Aosong Electronic Temperature and Humidity
Sensor Design Guide——Use temperature and humidity sensors correctly

Preface
Aosong electronic temperature and humidity sensor (hereinafter referred toas the sensor) is a temperature and humidity composite sensor with calibrated
digital/analog signal output, and applies dedicated digital module acquisition
technology and temperature and humidity sensor technology to ensure that the
product has extremely high reliability and excellent long-term stability. In orderto make full use of its excellent performance and features, this document
makes provisions for the design of some housings, PCBs, software, etc. andprovides assistance in the design stage.
Please note that unreasonable design of housing, PCB, etc. may causesignificant temperature and humidity deviation and increase in response time,
so please design strictly in accordance with the requirements of this document.

The most important design advice on accuracy
The accuracy of measurement depends not only on the accuracy of the sensor itself,
but also on the design of the sensing system. The sensor collects the temperature andhumidity from its direct environment, so the relativity between the local condition of thesensor and the measured condition is very important. Figures 1 to 4 illustrate the mostimportant design recommendations to ensure good sensor performance: contact
environment, air-tightness to the air inside the housing, small dead space volume, andisolation from heat sources. The following pages contain a more in-depth introduction.
1. The sensor is in good contact with the environment

Figure 1: The opening of the shell should have a good channel in contact with the environment, so that the sensor can exchange well withthe outside air.

2. The sensor is completely isolated from the dead space inside the housingwww.aosong.com

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Figure 2: The sensor is completely isolated from the dead zone air inside the housing, which minimizes the influence of the airtight air
inside the housing on the sensor

2. The closed dead zone around the sensor is small in size

Figure 3: The small dead volume enables the sensor to quickly adapt to changes in the environment.

3. The sensor is isolated from the heat source

Figure 4: The isolation of the sensor from the heat source in the PCB reduces the influence of internal heating on the sensor.

Precision introduction
When using the Aosong electronic sensor for measurement, the temperature andrelative humidity deviation between the sensor and the environment must be avoided.
Usually the root cause of temperature deviation is the heat source, while humidity deviation
is mainly caused by temperature deviation and slow response time. Please note that due tothe dependence of humidity on temperature, every temperature deviation will cause ahumidity deviation-relative humidity, a deviation of 1°C at 90%RH will result in a humidity
deviation of 5%RH.

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Aosong Electronic Temperature and Humidity Sensor Design Guide

Figure 5: The sensor measures the sensing element (RHL; TL).

In order to achieve accurate measurement, this local sensor condition (RHL; TL) mustcorrespond to the measured environmental condition (RHE; TE). For every temperature or
humidity change in the environment, the sensor needs a certain amount of time to balancewith the new environmental conditions. During this period, the sensor's reading may lag
behind the actual value. This is called the response time. In order to obtain accurate data, it
is recommended to reduce the response time of the sensor system as much as possible. If
the system must respond to rapid changes, then a sufficiently fast response time is critical.
How to implement a housing and PCB design to obtain accurate measurement andfast response time will be described in the following sections: Heated.
 Humidity response time.
 Temperature response time.
 Harsh environment design, examples.

1. Heat
An external heat source close to the sensor will cause the temperature to rise, thereby
reducing the RH reading. To avoid sensor be heated, please consider the following points:
 Thermal conduction: the sensor should be thermally isolated from all heat sources.
 Convection/radiation: protect the sensor from hot wind and heat radiation.
1) Heat conduction
The most common root cause of local heating of sensors is heat conduction fromnearby heat sources (power electronics, microprocessors, displays, etc.). Since heat
conduction occurs mainly through the metal on the PCB, it is recommended that the sensor
and the potential heat source be connected with a thinner metal wire while keeping asufficient distance. In addition, heat conduction can be reduced by grooving and removing(etching) all unnecessary metal on the PCB around the sensor (see Figure 6). Anotherpossible method to reduce heat conduction is to use flexible printing technology to connect
the sensor to a flexible printed circuit board (see Figure 10).

