Version 4.1 – July 2008
Users Guide SHT7x
1 Application Information
1.1
Operating Conditions
Sensor works stable within recommended normal range –
see Figure 4. Long term exposures to conditions outside
normal range may temporarily offset the RH signal (+3
%RH after 60h). After return to normal range it will slowly
return towards calibration state by itself. See Section 1.4.
“Reconditioning Procedure” to accelerate eliminating the
offset. Prolonged exposure to extreme conditions may
accelerate ageing.
Normal
Range
0
20
40
60
80
100
-40
-20
0
20
40
60
80
100
120
Temperature (°C)
Figure 4: Operating Conditions
1.2
Sockets and Soldering
For maintain high accuracy specifications the sensor shall
not be soldered. Sockets may be used such as “Preci-dip /
Mill-Max 851-93-004-20-001” or similar.
Standard wave soldering ovens may be used at maximum
235°C for 20 seconds. For manual soldering contact time
must be limited to 5 seconds at up to 350°C7.
After wave soldering the devices should be stored at
>75%RH for at least 12h to allow the polymer to re-
hydrate.
In no case, neither after manual nor wave soldering, a
board wash shall be applied. In case of application with
exposure of the sensor to corrosive gases the soldering
pads of pins and PCB shall be sealed to prevent loose
contacts or short cuts.
1.3
Storage Conditions and Handling Instructions
It is of great importance to understand that a humidity
sensor is not a normal electronic component and needs to
be handled with care. Chemical vapors at high
concentration in combination with long exposure times
may offset the sensor reading.
For these reasons it is recommended to store the sensors
in original packaging including the sealed ESD bag at
following conditions: Temperature shall be in the range of
7 235°C corresponds to 455°F, 350°C corresponds to 662°F
10°C – 50°C (0 – 80°C for limited time) and humidity at 20
– 60%RH (sensors that are not stored in ESD bags). For
sensors that have been removed from the original
packaging we recommend to stored them in ESD bags
made of PE-HD8.
In manufacturing and transport the sensors shall be
prevented of high concentration of chemical solvents and
long exposure times. Out-gassing of glues, adhesive tapes
and stickers or out-gassing packaging material such as
bubble foils, foams, etc. shall be avoided. Manufacturing
area shall be well ventilated.
For more detailed information please consult the
document “Handling Instructions” or contact Sensirion.
1.4
Reconditioning Procedure
As stated above extreme conditions or exposure to solvent
vapors may offset the sensor. The following reconditioning
procedure may bring the sensor back to calibration state:
Baking:
100 – 105°C at < 5%RH for 10h
Re-Hydration:
20 – 30°C at ~ 75%RH for 12h 9.
1.5
Temperature Effects
Relative
humidity
reading
strongly
depends
on
temperature. Therefore, it is essential to keep humidity
sensors at the same temperature as the air of which the
relative humidity is to be measured. In case of testing or
qualification the reference sensor and test sensor must
show equal temperature to allow for comparing humidity
readings.
The packaging of SHT7x is designed for minimal heat
transfer from the pins to the sensor. Still, if the SHT7x
shares a PCB with electronic components that produce
heat it should be mounted in a way that prevents heat
transfer or keeps it as low as possible.
Furthermore, there are self-heating effects in case the
measurement frequency is too high. Please refer to
Section 3.3 for detailed information.
1.6
Light
The SHT7x is not light sensitive. Prolonged direct
exposure to sunshine or strong UV radiation may age the
housing.
8 For example, please check www.sirel.ch
9 75%RH can conveniently be generated with saturated NaCl solution.
100 – 105°C correspond to 212 – 221°F, 20 – 30°C correspond to 68 – 86°F
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