DS1923 Maxim, DS1923 Datasheet - Page 53

• Manufacturer Part Number: DS1923
• Description: The DS1923 temperature/humidity logger iButton® is a rugged, self-sufficient system that measures temperature and/or humidity and records the result in a protected memory section
• Manufacturer: Maxim

The data for the software correction of humidity is taken
at 25°C. Since the temperature characteristics of the
humidity sensor are known, humidity readings taken at
other temperatures can be corrected, provided the
temperature at the time of the humidity conversion is
also known. Therefore, to obtain the most accurate
humidity results, both temperature and humidity should
be logged.
Capacitive humidity sensors read higher humidity val-
ues than the actual humidity level when they are
exposed to a high-humidity environment for an extend-
ed time period. The DS1923’s humidity sensor pro-
duces readings that are higher than the actual humidity
when exposed to humidity levels of about 70%RH and
higher. This shift continues to increase while the device
remains at 70%RH and above. This effect is called sat-
uration drift, or hysteresis. This drift is reversible.
Readings return to their regular level when the DS1923
is removed from a high-humidity environment.
It is possible to compensate for most of the error intro-
duced by the saturation drift by post-processing tem-
perature and humidity logs using the equation below,
which is based on laboratory tests and curve-fitting
techniques.
T = 70°C
Hcorr = 24.445%RH
SAMPLE INPUT DATA
RH Temperature Compensation
NAME
K
T
Hygrochron Temperature/Humidity Logger
Software Saturation Drift
Temperature at the time of humidity conversion.
Humidity sensor conversion constant.
Linear compensation, enumerator.
Quadratic compensation, enumerator.
Linear compensation, denominator.
Quadratic compensation, denominator.
______________________________________________________________________________________
HTcorr = (24.445 x 0.0307 + 0.0035 x 45 - 0.000043 x 45
HTcorr = 30.291%
= 0.00001/°C
iButton with 8KB Data-Log Memory
Compensation
Numerical RH Temperature Compensation Example
APPLICATION OF CORRECTION COEFFICIENTS TO SAMPLE READING
FUNCTION
Temperature compensation uses the following equation:
Hcorr is the humidity reading with the software correc-
tion algorithm for humidity already applied, as
explained in the previous section. The function and val-
ues of the other parameters are explained in the table
below.
ARH
T
N
HTcorr The humidity reading at the end of the N
The numbers in the equation are derived from curve fit-
ting. They apply to a time scale in hours.
HScorr
k
-
k
x (T - 25°C)
The average software-corrected and tempera-
ture-compensated humidity reading of the k
hour that the device is continuously exposed
to 70%RH or higher.
The average software-corrected temperature
reading of the k
continuously exposed to 70%RH or higher.
The number of hours that the device is contin-
uously exposed to 70%RH or higher.
hour with the software correction algorithm for
humidity and temperature compensation
already applied. See the Software Correction
Algorithm for Humidity and RH Temperature
Compensation sections for details.
HTcorr
HTcorr = (Hcorr x K +
(in °C)
0.0307
0.0035/°C
0.000043/°C
>15°C: 0.00001/°C
0.000002/°C
15°C: -0.00005/°C
2
)/(0.0307 + 0.00001 x 45 - 0.000002 x 45
2
)/(K + x (T - 25°C) -
k
N
1
0 0156
2
2
.
th
VALUE
1
hour that the device is
ARH
T T
x (T - 25°C)
k
k
25 100
2 54
.
x (T - 25°C)
/
0 3502
.
2
2
)
k
53
)
th
th