FAQ

Mass flow controllers

Communication interface

Sensirion MFC/MFMs feature the following analog and digital communication interfaces:

- Analog: 0–5 V, 0–10 V, 4–20 mA
- Digital: RS485, DeviceNet, IO-Link

The standard Sub-D 9-pin connector is the physical interface for both analog and digital communication.

Sensor performance

Sensirion MFCs directly measure the number of gas molecules (or gas mass) flowing past the sensor and therefore only show a very minor dependency on deviations in pressure and temperature from the calibration conditions. The respective temperature and pressure coefficients can be gathered from the datasheets. A change in gas temperature can cause both zero point and span errors. By contrast, a deviation in pressure may cause a span error only.

Pressure drop at full flow: This figure refers to the pressure reduction that the flow experiences due to obstructions such as the valve and the orifice at the full-scale flow rate. Consequently, the differential pressure between input and output needs to be greater than this value in order for the flow to reach full scale.

Maximum input pressure: This figure refers to the maximum pressure that can be applied at the upstream input. This results from the mechanical limitations of the seals. Above this value, the seals and valve may fail and the device may become less secure as a result.

Maximum differential pressure: This figure refers to the maximum pressure differential across the valve. If this value is exceeded, the pressure differential causes leakage through the valve even with zero flow and unstable control.

Sensirion’s patented CMOSens® technology encompasses several aspects. A highly innovative new measuring technology – employing the symmetric arrangement of two temperature sensors around a heating element – enables gas flow to be measured very fast and accurately. Another key aspect is the patented CMOS evaluation circuitry integrated on the same chip, which allows programmable and highly precise amplification and evaluation of the generated analog sensor signal.

Most measuring technologies used by competitors rely on steel capillaries to measure the upstream and downstream temperature, which is then used to deduce the mass flow. However, owing to the fact that such steel capillaries have a high thermal mass, the sensors measure changes in temperature relatively slowly, which in turn means the flow rate is also controlled at slow speeds.

By contrast, Sensirion’s innovative method of integrating both temperature sensors and heating elements onto a single chip results in a significantly lower thermal mass and thus records changes in temperature relatively quickly. Consequently, the device is capable of controlling flow more quickly and thus achieves best-in-class settling times.

The control range is defined as the ratio between the maximum and minimum flows that the device can control. For example, a control range of 1000:1 means that flow can be controlled from 0.1 to 100% FS.

Sensirion MFCs directly measure the number of gas molecules (or gas mass) flowing past the sensor and therefore only show a very minor dependency on deviations in pressure and temperature from the calibration conditions. The respective temperature and pressure coefficients can be gathered from the datasheets. A change in gas temperature may cause both zero point and span errors. By contrast, a deviation in pressure may cause a span error only.

Changing the reference temperature affects the volume at a constant pressure of 1,013.25 mbar. This volume change is governed by the Gay-Lussac gas law.

If you want to convert a normal liter into a standard liter, the formula is as follows:

V/T=constant (Gay-Lussac law).

Example: standard liter/standard temperature in K=normal liter/normal temperature in K -> standard liter=293.15/273.15 * normal liter=1.0732 normal liter.

Thus, a mistake with the reference temperature can result in a 7.3% error.

At temperatures slightly higher than the operating limit specification, the device faces the following issues:

Reduced accuracy: Temperature compensation is active only for the specified temperatures.

The full-scale flow cannot be reached: At higher temperatures, magnetization is reduced, which results in lower flow rates for the same valve voltage/current applied.

The leakage rate through the closed valve should be similar, since the valve is normally closed.

The MFC still works at higher temperatures notwithstanding these limitations. Please note that these relatively minor limitations are valid only for temperatures slightly above the maximum operating temperature. For even higher temperatures, e.g. 50°C above the operating limit, further reductions in performance can be expected.

Please contact Sensirion if MFCs are to be used outside the MFC specifications.

Mechanical integration

It is possible to connect the sensor using the 90° Legris connector. Internal tests have shown that its influence on performance is less than 1% m.v.

Sensor evaluation

To set up the MFC and make your first measurements, please follow the instructions in the Viewer Software Manual. The set-up time can be shortened by using the optional Evaluation Kit, which also contains a connection cable.

Gas flow sensors

Sensor performance

Changing the reference temperature influences the volume at a constant pressure of 1,013.25 mbar. This volume change is governed by the Gay-Lussac gas law.

