SHT1x / SHT7x Relative Humidity & Temperature Sensor System
www.sensirion.com
v2.01 March 2003
5/9
3 Converting Output to Physical Values
3.1 Relative Humidity
To compensate for the non-linearity of the humidity sensor
and to obtain the full accuracy it is recommended to convert
the readout with the following formula1:
2
RH
3
RH
2
1
linear
SO
c
SO
c
c
RH
•
+
•
+
=
SORH
c1
c2
c3
12 bit
-4
0.0405
-2.8 * 10-6
8 bit
-4
0.648
-7.2 * 10-4
Table 6
Humidity conversion coefficients
For simplified, less computation intense conversion formulas
see application note “RH and Temperature Non-Linearity
Compensation”.
The humidity sensor has no significant voltage dependency.
#
#
#
!
"
$
&
SORH sensor readout (12bit)
Figure 10 Conversion from SORH to relative humidity
3.1.1
Compensation of RH/Temperature dependency
For temperatures significantly different from 25 °C (~77 °F)
the temperature coefficient of the RH sensor should be
considered:
linear
RH
2
1
C
true
RH
)
SO
t
(t
25)
-
(T
RH
+
•
+
•
= °
SORH
t1
t2
12 bit
0.01
0.00008
8 bit
0.01
0.00128
Table 7
Temperature compensation coefficients
This equals ~0.12 %RH / °C @ 50 %RH
1 Where SORH is the sensor output for relative humidity
3.2 Temperature
The bandgap PTAT (Proportional To Absolute Temperature)
temperature sensor is very linear by design. Use the
following formula to convert from digital readout to
temperature:
T
2
1
SO
d
d
e
Temperatur
•
+
=
VDD
d1 [°C]
d1 [°f]
SOT
d2 [°C] d2 [°f]
5V
-40.00
-40.00
14bit
0.01
0.018
4V
-39.75
-39.50
12bit
0.04
0.072
3.5V
-39.66
-39.35
3V
-39.60
-39.28
2.5V
-39.55
-39.23
Table 8
Temperature conversion coefficients
For improved accuracies in extreme temperatures with more
computation intense conversion formulas see application
note “RH and Temperature Non-Linearity Compensation”.
3.3 Dewpoint
Since humidity and temperature are both measured on the
same monolithic chip, the SHTxx allows superb dewpoint
measurements. See application note “Dewpoint calculation”
for more.