DS1923 datasheet(49/52 Pages) DALLAS

49 / 52 page

DS1923: Hygrochron Temperature/Humidity Logger iButton with 8kB Data Log Memory

49 of 52

Step 2. Execution

Tc = TcH/2 + TcL/512 - Offset

(convert from binary to °C)

Tcorr = Tc - (A * Tc

2 + B * Tc + C)

(the actual correction)

Numerical Correction Example

Converted Data from Calibration Memory

Error Values

Tr2 = -10.1297°C

Tr3 = 24.6483°C

Tc2 = -10.0625°C

Tc3 = 24.5°C

Err2 = 0.0672°C

Err3 = -0.1483°C

Err1 = Err2

Resulting Correction Coefficients

Application of Correction Coefficients to Sample Reading

B = -0.008741

A = 0.000175/°C

C = -0.039332°C

Tc = 22.500000°C

Tcorr = 22.647275°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.

SOFTWARE CORRECTION ALGORITHM FOR HUMIDITY

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 different humidity levels, as

shown below. The data is taken at 25°C.

Address

Designator

Description

0248h

Hr1H

Low reference humidity, high-byte

0249h

Hr1L

Low reference humidity, low-byte

024Ah

Hc1H

Conversion result at low reference humidity, high-byte

024Bh

Hc1L

Conversion result at low reference humidity, low-byte

024Ch

Hr2H

Medium reference humidity, high-byte

024Dh

Hr2L

Medium reference humidity, low-byte

024Eh

Hc2H

Conversion result at medium reference humidity, high-byte

024Fh

Hc2L

Conversion result at medium reference humidity, low-byte

0250h

Hr3H

High reference humidity, high-byte

0251h

Hr3L

High reference humidity, low-byte

0252h

Hc3H

Conversion result at high reference humidity, high-byte

0253h

Hc3L

Conversion result at high reference humidity, low-byte

The correction algorithm consists of two steps: preparation 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

execution 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 corrected 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.

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