If you want to convert a normal liter into a standard liter, the formula is as follows:

V/T=constant (Gay-Lussac law)

Example: standard liter/standard temperature in K=normal liter/normal temperature in K -> standard liter=293.15/273.15 * normal liter=1.0732 normal liter

Thus, a mistake in the reference temperature can result in a 7.3% error.

Differential pressure

Sensor performance

The o-ring that should be used is ultimately dependent on the application. Sensirion usually uses o-rings with the following specification: ID=3.50, W=1.50, Material=NBR.

The sensor is calibrated to work with air. If used with other gases, the accuracy of the sensor’s measurement results depends on the thermal properties of the gas. Note that this also applies to the zero point and temperature compensation.

Please avoid using the sensor with aggressive or etching substances, such as H₂O₂, NH₃, etc.

Unfortunately, in most cases using a conversion factor would not provide acceptable results. For high-volume applications, we are happy to discuss creating customized versions of our differential pressure sensors.

These sensors are highly suitable for a wide range of HVAC applications, and as a result they are increasingly being employed in this area. To find out more see the DP Sensors for HVAC Applications document, which is available from the DP sensor Download Center.

Sensor package

“SDP3x” is the brand name of this sensor family. This is why all sensors in this range have “SDP3x” marked on the housing (e.g. SDP37, SDP31).

Nevertheless, individual products can still be identified from their laser marking. This is done by replacing the “x” in SDP3x with the first digit of the product’s unique serial number.

For example, the laser markings “SDP3x 2243WP” and “SDP3x 62E878” are the products SDP32 and SDP36, respectively.

Electrical integration

For manual soldering, we recommend a maximum soldering iron temperature of 350°C and a maximum soldering time of 3 seconds.

Liquid flow

Communication interface

Since the digital output is realized as an open collector circuit, a pull-up resistor must be connected between the digital output and a Vhigh external voltage, which serves as the high-level voltage. The value of this high-level voltage may be chosen independently or be identical to the supply voltage in order to match the logic levels of your control system. See the SCC1 analog sensor cable datasheet.

Sensor performance

Sensirion does not offer recalibration services for sensors. Our sensor technology is designed without using moving parts and thus does not suffer from wear and tear. This means our sensors enjoy very good long-term stability, rendering periodic verification unnecessary from a technical perspective. However, we do recommend setting up an application-specific maintenance schedule, where the sensor’s performance is regularly checked within the application it is part of. Since such a schedule is highly dependent on the specific application, specialists in the application should be responsible for creating it.

There may be several reasons why the results of the liquid flow sensor are deviating from those of the reference sensor.

If the flow rate exceeds the specified flow range of the sensor, the flow signal can saturate and the sensor’s accuracy may decline. Operating within the flow range specifications will allow the sensor to reach its full performance capability.

Due to the thermal measurement principle, the thermal properties of the measured liquid can influence the flow measurement results. When using media other than the one the sensor is calibrated for (usually H2O or IPA), the sensor output may significantly change. Use the calibration field that best matches the principal component of your medium. Check the sensor’s datasheet for the available calibrations. To change the calibration field, click the Edit Default button in the upper-right part of the Sensor Viewer software interface and choose the appropriate calibration field.

There may be a problem with the reference sensor itself. Please check that it is working properly.

A high-frequency pulsation in your flow rate may adversely affect the usability of the flow data. If your flow rate is pulsating (e.g. due to a pumping mechanism as the flow source), try to eliminate or reduce this, e.g. by using fluid damping methods.

An air bubble caught inside the sensor’s flow channel or at the inlet of the sensor can change the signal output. Try to remove any air bubbles by flushing the sensor. Due to evaporation and especially at low flow rates (ul/min and below), it is very difficult to find fitting leaks through a visual inspection alone. One method to thoroughly test your flow path for leakages is to apply pressure to the liquid path and check whether the sensor measures any flow.

The SLQ-HC60 is calibrated for hydrocarbon (IPA) only and is not suitable for measuring water-based media. Depending on the required flow rate, the SLS-1500 or SLQ-QT500 cover the flow range from 3 to 80 ml/min for water. In combination with the SCC1 analog sensor cable, these sensors provide a 0–10 V analog output.

This stands for percentage (%) of the measured value (m.v.) or sensor reading.

