The principles of the LogiCO2 Safety Sensors and Grow Sensor
All CO2 sensors need to be tested for the correct function. Our Non-Dispersive Infrared (NDIR) CO2 sensors are no exception. The check for exact calibration in the field is done by exposing the sensor to a known gas that is not affected by pressure or temperature. If we expose the sensor to 100% Nitrogen the sensor should show 0% CO2. If the sensor shows another value, it must be calibrated.
The IR Gas sensing principle
The CO2 measurement is based on the well-known principle of infrared (IR) of radiation called the Non-Dispersive Infra-Red (NDIR) technique. This technique relies on the fact that molecules absorb light (electromagnetic energy) at spectral regions where the radiated wavelength coincides with the internal molecular energy levels. In accordance to well-known quantum mechanical theory in physical chemistry such energy resonances exist 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 specie.
This fact is the basis for gas sensing through spectral analysis. By detecting the amount of absorbing light, within just a small spectral region that coincides with resonance wavelength of the specie selected, one gets a measure of the number of molecules of this particular specie, free from interreferences of other species.
Well known properties of NDIR gas detection are:
- High selectivity – free from cross- interference
- Sensitive and accurate
- Environmentally resistant
- Able to stock over long time periods
- No over exposure problems (no negative memory effects or exposure hysteresis)
- Described by relatively simple physics (predictable)
The proprietary optical system is hermetically sealed from dust particles and has a mechanically rugged design. The detector is illuminated by a strong and stable optical signal, which minimizes the need to need for additional electrical amplification and so reduces any amplifier problems.
Moreover, the Logico2 solution involves a careful selection of components, together with computer supervised manufacturing and sensor burn-in of each individual unit, plus microprocessor based intelligence that automatically detects and corrects any changes during long term operation.
This Extended Automatic Baseline Correction algorithm, Unique for LogiCO2, can of course be turned off by a simple adjustment depending on how/where the sensor is used. The sensor must then be calibrated manually.
How Often Should A CO2 Sensor Be Manually Calibrated?
The more accurate the CO2 reading needed, the more often it should be Calibration checked and if needing Calibration, Calibrate. However, LogiCO2 recommends customers to put their sensors on a regular calibration check schedule.
For specific industries we recommend the following schedules:
- Scientific Experimentation – zero check/calibrate before each test
- Safety – zero check/ calibrate at least annually
- Beverage/Restaurant – zero check/calibrate at least annually
- Greenhouse – zero check/calibrate after each growing cycle (see below)
- Breweries and Soft Drink plants – zero check/calibrate at least annually
These are general guidelines. Calibration schedules may also be dictated by experimental protocols or by particular industrial standards.
In addition, please reference your devices calibration procedure in the product manual for additional guidelines depending on your specific gas detector/monitor.
It is important to know that a sensor with activated Extended Automatic Baseline Correction can be calibrated in the same simple way, as a sensor with deactivated Extended Automatic Baseline Correction.
Calibration principle in LogiCO2’s products each gas sensor measurement channel x is assigned to one 16-point calibration table, which contains a sensor specific linearization curve for the raw data. Calibration of individual channels, allowing for unit-to-unit variations as compared to a fixed reference linearization 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 linearization takes place. ZEROx normalize the raw data to the counts value 61440 (= system reference level) when no absorbing gas is present. This is conveniently done with a zero-gas exposed into the sensor cell. Alternatively, the raw data normalization 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 as compared to when a true zero gas is used. 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 performance rather than improve it. Especially at altitude where the span test gas will not give a correct reading. The factory span calibration is made in evacuated gas chambers under a very rigid control avoiding the loopholes otherwise easily made.
With Activated LogiCO2 Extended Automatic Baseline Correction Sensor Technology
- The LogiCO2 system has two options for Sensor adjustments: Self-Correcting which is the default setting, or Manual-adjustment.
- Self-Correcting can be turned off requiring a manual adjustment for calibration verification using nitrogen to determine “zero carbon dioxide” at minimum annually or according to industrial standards.
- The LogiCO2 Sensor Self-Corrects on two-month intervals.
- The default target value for the Sensor is 400 ppm.
- During the two-month cycle the system continuously monitors for the lowest ambient CO2 value which lasts greater than 60 seconds, and stores that value during the period.
- This lowest encountered ambient value becomes the reference level for adjusting the Sensor at the end of the two-month period.
- The maximum adjustment which can occur at the end of the two-month cycle is 150 ppm.
This means that a sensor with Activated Extended ABC functionality can have a maximum of error of 300 ppm after two months (8 weeks) if it has been exposed to about 1200ppm continuously under this period and never been exposed to ambient air. The alarm levels are 5000 ppm and 30 000 ppm. The error will be 6% at 5000 ppm and 1% error at 30 000 ppm if maximum correction occurs. Please note that OSHA allows 5000ppm for 8 hours (TWA).
If sensor is forgotten:
Theoretically, if 1200 ppm is the reference during a twelve-month period, it would max adjust 800 ppm up to 1200 ppm since the difference is 800 ppm between the target value 400 ppm where it started from the original default setting and 1200 ppm which was the highest level encountered for greater than 60 seconds.
If a sensor has theoretically adjusted 800 ppm and is then exposed to an outside atmosphere of 400 ppm the sensor would measure a value of -400 ppm, if this happens, the sensor adjusts the value back up to compensate this mistake, because a sensor cannot be exposed to a negative value CO2 since it does not exist. The sensor is constantly seeking the lowest value, that lasts longer than 60 seconds.