Datasheet SHT7x
www.sensirion.com
Version 4.1 – July 2008
8/11
for V3 sensors, still applies to V4 sensors and is given in
Table 7 for reference.
SORH
c1*
c2*
c3*
12 bit
-4.0000
0.0405
-2.8000E-6
8 bit
-4.0000
0.6480
-7.2000E-4
Table 7: V3 humidity conversion coefficients, which also apply
to V4.
For simplified, less computation intense conversion
formulas see Application Note “RH and Temperature Non-
Linearity Compensation”. Values higher than 99% RH
indicate fully saturated air and must be processed and
displayed as 100%RH13. Please note that the humidity
sensor has no significant voltage dependency.
0%
20%
40%
60%
80%
100%
0
500
1000
1500
2000
2500
3000
3500
SORH sensor readout (12bit)
Figure 13: Conversion from SORH to relative humidity
4.2
Temperature compensation of Humidity Signal
For temperatures significantly different from 25°C (~77°F)
the humidity signal requires a temperature compensation.
The temperature correction corresponds roughly to
0.12%RH/°C @ 50%RH. Coefficients for the temperature
compensation are given in Table 8.
(
) (
)
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 8: Temperature compensation coefficients14
4.3
Temperature
The
band-gap
PTAT
(Proportional
To
Absolute
Temperature) temperature sensor is very linear by design.
Use the following formula to convert digital readout (SOT)
to temperature value, with coefficients given in Table 9:
T
2
1
SO
d
d
T
⋅
+
=
13 If wetted excessively (strong condensation of water on sensor surface),
sensor output signal can drop below 100%RH (even below 0%RH in some
cases), but the sensor will recover completely when water droplets
evaporate. The sensor is not damaged by water immersion or condensation.
14 Coefficients apply both to V3 as well as to V4 sensors.
VDD
d1 (°C)
d1 (°F)
SOT
d2 (°C)
d2 (°F)
5V
-40.1
-40.2
14bit
0.01
0.018
4V
-39.8
-39.6
12bit
0.04
0.072
3.5V
-39.7
-39.5
3V
-39.6
-39.3
2.5V
-39.4
-38.9
Table 9: Temperature conversion coefficients15.
4.4
Dew Point
SHT7x is not measuring dew point directly, however dew
point can be derived from humidity and temperature
readings. Since humidity and temperature are both
measured on the same monolithic chip, the SHT7x allows
superb dew point measurements.
For dew point (Td) calculations there are various formulas
to be applied, most of them quite complicated. For the
temperature range of -40 – 50°C the following
approximation provides good accuracy with parameters
given in Table 10:
(
)
T
T
T
m
100%
RH
ln
m
T
T
T
m
100%
RH
ln
T
T
RH,
T
n
n
n
d
+
⋅
−
−
+
⋅
+
⋅
=
Temperature Range
Tn (°C)
m
Above water,
0 – 50°C
243.12
17.62
Above ice,
-40 – 0°C
272.62
22.46
Table 10: Parameters for dew point (Td) calculation.
Please note that “ln(…)” denotes the natural logarithm. For
RH and T the linearized and compensated values for
relative humidity and temperature shall be applied.
For more information on dew point calculation see
Application Note “Dew point calculation”.
5 Environmental Stability
If sensors are qualified for assemblies or devices, please
make sure that they experience same conditions as the
reference sensor. It should be taken into account that
response times in assemblies may be longer, hence
enough dwell time for the measurement shall be granted.
For detailed information please consult Application Note
“Qualification Guide”.
15 Temperature coefficients have slightly been adjusted compared to datasheet
SHTxx version 3.01. Coefficients apply to V3 as well as V4 sensors.
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