AN5063
Application note
LPS33HW digital pressure sensor:
guidelines for system integration

Introduction
The purpose of this application note is to introduce guidelines for the system and hardware integrationof the LPS33HW water-resistant pressure sensor in the final customer application.
This document does not modify the content of the official datasheet. Please refer to the datasheet forparameter specifications.

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Contents

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Contents
1

System integration .......................................................................... 3

2

Mechanical design rules ................................................................. 52.1

Sensor placement ............................................................................. 52.1.1

Exposure to the environment ............................................................. 52.1.2

O-rings for sealing .............................................................................. 6

2.1.3

Heat propagation ................................................................................ 72.1.4

Mechanical stress ............................................................................... 92.1.5

Light exposure .................................................................................. 103Reference design: integration and housing on a PD .................. 114Accuracy over extended operating range ................................... 1256Robustness to corrosive agents .................................................. 13Robustness to high air overpressure stress tests...................... 14

7Revision history ............................................................................ 16

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1System integration

System integration
The integration of the LPS33HW pressure and temperature sensor in application systems
such as portable devices (PDs) like smartphones, wearable devices, weather stations orindustrial tools shall be implemented without compromising the performance of the sensor.
System integration can be done by looking at the main mechanical and geometricparameters and the factors that influence the performance of the sensor and thus
optimizing those.
The typical scenario for integration of pressure sensors is described in Figure 1: "Pressure
sensor system integration (a) standard pressure sensor, (b) LPS33HW water resistantpressure sensor " where the housing of the sensor has to be designed in order tocorrespond as much as possible to the pressure and temperature conditions of theenvironment under test (Px, Tx) and the conditions around the sensor sensing area
(represented by Ps, Ts), near the air inlet housing.
Figure 1: Pressure sensor system integration (a) standard pressure sensor, (b) LPS33HWwater resistant pressure sensor

The LPS33HW is featured as a water-resistant device which means that it can be exposed
to water pressure without damage and recovers after stress from water pressure. In waterresistant application contexts such as smart watches and bracelets, it is a common design
practice to have the pressure sensor in communication with the external environment
through one hole and to keep it isolated from the other devices placed in the hermetic
enclosure by means of an appropriate cylinder-shaped housing and the use of O-rings asshown in figure 1.b. More details are given in the next paragraphs.
In order to obtain reliable and consistent measurements, all parameters involved in themechanical design must be dimensioned to provide robustness to air or water
overpressure, to get maximum sensor exposure to the external environment, and to get thefastest response time, in terms of pressure and temperature, compatible with the required
design specifications.
Every change in an environmental condition under monitoring must be consistently
reflected in the sensor measurement as well as in the case of fast variations of pressure
and temperature. Therefore, the design integration must guarantee that the environmental
conditions match the sensing area conditions not only in steady-state (static conditions) but
also in dynamic conditions.
Deviations between the conditions under test and the conditions around the sensing area
are also influenced by heat sources, due to other devices close to the sensing area or theself-heating of the sensor. Changes in temperature are critical due to the fact that not only
the temperature is influenced, but changes in temperature will also determine pressure
deviations and, as a consequence, a slower response of the system.

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Based on the considerations above, the design optimization consists of determining the:•••placement of the sensor in the systemembodiment and housing of the sensor
protection of the sensor from dust, water, or chemical solvents by a sensor chamber(in the presence of a harsh environment)

The elements above are further described in the following sections of this document.

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Mechanical design rules

Mechanical design rules
For the mechanical design, the main constraints and features to be considered aredescribed in the following sections and provide a set of basic rules for good design
practices in order to have a successful integration of the sensor in the context of the finalapplication.

2.1

Sensor placement
The placement of the sensor has a direct impact on its performance in terms of sensor links
to the environment, thermal propagation mechanisms, and mechanical stress.