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Aosong Electronic Temperature and Humidity Sensor Design Guide

Figure 6: Sensor measuring sensing element (RHL; TL)a) The thin metal wire is connected to keep a sufficient distance from the heat source to avoid heat conduction. Please be careful toremove unnecessary metal on the PCB around the sensor. b) The hollow (white line) around the sensor reduces the heat conduction of
the PCB. c) Unnecessary metal, such as thick metal wire connection, will increase the heat transfer from the heat source to the sensor. d)
A nearby heat source will heat the sensor

2）Heat convection/heat radiation
Inside the electronic device, the air may be heated by the electronic components.
Avoid contact of hot air with the sensor by isolating the sensor. In addition, there should beenough heat to be transferred out of the equipment to avoid heating of the entire enclosure.

Figure 7: Schematic diagram of heat conductiona)A wall (orange) protects the sensor from hot air. The opening at the top avoids heating of the entire shell. b) The heated air is in directcontact with the sensor, which will cause the temperature reading to increase. c) Even hot air from nearby equipment may affect sensor
readings.

Do not expose the sensor to direct heat radiation (such as direct sunlight) to avoid heat.
If the radiation is strong, should be shielded by the complete enclosure (see Figure 8).

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Figure 8: Direct sunlight or other heat radiation may cause temperature readings to increase.

2. Humidity response time
In order to perform accurate humidity measurement, it is important that the humidity ofthe sensor matches the humidity of the environment when acquiring data. Therefore, thesensor should be in contact with the ambient air as much as possible. A housing design
with a large dead volume and/or small aperture may separate the sensor from theenvironment (see Figure 5), which may greatly increase the response time. In order toachieve fast response time, please consider the following points:
Place the sensor as close as possible to the environment.



A design that allows airflow through the sensor is better than a single-aperture design.

The volume of the dead zone is as small as possible, and the aperture is as large as
possible.



The filter membrane will slow down the humidity response, so do not use more than
one layer of membrane per hole.



Please make sure the dead zone is airtight, otherwise the humidity will spread.



There must be no material that can absorb humidity in the dead zone.

Materials that can absorb humidity should not be used as the housing, especially any
polyamide should be avoided.

1） Design an airflow as much as possible
If there is air flow through the sensor (see Figure 9a), the air in the dead zone space iscontinuously exchanged. Such a design can greatly shorten the response time. If there isno clear flow direction (e.g. in the living room), a design with multiple openings and possible
flow is preferred. If it is impossible to achieve the design of air flow through the sensor, thenthe following points become more important.
2）Dead zone volume
The larger the dead zone volume, the more air needs to be exchanged until theenvironment and sensor conditions match each other. A large dead zone volume will
greatly increase the humidity response time. It is recommended to keep the dead zone
volume as small as possible.
3）Aperture size
The ventilation hole is the connection between the environment and the sensor. The
larger the hole diameter, the faster the air exchange, so the response time of humidity iswww.aosong.com

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faster.
4）Filter membrane
The filter membrane can help protect the sensor from harsh environments. However,as the air exchange decreases, the reaction time may become slower. If a filter membraneis required, the dead volume and pore size become more critical.

Figure 9: Schematic diagram of different design optionsa) The defined airflow passes directly through the sensor, so the local conditions and environmental conditions at the sensor are quickly
balanced. If the flow direction is not defined, this design is not recommended because the dead zone volume is too large. b) The wall(orange) reduces the volume of the dead zone, and combined with the large aperture, there will be a faster response time. c) The volumeof the dead zone is small, and multiple openings are conducive to good air exchange. d-f) These designs have slower humidity responsetime due to the following reasons: d) The air flow staggers the sensor and the dead zone volume is too large. e) The aperture is too small
relative to the volume of the dead zone. f) Large volume of dead zone.

3.Temperature response time
Due to the heat of the equipment itself, its temperature reacts slowly to changes in theambient temperature. In order to achieve a fast temperature response time, the following
points should be considered.


The thermal coupling between the sensor and the measured environment (thermal
coupling refers to the heat transfer connection between the two) should be as strong as
possible.



The thermal coupling between the sensor and the housing heat (PCB) should be as
weak as possible.

