Clip-On Cap/Cable Evaluation Kit
Applicable to following sensors:
SFM3200-AW, SFM3300-AW/D, SFM3400-AW/D
Key content:1▪
Clip-on cap/cable evaluation kit description▪
Clip-on cap/cable use cases▪
Technical information on clip-on cap/cable▪
Recommended approach for customized solution
SummaryThis application note describes the clip-on cap interface for the proximal flow sensors SFM3300-AW/D,
SFM3400-AW/D and the expiratory sensor SFM3200-AW. These sensors have in common that they can easily
be swapped to allow for cleaning of the sensor (AW-type) or exchanged and disposal (D-type). Both types
therefore feature the clip-on interface which allows for user friendly and reliable connection by medical staff.
This application note provides relevant information about the evaluation kit, how to use it for initial feasibility andlater implement it in prototypes and the final product. Technical details and guidance are provided for customers
looking to implement a customized cap/cable solution for their respiratory device.

1 Introduction
Sensirion’s autoclavable, washable and single-use mass flow sensors are optimized for proximal flowmeasurements in respiratory applications with adults (SFM3300-AW/D), neonates (SFM3400-AW/D) and for
expiratory measurements requiring minimal pressure drop (SFM3200-AW).
Since these sensors are exposed to the exhaled media from the patient, they need to be cleaned or replaced
regularly, typically carried out by medical personnel. Therefore, these sensors require a user-friendly and reliableinterface with a cable which connects them to the respiratory device.
Sensirion’s clip-on cap/cable evaluation kit demonstrates the advantages of the proposed clip-on cap/cable
solution and offers various possibilities to evaluate the clip-on cap/cable firsthand and with little effort even in thecustomer’s own setup. This application note also provides information for customers on how to design or source acustomized solution tailored to their specific respiratory device.

1 Only the new cap/cable evaluation kit supports the pressure sensor as well as the interface documentation referred tobelow.
Please note that the “old” evaluation kit (clearly distinguishable by article number and without pressure port) supports only
flow evaluation using the Sensirion Viewer software and no interface documentation is available.
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2 Evaluation Kit Clip-On Cap/Cable
Sensirion’s evaluation kit cable is shown in Figure 1 below. The evaluation kit cable is composed of an USB
connector, a 2 m cable, and the clip-on cap, which houses a small microcontroller and a pressure sensor.

Figure 1. Evaluation kit cable and SFM3300-AW sensor.

2.1 Clip-On Cap/Cable Functionality
The clip-on cap incorporates a miniaturized PCB with a microcontroller which enables the following features andadvantages.
Pressure Sensor in Clip-On Cap
In respiratory applications the patient’s breathing pressure often must be measured as close as possible to thepatient, typically at a proximal location. In order to avoid having to loop back a thin tube carrying the pressure
signal back to the respiratory device (e.g. the ventilator), a fast, high resolution and sensitive pressure sensor has
been integrated into the re-usable clip-on cap (Type: AMS 5915-0200-D-B). This allows measuring the pressure
proximal to the patient while avoiding additional tubes running back to the ventilator, which delay and attenuate
the pressure signal.
The evaluation kit features the clip-on cap with an integrated pressure sensor and a pressure port2 which can beconnected to the breathing circuit (e.g. the WYE piece) with a short tube.

Figure 2. Clip-on cap featuring the pressure port. Flow and pressure readings are returned electrically to therespiratory device.

2 Appropriate connecter type: 158442 Luer-Tubing Adapter (Male) for Flexible Tubing (https://www.rct-online.de/en/)© Copyright Sensirion AG, Switzerland2/8

