test gas is calculated.