1）Thermal coupling between sensor and environment
In order to achieve a good thermal coupling between the sensor and the environment,
the sensor should be as close to the environment as possible-preferably in the corner or atleast at the edge of the device. The airflow of the ambient air will add additional coupling.
2）Thermal coupling of the sensor to the housing and the main PCB
In order to reduce the thermal coupling between the sensor and the housing/PCB, heat
conduction needs to be reduced, as described in the heated section above (see Figure 10).

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Figure 10: The sensor can be separated from the PCB with heat by reducing the PCB connection, hollowing out the place where there is
no wire connection, or using a flexible printed PCB.

4. Designed for harsh environments
1）Influence of dust
If used in a dusty environment, dust may enter the sensor, adhere to the surface of thesensor's moisture-sensitive layer, block the vents on the moisture-sensitive layer, and affectthe entry of water molecules, thereby affecting the humidity accuracy. Therefore, it isnecessary to pay attention to the dust-proof of the sensor, and use a filter film that canreach the dust-proof level to protect the sensor.
2）Influence of water droplets
In the outdoor environment, if it is rainy, water droplets may enter the sensor, which
may cause a short circuit of the sensor. In addition, the humidity is saturated at this time,
and the water inside the sensor is difficult to evaporate, which may cause the humidity to besaturated for a long time until the water evaporate completely. Therefore, pay attention to
the waterproof of the sensor, and use a filter membrane that can reach the waterproof levelto protect the sensor.
For various temperature and humidity sensors of Aosong Electronics. There is an IP67
filter membrane to protect the sensor, so that outside water and dust cannot directly enter
the sensor. Due to the extremely small package volume and the high air permeability of themembrane, the response time is the same as that of the same sensor without themembrane. Please note that in harsh environments, it may be necessary to apply aprotective coating, such as conformal coating, to avoid corrosion of the pad, but it must be
ensured that the coating does not touch the wet-felling part of the sensor, otherwise thehumidity will be inaccurate.
In addition, for the application environment that needs to use the filter membrane,
Aosong has designed an AF02 filter cap, which has the smallest dead space volume andcan quickly obtain the response time. See example 5 for specific applications.
3）High temperature effect
After reflow soldered or high-temperature soldered, the sensor will have low humidity
when heated. This phenomenon is normal and can be ignored. It will gradually return tonormal accuracy after using it in a normal environment. The recovery speed depends on
the humidity of the environment, the higher the humidity, the faster the recovery. If there isan urgent need to quickly restore the humidity accuracy, please refer to the recovery
process in the product specification for details.
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Aosong Electronic Temperature and Humidity Sensor Design Guide

For reflow soldering or manual soldering, please strictly follow the requirements of thespecification. The temperature should not be too high and the soldering time should not be
too long, otherwise the sensor may be damaged or the humidity may be permanently
shifted. When using a soldering iron or using a hot air gun, care should be taken to preventthe flux, rosin and other substances from splashing into the sensor, otherwise it will cause
poor humidity.
4）High humidity effectIf the sensor is used in a high humidity environment greater than 90% RH for a longtime, it may temporarily shift upward when the humidity drops, that means humidity value ishigher. This phenomenon is normal and can be ignored, and the accuracy will slowly return
to normal after use in a normal environment. If there is an urgent need to quickly restore thehumidity accuracy, please refer to the recovery process in the product specification for
details.
5）Impacts of extreme environmental containing corrosive chemical substances
If the sensor is used in an environment containing corrosive chemical substances, it
may be exposed to substances or components such as acetone, ammonia, hydrochloric
acid, nitric acid, chlorine, etc., which may cause the sensor to shift irreversibly. Therefore, it
is forbidden to be used in such environments.
6）Other extreme environments
If the sensor inevitably needs to be applied to the extreme environment that may cause
offset, it is recommended to design the sensor as a convenient replacement type, such as apluggable terminal type. To ensure accuracy, the sensor can be replaced regularly.
Here are some application examples:

Example 1: If no filter membrane is required, this is the most recommended design. It combines the above rules very well. The wall(orange) helps protect the sensor from the hot air and also reduces the dead space volume. The large opening allows good air exchange
and the hollowed groove reduces heat conduction through the PCB. Therefore, this design provides fast response time and low-impact
heating components.