External Heater
In specific applications it might be necessary to take scenarios of condensation or even icing of the sensor
element into consideration. To allow the sensor to operate in such harsh environments, a resistor has beenplaced on top of the flow sensor PCB which is intended to be used as a heater element. Heating power is limited
to 0.5 W and can be directly controlled by communication commands to values between 0 and 100%.
As heating the flow tube induces a thermal gradient over the sensing element, the set external heater power may
affect the sensor accuracy. Depending on the heater power applied and the accuracy requirements, additional
testing in the specific design-environment is recommended. Generally, it is recommended to only apply sufficient
power for the intended scenario, e.g. to avoid condensation, and not to continuously set the heater to 100%.
Sensor Supply Voltage On/Off Capability
Via communication commands to the microcontroller inside the clip-on cap the power supply to the sensor can beswitched on and off, to perform a hard reset.
Protocol/Bus Conversion
The sensor’s I2C protocol is intended for communication over short cable lengths (<10cm) only and therefore notideal to connect proximal sensors to a ventilator device with cable lengths typically extending beyond 1m.
In comparison the UART protocol is a much more robust communication protocol and better suited for signaltransport over longer distances. For this reason and in order to provide the functionalities mentioned above, amicrocontroller is incorporated inside the clip-on cap. This microcontroller reads the flow and pressure signals
from the flow and pressure sensor over a short distance via the I2C protocol and transmits the measurement
values using RS232 to the USB connector. Inside the USB connector the signal is then transformed into a USBsignal.

2.2 Clip-On Cap/Cable Evaluation Kit – Intended UseThe evaluation kit clip-on cap and cable was designed to allow for prototype testing as well as initial integration
into customer devices for feasibility testing. While allowing the clip-on cap to accompany the development
process as far as possible, several different use cases where considered:
Sensor Evaluation and Initial Testing with Sensirion’s Control Center Software
Sensor evaluation and initial testing is straight forward using our quick starter guide:
1.
2.
3.

Download install Sensirion’s Control Center Software:
https://www.sensirion.com/en/controlcenter/
Connect the sensor with the cap/connector cable to your computer
Start the software and run flow measurements

Proprietary Software on a PC interfacing via USB
The sensor readings can also be integrated into a customer specific application, e.g. a Simulink environment or
any other proprietary application. For details on the communication protocol, please refer to the Nicolay cap/cablecommunication protocol which can be downloaded from the following website:
https://nicolay.de/communication-protocol-flowsensor-cable-with-pressure-sensor
Installation of the FTDI USB driver might be required and can be downloaded from the following link:www.ftdichip.com/Drivers/VCP.htm
Prototype and interfacing via UARTThe sensor can also directly be connected to an electronic circuit board and interfaced with the UART RS232
interface. This requires cutting off the USB connector and follow the wiring diagram of section 2.3 for correctly
connecting the cable as well as the outlined communication protocol which is available as outlined in section 2.3.
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2.3 Key Characteristics of Clip-On Cap/Cable

Communication Protocol
The communication protocol (Nicolay flow meter connector with pressure sensor) is described in a separate
datasheet and provided by Nicolay. The PDF can be downloaded from the following Nicolay website:
https://nicolay.de/communication-protocol-flowsensor-cable-with-pressure-sensor

2.4 Order Information
The evaluation kit is best obtained from distribution.
▪
Name: SEK-SFM3xxx-AW/D Evaluation Kit Cable▪
Order Number: 3.000.518
For customized solutions, please refer to chapter 3.
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3 Recommendation for Customized Cap/Cable Solutions
Sensirion partnered with the company Nicolay for the development and manufacturing of our clip-on cap/cableevaluation kit. Nicolay has production tools readily available (e.g. molding tool for the clip-on interface) and offers
various options for customization of the clip-on cap/cable. The sealing inserted into the cap, which seals against
the PCB and protects the cap and sensor against spilled water is just one example of Nicolay’s expertise withcables for medical applications.
With many global projects supported with the Nicolay clip-on cap, this solution with the UART communication
protocol has been tested by many customers and qualified as a reliable solution. For any project going beyond
the initial feasibility, Sensirion recommends to reach out directly to Nicolay regarding a clip-on cap/cable solution
for our proximal and expiratory sensors. The contact details are the following:
Nicolay GmbH
Graf-Zeppelin-Str. 21,
72202 Nagold, Germany
Website: www.nicolay.deAdam Pawolka
E-Mail: a.pawolka@nicolay.dePhone: +49 745 282 3145

3.1 Available Options and Inquiry Form
The table below summarizes the most common cap/cable options offered by Nicolay. Please reach out directly toNicolay to discuss your preferences, requirements, and commercial aspects.
Configuration Options