2.1.1

Exposure to the environment
One key design rule is to maximize the exposure of the pressure sensor to the environment
where pressure and temperature are measured. In other words, the sensor has to bepositioned in such a way to react to rapid changes in environmental conditions and provide
a reliable measurement following the dynamics of environmental changes.
The integration design of the pressure sensor clearly impacts the overall response time.
Typically the design target is to match the pressure sensor performance according to thedatasheet. The guidelines below are recommended, with reference to Figure 2: "Pressuresensor integration and housing reference" for achieving this objective:
1.
2.

3.
4.

Place the sensor to get the best connection with the environment under test, as close
as possible to the vent apertures
A large dead volume will increase the response time, with a bigger contribution/delay
to the pressure response time. It is therefore recommended to minimize the volume,
trying to shape a tailored housing around the geometry of the sensor.
Vent apertures should be as large as possible.
The depth of the vent apertures must be minimized.

As a reference for system integration design, Figure 2: "Pressure sensor integration andhousing reference" describes an example of the above recommendations.
Figure 2: Pressure sensor integration and housing reference

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2.1.2

AN5063O-rings for sealing
In wearable and portable applications, it is necessary to add a sealing mechanism such as
an O-ring in order to avoid that direct water or other liquids reach the PCB or other zones of
the customer application as depicted in Figure 2: "Pressure sensor integration and housingreference". For such applications the LPS33HW provides the possibility to seal and protect
the embodiment by an O-ring placed at the groove location (mid part of the metal lid).
Simulation results show that the surface contact pressure between an O-ring of 1 mm
diameter and two contact sides (lid of the LPS33HW and plastic embodiment) is 1.45 Mpa
by applying the geometries in Figure 4: "Groove for O-ring, simulation with O-ring applied
to LPS33HW". Therefore, sealing can be guaranteed until fluidic hydrostatic pressure isless than contact pressure (i.e. 10 bar (1 Mpa) ≤ 1.45 Mpa).
Figure 3: O-ring placement

Figure 4: Groove for O-ring, simulation with O-ring applied to LPS33HWThe thickness of the O-ring needs to be sized in order to fit the housing and to apply asealing force for resisting to 10 bar fluidic pressure according to the device specification. Itis recommended to use an O-ring made of nitrile rubber 70 Shore A with an inner diameterfrom 1.15 mm (for example with thickness of 1.0 ± 0.03 mm) to 1.8 mm (for example withthickness of 0.8 ± 0.03 mm). The O-ring selection depends on the customer applicationgeometries, specifically the cylinder-shaped housing geometries. The information provided
above about the O-ring is given exclusively as general information and shall not be
considered as qualified for any customer application.

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2.1.3

Mechanical design rules

Heat propagation
The presence of heat sources near the sensor can cause deterioration of the performanceby modifying pressure and temperature measurements as well as generating thermal
gradients around the sensing area which affect the correct measurement in static anddynamic conditions.
From a physical point of view, these local sources act as a thermal capacitor placed inparallel to the thermal model of the LPS33HW and they can contribute to the local
temperature which is different from the environmental one.
Depending on the location of the heat sources and the propagation of the heating
mechanism, we can distinguish the propagation related to different mechanisms as
described in the following paragraphs.

2.1.3.1

Heat convection
Local thermal sources around the sensor can modify the pressure and temperature
measurement by radiating heat.
Typical sources are as follows:••••other sensors and devices close to the pressure sensor
power management devices
processors and microcontrollers
LCD displays that, in particular, provide a significant temperature gradient between theenvironment and the dead volume inside the system

Therefore the sensor has to be placed at the correct distance from these sources, and toguarantee the appropriate isolation, it is recommended to adopt inside the embodiment,
heat insulation structures as described in Figure 5: "Insulation from heat implemented forprotecting the sensor". A flexible PCB can be used to place the pressure sensor far fromthe main board and to position the LPS33HW as close as possible to the externalenvironment.
Figure 5: Insulation from heat implemented for protecting the sensor

