SHTxx, STSxx Design Guide Datasheet by Sensirion AG

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SHTxx and STSxx Design Guide
How to design-in a humidity and temperature sensor.
Overview: The Most Important Design-In Recommendations
1) Sensor has good access to environment
Figure 1: A large opening in the housing provides good access
to environment and allows for air exchange.
2) Sensor is sealed from air entrapped in housing
Figure 2: Sealing of the sensor compartment towards the
remaining housing minimizes the influence of entrapped air on
the sensor.
3) Dead volume enclosed around sensor is small
Figure 3: A small dead volume allows for rapid adaption to
changes in the environment.
4) Sensor is decoupled from heat sources
Figure 4: Decoupling of the sensor from heat sources in the
PCB minimizes the influence of internal heating on the sensor.
Introduction
The accuracy of a measurement does not just depend
on the sensor accuracy itself but also on the set up of the
sensing system. The SHTxx sensors sample relative
humidity and temperature of their direct environment. It
is thus important that the local conditions at the sensor
correspond to the conditions under test. Figure 1 to Figure
4 show the most important design-in recommendations
to ensure good sensor performance: access to
environment, sealing from entrapped air in the housing,
small dead volume and decoupling from heat sources.
The subsequent pages contain more in-depth
descriptions of the design-in recommendations together
with many practical examples.
For proper measurements using SHTxx sensors,
temperature and relative humidity (RH) deviations
between the sensor and the environment must be
avoided.
A usual root cause for temperature deviations are heat
sources, while RH deviations are mostly caused by
temperature deviations as well as slow response times.
Please note that every temperature deviation will cause
RH deviations due to the temperature dependence of the
Preface
The SHTxx are humidity and temperature sensors of
high quality. The digital interface and factory calibration
allows a fast and easy implementation as well as full
interchangeability. In order to take full advantage of their
outstanding performance and features a number of
housing and PCB design rules
need to be considered. This document lists this design
rules and provides help during design-in phase. Please
note that unbeneficial housing and/or PCB designs may
cause significant temperature and humidity deviations
as well as highly increased response times.
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relative humidity, i.e. a deviation of 1°C at 90%RH will
result in a 5%RH deviation. For further details please
check the application note “Introduction to Relative
Humidity”
1
.
Figure 5: The sensor measures the local conditions at the
sensing element (RHL; TL). In order to achieve good
measurements this local conditions need to correspond to the
conditions of the environment under test (i.e. RHE; TE).
For each temperature or humidity change of the
environment, the sensor requires a certain amount of
time to equilibrate with the new environmental
conditions. During this time the sensor readings may lag
behind the actual values. This is called response time.
To get precise data it is recommended to decrease the
response time of the sensor system as good as possible.
If the system must react on fast changes a sufficient fast
response time is crucial.
How to effectuate a housing and PCB design to get
accurate measurements with fast response times is
descried in the following sections.
Heating
Humidity Response Time
Temperature Response Time
Design for harsh Environments
Examples
1
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Heating
External heat sources close to the sensor will cause
increased temperature (and thus decreased RH)
readings. To avoid heating of the sensor please consider
the following:
Heat conduction: The sensor should be thermally
decoupled from all heat sources.
Heat convection / radiation: Shield the sensor from
heated air and heat radiation.
Heat conduction
The most common root cause for local heating of the
sensor is due to thermal conduction from a nearby heat
source (power electronics, microprocessors, displays,
etc.). As thermal conduction mostly occurs through the
metal on the PCB, thin metal lines and sufficient
distances between the sensor and potential heat
sources are recommended. Further, heat conduction
can be decreased by milling slits in - and removing
(etching) all unnecessary metal from the PCB around the
sensor (see Figure 6). Another possibility to decrease
heat conduction to the sensor is the use of a flex print to
connect the sensor to the PCB (see Figure 10).
Figure 6: a) Thin metal connections and sufficient distance to
the heat source helps to avoid heat conduction. Please note
to remove unnecessary metal on the PCB around the sensor.
b) The milled slits (white lines) around the sensor decrease the
thermal conduction through the PCB.
c) Unnecessary metal, such as thick metal connections will
increase heat transfer from the heat source to the sensor.
d) Heat sources in close proximity will heat the sensor
Heat convection / radiation
Inside of electronic devices the air might be heated up
by electronic components. Contact of heated air and the
sensor shall be avoided by shielding the sensor
RHL; TL
Device
housing
conditions
RHE; TE
Local
sensor
conditions
Sensor
‘” SENSIRION senses: COMPANY
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physically from all heat sources (see Figure 7).
Additionally, there should be a sufficient heat transfer out
of the devices to avoid the heating of the complete
housing.