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Aosong Electronic Temperature and Humidity Sensor Design Guide
Example 2: This is a simpler variant of Example 1. Because there is no air flow, the humidity response time is slow (depending on the
distance from the sensor to the opening). If necessary, add an extra slot on the PCB to shield the external heating on sensor.

Example 3: This is a more complicated version of Example 2, using flexible pcb for thermal isolation (but please note that violent bendingor pulling is prohibited when using flexible pcb, so as not to pull the sensor pad), the module can be connected through the way of lead ,There is also a filter membrane to protect the sensor. The short distance between the sensor and the environment under test improves
response time.

Example 4: This design shows an AHT25 in a tube with airflow. The thin PCB connection can separate the AHT25 from the tube well, andprovide a very fast thermal response time, and reduce the impact of the temperature deviation between the tube and the airflow.

Example 5: AF2 filter cap may help to design a compact housing. The filter membrane protects the sensor and housing from dust andwater. Due to the small volume between the sensor and the environment, a fast humidity response time can be achieved.

Please note that all the rules and recommendations described above are simplified
examples and may not apply to specific customer products. Therefore, careful design
evaluation is inevitable for each individual project. Please read the operating instructions
carefully during the design phase and before product release.

The most important design suggestions for temperature andhumidity system design
Aosong electronic temperature and humidity sensors are widely used in consumer
electronics, medical, automotive, industry, agriculture, meteorology and other fields, suchas: HVAC, dehumidifiers and refrigerators and other home appliances, testing andwww.aosong.com

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inspecting equipment and other related temperature and humidity detection and control
products . Most of the temperature and humidity sensors are used as core or importantdevices in the application, so the reliability of the temperature and humidity system design
is very important. Especially when using the sensor in high frequency, high noise, magnetic
field and other scenes that may cause interference, it is recommended to add a power
control design to avoid communication failure when the sensor is severely interfered,
thereby affecting the operation of the host.
The following points should be considered.
Hardware: The IO port is used to control the power supply in the hardware circuitdesign, and the hardware can be reset if necessary.
Software: There are multiple wrong judgments and restarts in the software acquisition
logic.

1. Hardware
When designing the hardware circuit, it is recommended to use VDD or GND toconnect to the IO port control method, as shown in Figure 11 and Figure 12, to illustrate theapplication of the digital IIC protocol sensor using the IO port to control the power supply.

Figure 11: IO port control VDD scheme, please note that if an external pull-up resistor is required, the pull-up VDD of the pull-up resistor
must be the same as the sensor power supply VDD.

Figure 12: IO port control GND scheme.

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2. Software
When designing the software, the acquisition frequency needs to follow therequirements of the sensor in the specification. For example, some require at least 2s tocollect once, otherwise it may cause communication failure. If data errors or communication
abnormalities occur during normal collection, the following points should be considered。
1) Multiple misjudgments.
When affected by external high frequency, high noise, electromagnetic interference,
etc., the communication with the sensor may have a short signal error, resulting inabnormal data readings. Therefore, it is recommended to add multiple error filtering on thesoftware to allow individual errors to occur, such as after continuous 10 times collecting,there is an error and then determine the sensor abnormality, and make the sensor
abnormality treatment.
2) If necessary, restart the sensor through the IO of the control power supply (VDD/GND).
On the basis of the above (1), if the error is still reported, consider that the sensor
enters an abnormal working mode under strong interference. At this time, you can pull
down VCC or pull up GND to the sensor through the IO port of the control power supply(VDD/GND) to turn off the power, and then pull up VCC or pull down GND again to power
on, and after the sensor is completely reset, go to collect data. It should be noted that if the
VCC control method is used, the communication port needs to be set to low when the VCC
is pulled down and the power is turned off, so as to prevent the voltage from backflow fromthe IO port, causing the sensor to work abnormally.

This manual may be changed at any time without notice.

All rights reserved ©2021,

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