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Looking at a section of the sensor housing, Figure 6: "Top view of the sensor housing: on
the left a correct design with insulation from heat, on the right an incorrect design" depicts acorrect design with the insulation structure on the left; the heat source is far from thesensor and a thermal protection structure is placed in the middle. On the right an incorrect
design is described, causing heating of the sensor because of the heat radiating from thecomponent nearby.
Figure 6: Top view of the sensor housing: on the left a correct design with insulation fromheat, on the right an incorrect design

The solution of a cooling channel is not applicable for water-resistant applications. It isimportant to emphasize that the LPS33HW has an embedded quadratic compensation thatallows minimizing the effect of temperature in the measurement of pressure. In this context
the recommended design is just to introduce a thermal insulation layer but without a coolingchannel.

2.1.3.2

Heat conduction
Thermal conduction mostly occurs through the metal lines on the PCB and PCB itself.
In order to reduce this effect, we recommend adopting thin metal lines around the sensor at
an appropriate distance from the sensor and potential heat sources, avoiding metal areas
near and under the device.
A good design rule is provided in Figure 7: "Examples of correct sensor placement on thePCB to obtain appropriate isolation from heat sources" as an example of a proper design.
On the left the devices generating heat are positioned as far as possible from the sensor.
Thinner metal lines have been used which contribute to minimizing the thermal effect.
Figure 7: Examples of correct sensor placement on the PCB to obtain appropriate isolationfrom heat sources

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Mechanical design rules
Figure 8: Example of incorrect sensor wiring on the PCB

In Figure 8: "Example of incorrect sensor wiring on the PCB", the left side of the figure in
the example shows that the wrong size of metal lines have been used. The bigger
dimensions provide a higher level of heat conduction. On the right, the incorrect placementof the sensor, close to a device generating too much heat, deteriorates the performance ofthe sensor. In both cases of thermal mechanism propagation, an infrared-based thermal
analysis of the whole system, running under different operating conditions, is the rightapproach for identifying the appropriate sensor location.

2.1.4

Mechanical stress
Placement of the sensor shall be such to avoid any mechanical force applied to the sensor,
due directly to an incorrect mechanical system design or manufacturing tolerances, or due
indirectly to user interaction with the system as in the case of wearables or portabledevices.
In other words, the final assembly must be free of hard contact or shearing forces between
the LPS33HW and the cylinder of the housing.
Figure 9: Incorrect configuration for mechanical stress

Figure 9: "Incorrect configuration for mechanical stress" above shows examples ofincorrect integration where the embodiment structure is directly in contact with the sensor
package, causing mechanical stress that can cause deterioration of the performance of thesensor. Thus, a minimal clearance has to be maintained to avoid any force applied to thesensor, also taking into consideration manufacturing tolerances.

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2.1.5

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Light exposure
Direct exposure of the sensor to light shall be avoided.

2.1.5.1

Sensor embodiment and housing
The sensor housing in the system shall match as much as possible the recommendations
previously highlighted for sensor placement and, additionally, shall provide all the features
required of the specific application (waterproof, water-resistant or resistant to a harshenvironment).
Furthermore, the design of the customer application shall guarantee the circulation of air
from the environment (outside) to the pressure sensor (inside). The more efficient the aircirculation is in this path, the better the performance will be as well.
The air path shall be well-identified and sized in order to maximize the airflow, and, as aresult, the final performance of the integrated system.
Figure 10: "Example of correct sensor embodiment and housing" represents a summary ofa correct example of sensor embodiment and housing. It also includes milling slits toincrease thermal decoupling which is a solution for the specific case where the devices
around the pressure sensor are generating too much heat.
Figure 10: Example of correct sensor embodiment and housing

2.1.5.2

Sensor protection
An optional filter can be adopted as a protection of the sensor from dust or from chemicalsolvents in the presence of a harsh environment. The key parameter for this kind ofimplementation is the appropriate choice of the membrane according to customer design
requirements and taking into account that the membrane material may cause a slower
response time, in particular in terms of pressure response time.