Figure 7: a) A wall (orange) shields the sensor from the heated
air. The opening on the top avoids the heating of the complete
housing. b) The heated air gets in direct contact with the
sensor which will cause increased temperature readings. c)
Even heated air from nearby devices may influence the sensor
readings.
Do not expose the sensor to direct heat radiation (e.g.
direct sunlight) to avoid heating. If the radiation is strong
the complete housing should be shielded from the
radiation (see Figure 8).
Figure 8: Direct sunlight or other heat radiation may cause
increased temperature readings.
Humidity Response Time
For proper humidity measurements it is important that
the humidity at the sensor matches the one of the
environment while acquiring data. Therefore, the sensor
should be connected as well as possible to
environmental air. Housing designs with a large dead
volume and/or small aperture may act as a separation of
the sensor and environment (see Figure 5) which may
result in highly increased response times. In order to
achieve fast response times please consider the
following:
Place the sensor as close to the environment as
possible.
A design which allows an airflow over the sensor is
preferred to a design with a single aperture.
The dead volume should be as small as possible
The aperture(s) should be as large as possible
Filter membranes will slow down humidity response.
Never use more than one membrane per aperture.
Make sure that the dead volume is sealed airtight,
otherwise humidity will diffuse.
There should be no material which can absorb
humidity inside of the dead volume.
There should be no material which can absorb
humidity used as a casing. Especially any
polyamide should be avoided.
Design with a possible Airflow
If there is an airflow over the sensor (see Figure 9 a), the
air inside of the dead volume is exchanged constantly.
Such a design is favourable in terms of response times.
Even if there is no defined flow (e.g. in a living room) a
design with multiple openings and a possible flow is
preferred. If there is no possibility to realize a design with
airflow over the sensor, the following points become
more important.
Dead Volume
The larger the dead volume the more air needs to be
exchanged until the environmental and sensor
conditions match each other. Large dead volumes will
drastically increase the humidity response time. It is
recommended to keep the dead volume as small as
possible.
Aperture Size
The aperture is the connection between environment
and sensor. A bigger aperture allows a faster air
exchange and therefore better humidity response times.
Filter Membranes
Filter membranes may help to protect the sensor from
harsh environments. But as they decrease the air
exchange the response time may be slower. If a filter
SENSIRION THE season COMPANY a) bi c) thin PCB milled slits flex connection For more information on the Datasheet Filter Membrane cap is designed with a d information about the www.sensi n.00mlsf2
SHTxx Design Guide
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membrane is required, the size of the dead volume and
the aperture become more critical.
Figure 9: Schematic view of different design-ins. a) The
defined airflow goes directly over the sensor and therefore the
local conditions at the sensor equilibrate quickly with the
environmental conditions. If there is no defined flow this design
is not recommended as the dead volume is too big. b) The
walls (orange) reduce the dead volume which will lead in
combination with the large aperture to fairly good response
times. c) The small dead volume, and multiple openings
enable a good air exchange. d-f) These designs will have slow
humidity response times due to the following reasons: d) The
airflow misses the sensor and the dead volume is large. e) The
aperture size is too small in respect to the dead volume. f) The
dead volume is large.
Temperature Response Time
Due to the thermal mass of a device it temperature
reacts slow on changes of environmental temperature.
In order to achieve fast temperature response times the
following points should be considered.
Thermal coupling of the sensor to the environment
under test should be as strong as possible.
Thermal coupling of the sensor to the thermal mass
of the housing (PCB) should be as weak as
possible.
Thermal coupling of the Sensor to the Environment
To achieve a good thermal coupling between the sensor
and the environment the sensor should be placed as
close to the environment as possible best at a corner
or at least at the edge of the device. An airstream of
ambient air will additionally increase the coupling.
Thermal coupling of the sensor to the thermal mass
of the housing and the main PCB
In order to get a good decoupling of the sensor and the
housing / PCB the heat conduction needs to be reduced
as described in the heating section above (see Figure
10).
Figure 10: The sensor may be thermally decoupled from the
PCB by small PCB connections or with a flex.
Designs for harsh Environments
For selected versions of the SHT3x there is a filter
membrane available which protects the sensor opening
form water and dust according to IP67. Due to the
minimal package volume and the membranes high
vapour permeability, the response time is identical to the
sensor without membrane. Please note that for
applications in harsh environments it may be necessary
to apply conformal coating to avoid corrosion of the
solder pads.
For more information on the SHT3x with filter membrane:
Datasheet Filter Membrane
Alternatively, the SF2 filter cap can be used to achieve
water and dust tight housings with good response times
(see Example). In order to achieve fast humidity
response time the SF2 filter cap is designed with a
minimal dead volume. Detailed information about the
SF2 filter cap may be found on: www.sensirion.com/sf2
SENSIRION THE season counuv Example 3: This is a more sophisticated version of Example
SHTxx Design Guide
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Examples
This chapter shows some different designs for different
applications.