In principle this is possible. To do so, select the standard calibration field that best matches your fluid’s chemical composition. For example, the water calibration can be used for saline solutions or the hydrocarbon calibration can be used for lubricant oils. A matching calibration field increases the possibility of a linear response from the sensor. A good rule of thumb is to use the IPA calibration for all liquids that do not contain any water.

Please be sure to mention the medium you are planning on measuring when talking to your Sensirion contact. Sensirion’s liquid flow sensors can be used for liquids other than water or IPA. Please contact us for further assistance on this topic.

Regardless of the fluid, your liquid flow meter is capable of achieving very high repeatability. Typical values for repeatability range from 0.8% to 1.5% for all media, depending on the liquid flow meter used. This allows you to use the sensor as a very precise relative gauge when liquid flow is required to stay within an acceptable range. In such a case, the sensor’s output can be compared to set maximum and minimum flow rates and used as upper and lower thresholds for reference. The liquid flow meter can then monitor process repeatability within verified acceptable limits. Since absolute values may vary from sensor to sensor, this tolerance band must be individually set for each flow meter.

Sensirion’s liquid flow meters are 100% factory calibrated with at least one standard liquid (typically H₂O or IPA). See the datasheet for your sensor for further information.

Make sure the fluidic system is not influenced by mechanical disturbances. Vibration or movement of the tubing or the sensor itself can have an impact on the liquid inside the fluidic system, which is visualized on the sensor’s output signal. The sensor is highly sensitive and fast, so it is possible that it is showing effects that had not been observed previously. Due to their low mass, microfluidic systems tend to be highly dynamic. The sensor itself is not sensitive to movement or vibration itself (at least to a reasonable level), but it does sense the real flow caused by the vibration.

There may be several causes for this effect, some of which may be linked to the environmental conditions of your set-up. These can include, for example, a change in the physical properties of the medium used or a strong temperature change, a proceeding blockage of the fluidic path or the flow source (e.g. due to the pump not working properly) or even a system leakage. Make sure the environmental conditions and physical properties of the medium are consistent. Check the fluidic path and fluid supply for possible errors.

Analyze each part of the fluidic system in order to understand the effects monitored. In most cases, the sensor is accurate and its results can be trusted. Remember that the sensor is highly sensitive and fast, and therefore may show effects that had not previously been observed. Due to their low mass, microfluidic systems tend to be highly dynamic. At a truly constant flow, the sensor’s noise signal should be almost the same as at zero flow. To observe this for yourself, use a pressure difference to generate a constant flow. Contact Sensirion for additional support if the observed behavior is still not well understood. Please provide flow data, graphs and a description of your fluidic system set-up.

This is normal. The sensor’s measurement accuracy is just as high at zero flow. For flows around zero, the accuracy specification as “% of full scale” applies. Below is an example with the liquid flow sensor SLI-2000.

Example: The specification for the SLI-2000 around zero flow is “Accuracy 0.2% of full scale”. The full-scale flow rate is 5,000 ul/min. At and below a flow rate of 200 ul/min, the calculated absolute accuracy of full scale results in ±10 ul/min.

For details about the different liquid flow meters, please consult the relevant datasheets.

There is no strict limit below which the liquid flow meters are unable to measure the flow rate. To answer this question, it is necessary to consider the sensor’s repeatability and the required accuracy of the measurement at a given low flow rate. The relative accuracy is either specified in % m.v. (of the measured value) or as a % of full scale (whichever error is larger). The sensor’s repeatability is basically its resolution. Looking at the relative accuracy, there is a particular flow rate at around 5% to 10% of full scale where the absolute measurement accuracy of “m.v.” and “full scale” are equal. For flow rates below this value, the error of the full scale is valid and remains valid down to zero flow.

Example: The specification for the SLI-2000 is “Accuracy below full scale 5.0% m.v. or 0.2% of full scale” (whichever error is larger). The full scale flow rate is 5,000 ul/min. At a flow rate of 200 ul/min, both the calculated absolute “accuracy of full scale” and “accuracy of m.v.” result in 10 ul/min. Thus, the possible measurement error below 200 ul/min remains constant at ±10 ul/min.

The sensor’s repeatability reveals its resolution. As explained above, at a flow rate of 200 ul/min the absolute measurement accuracy is ±10 ul/min. But as the sensor has a repeatability of 1 ul/min (0.02% of full scale), and this remains true even at much lower flow rates than 200 ul/min, e.g. at 50 ul/min, the repeatability remains at 1 ul/min.

