Datasheet SHT1x
(SORH) with the following formula with coefficients given in
Table 6:
Bit Type Description
Default
0
7
reserved
End of Battery (low voltage
detection)
‘0’ for VDD > 2.47
‘1’ for VDD < 2.47
No default value,
bit is only updated
after a
2
RHlinear c1 c2 SORH c3 SORH (%RH)
6
R
X
measurement
SORH
12 bit
8 bit
c1
c2
c3
5
4
3
2
1
reserved
0
0
0
0
0
-2.0468
-2.0468
0.0367
0.5872
-1.5955E-6
-4.0845E-4
reserved
For Testing only, do not use
Table 6: V4 humidity conversion coefficients
R/W Heater
off
R/W no reload from OTP
reload
The values given in Table 6 are optimized coefficients for
V4 sensors. The parameter set for V3 sensors, which has
been proposed in earlier datasheets, still applies and is
provided by Sensirion upon request.
’1’ = 8bit RH / 12bit Temp.
resolution
’0’ = 12bit RH / 14bit Temp.
resolution
12bit RH
14bit Temp.
0
R/W
0
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.
Table 5: Status Register Bits
Measurement resolution: The default measurement
resolution of 14bit (temperature) and 12bit (humidity) can
be reduced to 12 and 8bit. This is especially useful in high
speed or extreme low power applications.
100%
80%
60%
40%
20%
0%
End of Battery function detects and notifies VDD voltages
below 2.47V. Accuracy is 0.05V.
Heater: An on chip heating element can be addressed by
writing a command into status register. The heater may
increase the temperature of the sensor by 5 – 10°C12
beyond ambient temperature. The heater draws roughly
8mA @ 5V supply voltage.
0
500
1000 1500 2000 2500 3000 3500
SORH sensor readout (12bit)
Figure 18: Conversion from SORH to relative humidity
For example the heater can be helpful for functionality
analysis: Humidity and temperature readings before and
after applying the heater are compared. Temperature shall
increase while relative humidity decreases at the same
time. Dew point shall remain the same.
4.2 Temperature compensation of Humidity Signal
For temperatures significantly different from 25°C (~77°F)
the humidity signal requires temperature compensation.
The temperature correction corresponds roughly to
0.12%RH/°C @ 50%RH. Coefficients for the temperature
compensation are given in Table 7.
Please note: The temperature reading will display the
temperature of the heated sensor element and not
ambient temperature. Furthermore, the sensor is not
qualified for continuous application of the heater.
RHtrue
t1 t2 SORH RHlinear
SORH
12 bit
8 bit
t1
t2
OTP reload: With this operation the calibration data is
uploaded to the register before each measurement. This
may be deactivated for reducing measurement time by
about 10ms.
0.01
0.01
0.00008
0.00128
Table 7: Temperature compensation coefficients14
4.3 Temperature
4 Conversion of Signal Output
The band-gap PTAT (Proportional To Absolute
Temperature) temperature sensor is very linear by design.
4.1 Relative Humidity
For compensating non-linearity of the humidity sensor –
see Figure 18 – and for obtaining the full accuracy of the
sensor it is recommended to convert the humidity readout
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.
12
14
Corresponds to 9 – 18°F
Coefficients apply both to V3 as well as to V4 sensors.
www.sensirion.com
Version 4.3 – May 2010
8/11
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