Sensirion Humidity Sensors Design Guide V1

Sensirion%20Humidity%20Sensors%20Design_Guide%20V1

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SHTxx and STSxx Design Guide

How to design-in a humidity and temperature 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: 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 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 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

.

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

Heating

External heat sources close to the sensor or internal

heating by heat dissipation of the sensor itself will

cause increased temperature (and thus decreased RH)

readings. To avoid heating of the sensor please

consider the following:

 Self heating: The sensor should be less then 10%

in active state.

 Heat conduction: The sensor should be thermally

decoupled from all heat sources.

 Heat convection / radiation: Shield the sensor from

heated air and heat radiation.

1

www.sensirion.com/data_overview

local sensor

conditions

RHL; TL

device housing

environmental

conditions

Sensor

RHE; TE

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

advangtage 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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Self heating

Due to the tiny size of the sensing elements even the

small power consumption of the SHTxx sensor might

cause self heating. As the power consumption is

significantly increased while in active state (measuring

+ communicating) it is recommended to be less then

10% of the time in active state (see Figure 2) in order to

avoid self heating. The number of readings that can be

done per second depends on the resolution of the

measurement and the sensor type. Please check the

datasheet of the according sensor at

www.sensirion.com/humidity for further details.

Figure 2: Using the sensor more then 10% in active state

may cause internal heating which results in a temperature

and humidity deviation.

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 3). Another possibility to

decrease heat conuction to the sensor is the use of a

flex print to connect the sensor to the PCB (see Figure

8).

Figure 3: 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

physically from all heat sources (see Figure 4).

Additionally there should be a sufficient heat transfer

out of the devices to avoid the heating of the complete

housing.

Figure 4: a) A wall of the housing (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 5).

Active

Sleep or Off

< 10%

> 10%

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Figure 5: 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 1) 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 (see Figure 6) 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 then one membrane per

aperture.

 There should be no material which can absorb

humidity inside of the dead volume.

Figure 6: This figure shows a schematic view of a sensor

design-in. The volume around the sensor which is separated

from the environment is called dead volume. The aperture is

the cross section area which connects the dead volume and

the environment. This example has an additional filter

membrane which may help to protect the sensor (see below)

but will slow down the response time.

Design with a possible Airflow

If there is an airflow over the sensor (see Figure 7 a, d

and e), 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 terms becomes 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 volume s will

drastically increase the humidity response time. It is

highly 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 will be slower. If a filter

cap is required the size of the dead volume and the

aperture becomes more important.

dead volume

aperture

device housing

environment under test

Sensor

optional filter

membrane

a)

b)

11

A5Z

11

A5Z

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Figure 7: Schematic view of different design-ins. a) This is

the preferred design if ever possible. The small dead volume,

sufficient aperture size and the air flow allow a fast humidity

response time. b) The walls (orange) reduce the dead

volume which will lead in combination with the large aperture

to fairly good response times. c) 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. d-f) This

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 terms 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 8).

Figure 8: The sensor may be thermally decoupled from the

PCB by small PCB connections or with a flex.

Designs for harsh Environemts

For the SHT1x and the SHT2x series there are filter

caps available which can be used to achieve water and

dust tight housings as well as good response times

(see Example 5 + 6). In order to achieve fast humidity

response time they are design with a minimal dead

volume. Detailed information about these filter caps

may be found on:

SF1 (for SHT1x): www.sensirion.com/sf1

SF2 (for SHT2x): www.sensirion.com/sf2

71

B2G

a)

b)

c)

A5Z

11

thin PCB

connection

milled slits

flex

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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 two openings

allow an airflow over the sensor 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 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 SHT71 inside of a tube

with an airflow. The thin PCB connection decouples the

SHT75 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 SF1 filter cap (for SHT1x)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.

Example 6: The SF2 filter cap (for SHT2x) provides the same

benefits like the SF1 (see Example 5).

SHT1x

SHT2x

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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.

Revision History

Date

Revision

Changes

24 June 2010

1.0

Initial release (DBO)

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