In process control applications, the repeatability is very often more important than the accuracy of the actual flow rate. In such a case, you can basically rely on a repeatability “accuracy” of 1 ul/min (in the case of the SLI-2000).

Sensor package

No, it is not. The sensing element is not wetted due to the specific design of Sensirion’s liquid flow sensors, whereby the sensor measures the liquid flow rate through the wall of a straight capillary. The wetted material list for your sensor can be found in the Mechanical Specification section on the datasheet.

Mechanical integration

A basic set of fluidic connectors is included in your Flow Meter Kit. If other connectors are needed, we recommend purchasing them from reputable fluidic connector manufacturers such as Idex, Vici or Nordson Value Plastics. The sensor’s datasheet contains details on the fluidic connectors suitable for your specific model. For more details, see the Sensor Ports and Tubing Connections application note in the Download Center.

Sensor evaluation

Check that the USB driver is providing a virtual COM port and you are using the correct number. Ensure that the sensor cable is properly plugged in.

Ensure all other programs using COM ports are closed. It may be necessary to reboot the computer.

Ensure the operating system of your PC corresponds to the requirements specified in the operating guidelines. When plugging in the USB cable, the Virtual Com-Port (VCP) driver should be installed automatically. After the driver has been successfully installed, the device appears in the device manager as USB serial port.

If this does not happen, please install the necessary VCP driver from the following link:

Particulate matter

Sensor performance

Particulate matter (PM) can be found in both indoor and outdoor environments.

Indoor environments:

Common PM sources include sprays and smoke (cigarette, candle, incense, etc.).

Regular dust, which builds up over time when surfaces are not cleaned, is also a very common source of PM. This dust can be also seen by the naked eye when sunlight enters through a window and many particles can be observed flying through it.

Cooking might also generate many particles. For example, boiling oil creates quite high concentrations of PM in the air.

Outdoor environments:

In outdoor environments, PM is normally generated through the combustion of solid and liquid fuels, such as for power generation, domestic heating and in vehicle engines.

In towns and cities, emissions from road vehicles are an important source of PM2.5. Consequently, levels of PM2.5 (and population exposure) are often much higher close to roadsides than in places located further away from road infrastructure.

In some places, industrial emissions can also be important sources of PM, as can the use of non-smokeless fuels for heating and other domestic sources of smoke such as bonfires.

Under some meteorological conditions, air polluted with PM2.5 may circulate over neighboring islands or land – a condition known as the long-range transportation of air pollution.

The “x” in PMx stands for particle matters with a diameter equal to or smaller than the value of “x”, measured in micrometers. Therefore PM2.5 defines inhalable particles with diameters of 2.5 micrometers and smaller. PM10 defines inhalable particles with diameters of 10 micrometers and smaller.

PM is a mixture of airborne solid particles and liquid droplets. It can be inhaled and can cause serious health problems. The smaller the particles are, the deeper they can penetrate into the respiratory system and into the bloodstream after inhaling them. Historically, PM values are measured in μg/m3.

CO₂

Sensor performance

Empirical data shows that CO₂ concentration drops to levels very close to 400 ppm from time to time, even if the ventilation is not constant. This is often observed over weekends, for example. Even if the air does not seem fresh on Monday morning, the CO₂ concentration will be close to 400 ppm due to the absence of people. The SCD30 ASC algorithm is optimized to find these minima and trigger recalibration accordingly. A built-in consistency check makes sure that no false recalibration is triggered by the ASC. Finally, the SCD30 has superb long-term stability due to its dual-channel technology. Even if the ASC does not trigger recalibration over a longer time period (e.g. 2 months) the CO₂ output remains very accurate.

All SCD30 sensors are factory calibrated. However, since NDRI-based CO₂ sensors are delicate optical instruments, they can be offset due to experiencing mechanical stress during handling and assembly. If offsetting occurs, apply the Forced Recalibration (FRC) or Automatic Self Calibration (ASC) procedures to recalibrate the sensor.

Ideally, a sensor calibration should be performed immediately after its installation to compensate for potential mechanical stress during installation.

Humidity

Communication interface

The most likely explanation is that the ACK is missing after transmission of the MSB. The master must acknowledge the successful reception of the first 8 bits by pulling the SDA low on the 9th clock and then releasing the SDA once again.

