Field calibration for SCD30
CO2, humidity, and temperature sensor
Preface
NDIR based CO2 sensors such as SCD30 are delicate
optical systems. Special care has to be taken during
handling of the sensors. Mechanical stresses on the
optical cavity could change physical properties of thesensor affecting the calibration and thus, resulting in
reduced accuracy.
SCD30 features two calibration algorithms to restore fullaccuracy, in cases where mechanical stress alters
physical properties of the sensor.
This document explains how to use the field calibrationalgorithms ASC (automatic self-calibration) and FRC(forced re-calibration) to restore full accuracy of theSCD30. Implementing these algorithms in a production
environment and at end user is discussed separately.

Overview

Providing a valid reference value is crucial for correctmanipulation of the calibration curve. The reference value
can be generated in three ways - from a calibrated
reference sensor; by exposing the SCD30 to an
environment with controlled CO2 concentration; or from
fresh outside air (= 400 ppm). It is recommended that CO2concentration values serving as reference for FRC, be in
the range 400 ppm to 2000 ppm.

The algorithms ASC and FRC adapt the calibration of theSCD30, which allows for compensation of the sensors’ drift
over the entire concentration range. Both algorithms require
only a single reference value for the desired manipulation.
It is important to note that FRC and ASC do not differ in theapplied adaption of the calibration – using the samereference value for ASC and FRC will result in an identicalmanipulation of the calibration.

Figure 1: Workflow to restore sensor accuracy by ASC or FRC.

The major difference between ASC and FRC is the sourceof the reference value. FRC requires an input from the hostsite whereas ASC generates a reference valueautonomously from historical sensor data.
For calibration in a production line, we recommend to apply
FRC after assembly of the device. In applications where theend device is placed in well-ventilated buildings, werecommend to activate ASC to account for potential sensor
drifts. If conditions at the end user do not fulfill therequirements necessary for ASC (see below), FRC
provides means to mitigate deviations.

FRC Field-Calibration Algorithm
In case of forced re-calibration (FRC), the user has to feeda reference value from the host side to SCD30. The value
provided by the host is used for updating the calibration torestore full accuracy. The latest reference value is saved ina non-volatile memory on the sensor device and will persist
until it is overwritten either by a new reference value that issupplied via FRC or by finding a new reference value byASC (see Figure 1).

1 Contact your local Sensirion sales representative for a recommended

The effect of FRC takes place immediately. FRC can be
executed multiple times at arbitrary intervals. Before
applying FRC, SCD30 needs to be operated for 2 minuteswith the desired measurement period in continuous mode.
Please find below an example demonstrating FRC
implementation.

Applying FRC in Production LineA reliable reference value and correct FRC execution canbe ensured by the following method in a production
environment, e.g. in the course of an end test process:Locate the device in an environment with air having a stable
CO2 concentration in the range of 400 ppm to2000 ppm. This can be accomplished by eithera) Setting and controlling a known CO2 concentration
in a sealed environment with the set CO2
concentration acting as the reference value forFRC. Orb) Keeping SCD30 away from CO2 sources anddrains like humans, open windows, and ventilation
slots. A calibrated reference in close proximity tothe SCD30 gives the reference value for FRC1.
Operate the SCD30 in the completely assembled end user
device in either one of the two above describedenvironments. The measurement period needs to be thesame as that used in the end user application. After 2
minutes settling time execute FRC via the digital interface2.

2 See interface description for more details.

reference device.

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Applying FRC at End User

ASC Field-Calibration Algorithm

At the end user, a CO2 reference as described above isusually not available. In that case, fresh air from the outside
can be used as a reference. Outside air typically has a CO2
concentration of 400 ppm. This can be used to apply FRC
at end user as follows:

The automatic self-calibration (ASC) algorithm
autonomously generates a reference value by analyzing themeasured CO2 concentration over a period of time. Thisreference value is in turn employed for updating of thecalibration curve. ASC can be activated and deactivatedeither via the digital interface or the ControlCenter software
for the Sensirion SEK EvalKit. ASC is an optional featureand is deactivated by default. Both the activation status andthe latest reference value are saved in an on-board nonvolatile memory.

Offer an interactive calibration procedure via the firmwareof the end user device for the following steps:a) The end user has to expose the device to outside
air, e.g. by placing it close to an open window oroutside. Direct sun light, extreme temperatures,and strong air flow have to be prevented.
b) The user has to confirm the stability of ambientconditions via the user interface, upon which theSCD30 will be operated. The operation mode hasto be the same as in normal end user operationmode.
c) After 2 minutes, apply FRC with the reference
value 400 ppm via the digital interface.
d) Confirm to end user that calibration procedure iscompleted.

Applying FRC via Sensirion SEK Evaluation Kit
For testing purposes, the FRC feature can be executed byusing the Sensirion SEK EvalKit by going through thefollowing procedure:a) Connect the SCD30 to the SEK EvalKit and start
the ControlCenter software.
b) Open the Setting dialog box of SCD30:

The correct application of ASC requires the SCD30 to be
regularly exposed to air with CO2 concentration = 400 ppm,
as is the case in well ventilated buildings over night without
human presence. SCD30 needs to be operated incontinuous measurement mode, powering down the sensor
will delay identification of a new reference value. The
reference value will be overwritten either with a newreference value externally supplied via FRC or internallysupplied from the ASC (see Figure 1).

ASC working principle
ASC assumes that the lowest CO2 concentration theSCD30 is exposed to corresponds to 400 ppm. The sensor
estimates the most likely reading corresponding to thisbackground level and identifies this as 400ppm. Using thisreference value, the very same manipulation is triggered
when applying FRC with the reference value of 400 ppm.
Generating a reference value not corresponding to 400 ppm
induces an erroneous update of the calibration and reduces
sensor accuracy. To prevent a faulty self-calibration, ASC
employs an internal self-consistency check.

Switching ASC on and off at End User
Activate ASC feature via the digital interface2 before
delivery of the end user device.

Applying ASC via Sensirion SEK Evaluation Kit
For testing purposes, the ASC feature can be switched onand off via the Sensirion SEK EvalKit by going through thefollowing procedure:c) Input the reference value in FRC panel:d) Hit OK and confirm FRC implementation.

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Connect the SCD30 to the SEK EvalKit and start theControlCenter software.
a) Open the Setting dialog box of SCD30c) Hit OK.
Note that the sensor needs to be operated for typical 7 days
so that ASC can be applied.

b) Toggle the ASC setting.

Example: Applying FRC Procedure on Field-Data.
Figure 2 shows the effect of FRC on a sample set of SCD30. Eight sensors were exposed to mechanical stress by handling,shock, and drop testing. The resulting spread in outputs (deviation from a calibrated reference) can be seen in Figure 2 (left)
over the range 400 ppm to 10000 ppm. Although most sensors show only minor deviations, two sensors cannot fulfill the specs(dashed lines). Figure 2 (right) shows the accuracy of the same sensors after implementation of FRC in a 400 ppm environment(reference value = 400 ppm). Note that all sensors were in spec over the entire concentration range after re-calibration with asingle reference value. ASC will result in the same accuracy given the conditions and assumptions for ASC are fulfilled.

Figure 2 (left): Accuracy of CO2 concentration measurements at different setpoints for 8 stressed SCD30 sensors. (right): Accuracy fromsame sensors after applying FRC. Application of FRC restores full accuracy over the whole measurement range.

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Revision History
DateApril 2018

Version0.1Page(s)allChanges
Initial version

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