Customer Selection

Cable length (10cm – 400cm, default 2m)___________cm

Select cable exit side○ left○ right

Cap with or without pressure sensor○ with pressure sensor○ without pressure sensor

Select PCB communication protocol○ RS485○ RS232

Select device connector interface○ Nicolay mini7 plug○ Nicolay mini12 plug
○ USB connection○ Open end cable (pigtail)○ other: __________

Please indicate order quantity (MOQ = 100pcs)___________pcs© Copyright Sensirion AG, Switzerland

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4 Own Clip-on Cap/Cable Design
In this chapter details are provided for customers preferring to design their own cap/cable clip-on. Please notethat Sensirion does not have a molding tool and solely offers the evaluation kit based on Nicolay’s solution. As
explained in section 2.1 it is recommended to integrate electronics inside the cap which allow to implement thefollowing functionality:▪
▪
▪
▪

Conversion of the communication protocol form I2C to robust long-range protocol (RS485/RS232 etc.)
Hard reset by switching sensor supply voltage on and off
Control of the external heater resistor
Communication with flow and optional pressure sensor

The connector interface of the proximal and expiratory sensors is designed to enable easy and user-friendly
connecting and disconnecting of the sensor. Sensirion’s proposed connecting method is to utilize a connector-cap
which attaches mechanically to the sensor with a clip-on interface and allows electrical connection via springloaded pogo-pins and the pads on printed circuit board (PCB).

CapSensor PCBSensor
Housing

Figure 3. Cross sectional view of clip-on cap and sensor housing
Figure 3 shows a cross sectional view with the drawing plane perpendicular to the flow direction of the flow
sensor and an attached cap. The sensor housing is colored dark grey, the PCB green and the connection-cap inlight blueish grey. To allow easy and secure fastening and removal of the sensor, the sensor connector interface
is designed with a simple snap-fit mechanism. The CAD file of the connector interface can be downloaded fromSensirion at the following link: www.sensirion.com/file/stepfile_sfm_cap
The following figure shows the top view of the sensor’s connector interface and the location of the electrical pads.
These pads on the PCB are best contacted via spring-loaded pins. The recommended component is theconnector Preci-Dip 811-S1-006-10-014101.

1234
5
6
Figure 4. PCB pad layout of sensor© Copyright Sensirion AG, Switzerland

6/8HEATData
GNDSCKVDD
HEAT_GND

Heater resistance
I2C, data
I2C, ground
I2C, clock
Supply voltage
Heater resistanceThe heater resistor on the top side of the PCB is specified to 51 Ohms and a maximum 0.5 W heating power.
Therefore, the supplied voltage drop over the resistor applied with pin 1 and 6 should never exceed 5 V,
respectively the current should not exceed 99 mA for full heating power.

5 Revision History
DateAuthor

VersionJan 2021

LOEH

1.0

Sept 2021

PSIM

1.1

Changes
Entire new version with description of evaluation kit details. Previousversion is now covered in chapter “Own Clip-on Cap/Cable Design”.
Removed preliminary disclaimer

6 Headquarters and Subsidiaries
SENSIRION AG
Laubisruetistr. 50
CH-8712 Staefa ZH
Switzerland

Sensirion Korea Co. Ltd.
phone: +82 31 337 7700 3
info@sensirion.co.kr
www.sensirion.co.kr

phone: +41 44 306 40 00
fax:+41 44 306 40 30
info@sensirion.com
www.sensirion.com

Sensirion China Co. Ltd.
phone: +86 755 8252 1501
info@sensirion.com.cn
www.sensirion.com.cn

Sensirion Inc., USA
phone: +1 805 409 4900
info_us@sensirion.comwww.sensirion.com

Sensirion AG (Germany)
phone: +41 44 927 11 66
info@sensirion.com
www.sensirion.com

Sensirion Japan Co. Ltd.
phone: +81 3 3444 4940
info@sensirion.co.jp
www.sensirion.co.jp© Copyright Sensirion AG, Switzerland

To find your local representative, please
visit www.sensirion.com/contact

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© Copyright Sensirion AG, Switzerland

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