To avoid experiencing communication problems, we recommend that cables should be no longer than 10 cm. As bus length (cable length) increases, the likelihood of capacitive crosstalk and insufficient Electromagnetic Compatibility (EMC) immunity also increases.

It is possible to use longer cables, but in such cases additional measures are often required to safeguard high performance levels. If you are experiencing problems, the following measures might improve communication:

- Use low-transmission frequencies; e.g. 10 kHz.
- Avoid running DATA and SCK next to each other; e.g. run them at the edges of a flat ribbon cable.
- Reduce the value of the pull-up resistor, e.g. to 3k.
- Use shielded cables.
- Use the CRC check feature; see also the CRC Check application note in the Download Center.

To verify error-free data transmission it is recommended to run a CRC check as described in the datasheet or application note for the relevant sensor. The relevant documents can be found in the Download Center on our website.

Since the interface consists of fully static logic, there is no minimum clock (SCL) frequency.

Sensor performance

Sensirion is unable to recommend time intervals for individual testing devices since too many external factors are at play. If your product is subject to regulations demanding verification within a certain time frame, it is advisable to make the sensor exchangeable so that it can be replaced if necessary.

The deviation measured against the reference may be characterized by an average value and a coverage factor “k” (k=1 is equivalent to standard distribution σ in the case of normal distribution). For typical accuracy tolerances at a certain log point, Sensirion understands that for a sample, such as a batch, average values of ±2k are located inside specified limits. In other words, 95% of sensors measure within this typical range.

More information can be found in the Sensor Specification Statement application note.

This is the time taken to achieve 63% of a step function, in this case a change in humidity from 10% RH to 90% RH. The value of 8 s is valid at 25°C and 1 m/s airflow; at lower temperatures, the response time will be slower, while at higher temperatures, the response time will be faster. The sensor adapts to ambient conditions even when it is not powered.

Extreme conditions – e.g. very low or very high humidity or exposure to solvents – can offset the sensor. Applying the reconditioning procedure may bring the sensor back to its calibration state. Please note that it is not essential to apply reconditioning after soldering. Leaving the sensor for a couple of days at 50–70% RH will bring the sensor back to specification. However, immediately after soldering and without rehydration, the sensor usually shows an offset of about -2% to -3%.

The reconditioning procedure comprises two steps:

- Baking: 100–105°C at < 5%RH for 10 hours
- Rehydration: 20–30°C at ~ 75%RH for 12 hours

For more details, please see the handling instructions available on our website.

As with all polymer-based capacitive humidity sensors, SHTxx sensors are sensitive to chemical exposure. Sensirion does not provide a chemical sensitivity chart. Compliance with the handling instructions is recommended to ensure correct functioning of the sensor.

Beside the nature of the substance itself, the duration of the exposure, the concentration of the contaminating substance and the temperature are critical factors in contamination. As a rule of thumb, air that humans can breathe over a long period without suffering any harm to health is not expected to contaminate the SHTxx.

If a deviation in temperature is observed, please be aware that any deviation from the specification must be larger than the sum of the specified accuracy tolerances of the tested sensor and reference sensor. Please make sure that the reference sensor is performing well.

The possible causes of such an effect may include the presence of heating or cooling elements close to the sensor, too many subsequent measurements (self-heating), housing that slows down the response time, using wires to connect the sensor or a missing decoupling capacitor between the VDD and GND.

Possible solutions might include disconnecting the sensor from the heating element by adding slits into the PCB or connecting the sensor to the rest of the PCB only via narrow bridges, ensuring that the sensor is not mounted directly on to heat sources or heat sinks, reducing sampling, shortening the cables and/or using a decoupling capacitor (typ. 100nF) so that the VDD and GND pins of the sensor are as close together as possible.

Guidelines for implementing sensors can be found in the Design Guide.

The sensors are designed to measure air humidity, not soil humidity. Calculating the humidity inside soil using values measured close to the soil is theoretically possible but complex. As we have not yet investigated using sensors for this kind of application, we are unable to offer support for this topic.

This cannot be performed directly. Our sensors measure relative humidity, but such values can be converted to absolute humidity or dew point if needed. Please see the Humidity at a Glance application note available in the Download Center.

Sensor package

You can find certificates for our humidity and temperature sensors in the Download Center.

Mechanical integration

Along with the casing, the filter cap provides protection against water immersion as well as against dust and particles forming on the sensor.