Example 1: This is the most recommended design if no filter
membrane is required. It well combines the rules above. The
wall (orange) helps to shield the sensor from the heated air as
well as it decreases the dead volume. The large opening
allows for a good air exchange and the milled slits reduce
thermal conduction through the PCB. Therefore this design
provides fast response times as well as low influences from
heating parts.
Example 2: This is a more simple variation of Example 1. As
there is no airflow the humidity response time is slower
(depends on the distance of the sensor to the opening). With
additional slits in the PCB the sensor could be shielded from
external heating if required.
Example 3: This is a more sophisticated version of Example
2 using a flex pcb for thermal decoupling. Additionally there is
a filter membrane to protect the sensor. The short distance
between the sensor and the environment under test improves
response times.
Example 4: This design shows an SHT85 inside of a tube with
an airflow. The thin PCB connection decouples the SHT85
very well from the tube and grants a very fast thermal response
time as well as reduced influence from temperature deviations
between the tube and the airflow.
Example 5: The SF2 filter cap may help to design tight
housings. The filter membrane protects the sensor and the
housing from dust and water. Due to the very small volume
between the sensor and the environment fast humidity
response times can be achieved.
Final remark
Please note that all rules and suggestions of this
application note are term of simplified examples and may
be not applicable for specific customer products.
Therefore, it is inevitable to carefully evaluate the design-
in separately for each individual project. Please also read
carefully the handling instructions during design-in
phase and before production release.
SENSIRION was sense»: COMPANY Date Revision Changes 24 June 2010 1.0 Initial release 27. May 2019 1.1 Updated from page and figures
SHTxx Design Guide
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Revision History
Date
Revision
Changes
24 June 2010
1.0
Initial release
27. May 2019
1.1
Updated front page and figures
Important Notices
Warning, Personal Injury
Do not use this product as safety or emergency stop devices or in any other application where failure of the product could result in personal
injury. Do not use this product for applications other than its intended and authorized use. Before installing, handling, using or servicing
this product, please consult the data sheet and application notes. Failure to comply with these instructions could result in death or serious
injury.
If the Buyer shall purchase or use SENSIRION products for any unintended or unauthorized application, Buyer shall defend, indemnify and hold
harmless SENSIRION and its officers, employees, subsidiaries, affiliates and distributors against all claims, costs, damages and expenses, and
reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized
use, even if SENSIRION shall be allegedly negligent with respect to the design or the manufacture of the product.
ESD Precautions
The inherent design of this component causes it to be sensitive to electrostatic discharge (ESD). To prevent ESD-induced damage and/or degradation,
take customary and statutory ESD precautions when handling this product.
See application note “ESD, Latchup and EMC” for more information.
Warranty
SENSIRION warrants solely to the original purchaser of this product for a period of 12 months (one year) from the date of delivery that this product
shall be of the quality, material and workmanship defined in SENSIRION’s published specifications of the product. Within such period, if proven to be
defective, SENSIRION shall repair and/or replace this product, in SENSIRION’s discretion, free of charge to the Buyer, provided that:
notice in writing describing the defects shall be given to SENSIRION within fourteen (14) days after their appearance;
such defects shall be found, to SENSIRION’s reasonable satisfaction, to have arisen from SENSIRION’s faulty design, material, or workmanship;
the defective product shall be returned to SENSIRION’s factory at the Buyer’s expense; and
the warranty period for any repaired or replaced product shall be limited to the unexpired portion of the original period.
This warranty does not apply to any equipment which has not been installed and used within the specifications recommended by SENSIRION for the
intended and proper use of the equipment. EXCEPT FOR THE WARRANTIES EXPRESSLY SET FORTH HEREIN, SENSIRION MAKES NO
WARRANTIES, EITHER EXPRESS OR IMPLIED, WITH RESPECT TO THE PRODUCT. ANY AND ALL WARRANTIES, INCLUDING WITHOUT
LIMITATION, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE, ARE EXPRESSLY EXCLUDED AND
DECLINED.
SENSIRION is only liable for defects of this product arising under the conditions of operation provided for in the data sheet and proper use of the
goods. SENSIRION explicitly disclaims all warranties, express or implied, for any period during which the goods are operated or stored not in
accordance with the technical specifications.
SENSIRION does not assume any liability arising out of any application or use of any product or circuit and specifically disclaims any and all liability,
including without limitation consequential or incidental damages. All operating parameters, including without limitation recommended parameters,
must be validated for each customer’s applications by customer’s technical experts. Recommended parameters can and do vary in different
applications.
SENSIRION reserves the right, without further notice, (i) to change the product specifications and/or the information in this document and (ii) to
improve reliability, functions and design of this product.
Copyright© 2019, by SENSIRION.
CMOSens® is a trademark of Sensirion
All rights reserved
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