R36US0002EU0100 Rev 1.00
Page 8
Aug 26, 2021
RTD120D-EVK Evaluation Kit Software Manual
such as for calculating CO
2
concentration or calibrating the kit so that accurate CO
2
concentration determination
is possible.
Calibration consists of allowing various gas mixtures of known CO
2
concentration to enter the gas chamber
(where CO
2
concentration is measured), and storing in memory the resulting “raw” values that result from those
gas mixtures, associating those values with the known CO
2
concentration of each gas. For example, gas with no
CO
2
(concentration of 0 ppm) results in a raw value that gets stored in a variable “raw_1”, gas with a CO
2
concentration of 1000 ppm results in another raw value that gets stored in a variable “raw_2”, and so on. At least
three such values must be stored: raw_1, raw_2, and raw_3 corresponding to CO
2
concentrations of 0ppm,
1000ppm, and 400ppm, respectively. For more accuracy in ranges above 1000ppm, a fourth raw value (raw_4)
can optionally be stored as well, which corresponds to a CO
2
concentration of 2000ppm. These three or four raw
values get stored in flash memory, and are used later when the kit is determining CO
2
concentration of unknown
gases (such as ambient air in the vicinity of the kit). This is done by computing raw readings and applying a
function that interpolates the CO
2
concentration from the known concentrations associated with the stored raw
values.
2.5.2
Calibration Procedure
When the EVK is used with DAQFactory running on a PC, real-time display of a graph of CO
2
concentration and
other kit parameters can be viewed. That data can also be processed, analyzed, and logged for further processing
and use with other tools.
A useful capability provided by the GUI is to allow the calibration procedure to be performed in a more interactive,
visual manner. For this, a control file must be loaded by DAQFactory which defines user-interface elements such
as live graphs and buttons to aid in performing calibration.
From DAQFactory, open the control file “calibrate.ctl”. This file is included in the GUI download from the
product page. The initial display of the calibration GUI consists of several graphs, live value
displays, and buttons. The graphs include the following:
▪ Raw value
▪ Raw value peak-to-peak variation
▪ CO
2
concentration
At a high level, calibration is performed by determining and storing “raw” values for several reference CO
2
concentrations. For each reference value, the following steps should be completed:
1. Apply a calibration gas of the reference concentrations to the kit.
2. Watch the graphs to determine when the “raw” reading for the current reference concentration has stabilized
(stopped changing).
3. Press the appropriate button to accept the “raw” value, which will be saved for later storing in flash memory.
This value will be associated with the current CO
2
reference concentration.
After calibration points have been found for all reference concentrations, store the values into flash memory so
that they will be retained after the kit has been powered down. To do so, complete the following:
1. To calibrate a kit using the GUI, first connect the kit to a PC running DAQFactory with the supplied USB cable.
2.
If the kit was already warmed up in advance, then the following step is not necessary
:
Wait at least 10 minutes for the gas chamber to warm up. Since the kit has a lamp that turns on and off at a
regular constant rate, the internal temperature will rise slightly above ambient but soon it will reach a steady
state temperature. The temperature is typically stable enough after 5 minutes, but 10 minutes is
recommended.