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3

Reference design: integration and housing on aPD

Reference design: integration and housing on a PD
The example below describes how sensor placement is implemented by following the basic
rules described in this document; in other words by mounting the sensor as far as possible
from the main sources of heat present on the board such as display LCDs andmicrocontrollers that represent the more critical sources of heat. Figure 11: "Integration ofthe LPS33HW in a sensor chamber with vent apertures" depicts the integration of thesensor in a sensor chamber insulated from heat and with one vent aperture sealed by anO-ring.
Figure 11: Integration of the LPS33HW in a sensor chamber with vent apertures

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4AN5063

Accuracy over extended operating range
The LPS33HW datasheet specifies the full accuracy operating range from 260 mbar to1260 mbar. The absolute accuracy variations in the range from 1200 mbar to 2 bar are
indicated in the graphs below (Figure 12: "LPS33HW absolute accuracy up to 2 bar (4sigma) from 10 to 40 degrees Celsius") as the average of a relevant number of samples
and the output of the pressure.
Figure 12: LPS33HW absolute accuracy up to 2 bar (4 sigma) from 10 to 40 degrees Celsius

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5Robustness to corrosive agentsRobustness to corrosive agentsThe LPS33HW was evaluated in a swimming pool and sea water to correspond to theapplicative environments. Hot chlorine, bromine, and salt water tests were run on theLPS33HW which showed high robustness and no impact in accuracy nor otherperformance (see Figure 13: "Chlorine, bromine and salt water tests"). Specifically, thetesting protocol included 34 cycles (34 days) where each test was composed of animmersion for 6 hours in each water condition and for the following 18 hours dried in aclimatic chamber at 60°C, 60% rH.
Further tests with detergent water (commercial shampoo, hand soap and water mixed in)were performed, demonstrating the robustness of the LPS33HW to these kinds of potential
corrosive agents as well.
Figure 13: Chlorine, bromine and salt water tests

Finally, the LPS33HW even showed robustness to n-Pentane liquid for 72 hours. This test
was performed in order to demonstrate the suitability of the LPS33HW for gas meter
applications or other industrial applications. No physical damage was observed. The
following table shows that the accuracy did not change after the exposure of the pressure
sensor to this agent.
Table 1: Accuracy before and after n-Pentane liquid for 72h
Mean P [sens]

Mean P [ref]Accuracy P

992.42 hPa

993.95 hPa-1.54 hPa

992.68 hPa

993.96 hPa-1.28 hPa

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Robustness to high air overpressure stress tests

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Robustness to high air overpressure stress tests
The LPS33HW was submitted to 40 test cycles where each cycle lasted 10 minutes at 10
bar air overpressure and 10 minutes of ambient pressure. The device demonstrated
robustness to these particular conditions. The device under test was immersed in water inorder to observe potential leakage as bubble formations (Figure 14: "Four LPS33HW
immersed in water with 3 mm tube for air overpressure"). Further tests were run on thewater-resistance specification (15 bar) and again the device showed no failures.
The device was tested by plugging a tube of 3 mm that fits perfectly with the LPS33HW
metal lid. Air overpressure was forced into the device hole while the device was immersed
in water in order to verify leakage by observing potential bubble formations.
Figure 14: Four LPS33HW immersed in water with 3 mm tube for air overpressure

In most of the real-use cases, the 10 bar overpressure lasts less than 2 seconds a and thenthe ambient pressure is applied for 2 seconds again (e.g. operating during swimming withshort and repeatable hydro-dynamic pressure variations), the device shows a negligible
recovery time of the initial performance as shown in Figure 15: "LPS33HW recovery
emulating swimming operating mode".

a Some recovery time might be necessary in case of prolonged overpressure conditions.

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Robustness to high air overpressure stress tests
Figure 15: LPS33HW recovery emulating swimming operating mode

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Revision history

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Revision history
Table 2: Document revision history

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Date

Version

28-Aug-2017

1

Changes
Initial release

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