DS1923 Maxim, DS1923 Datasheet - Page 51

• 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 accuracy of humidity conversion results (forced
conversion as well as logged data) can be improved
through a correction algorithm. The data needed for
this software correction is stored in the calibration
memory (memory page 18). It consists of reference
humidity (Hr) and conversion result (Hc) for three differ-
ent humidity levels, as shown below. The data is taken
at 25°C.
Note: The software correction requires floating point arithmetic (24-bit or better). Suitable math libraries for microcontrollers are
found on various websites and are included in cross-compilers.
Tr2 = -10.1297°C
Tr3 = 24.6483°C
Tc2 = -10.0625°C
Tc3 = 24.5°C
B = -0.008741
A = 0.000175/°C
C = -0.039332°C
CONVERTED DATA FROM CALIBRATION MEMORY
Software Correction Algorithm
RESULTING CORRECTION COEFFICIENTS
ADDRESS
024Ah
024Bh
024Ch
024Dh
0248h
0249h
024Eh
024Fh
0250h
0251h
0252h
0253h
Hygrochron Temperature/Humidity Logger
______________________________________________________________________________________
iButton with 8KB Data-Log Memory
for Humidity
DESIGNATOR
Hc1H
Hc2H
Hc3H
Hc1L
Hc2L
Hc3L
Hr1H
Hr2H
Hr3H
Hr1L
Hr2L
Hr3L
Numerical Temperature Correction Example
Err2 = 0.0672°C
Err3 = -0.1483°C
Err1 = Err2
Tc = 22.500000°C
Tcorr = 22.647275°C
Low reference humidity, high byte.
Low reference humidity, low byte.
Conversion result at low reference humidity, high byte.
Conversion result at low reference humidity, low byte.
Medium reference humidity, high byte.
Medium reference humidity, low byte.
Conversion result at medium reference humidity, high byte.
Conversion result at medium reference humidity, low byte.
High reference humidity, high byte.
High reference humidity, low byte.
Conversion result at high reference humidity, high byte.
Conversion result at high reference humidity, low byte.
The correction algorithm consists of two steps: prepara-
tion and execution. The preparation step first converts
humidity data from binary to decimal %RH format. Next,
three coefficients A, B, and C are computed. In the exe-
cution step the humidity reading as delivered by the
DS1923 (raw data) is first converted from the low/high-
byte format (HcL, HcH) to %RH (Hc) and then correct-
ed to Hcorr. Once step 1 is performed, the three
coefficients can be used repeatedly to correct any
humidity reading and humidity log of the same device .
APPLICATION OF CORRECTION COEFFICIENTS
TO SAMPLE READING
ERROR VALUES
DESCRIPTION
51