Sensirion Humidity/CO2 Sensors FAQ

This post will cover some common questions Sensirion has received and answered on their humidity/CO2 sensors.

The ASC of the SCD30 is a pretty useful functionality, the only problem we find is forcing the end user to expose the device to “fresh air”. We can’t ensure regular fresh air conditions if a device is placed statically in an office with nonstable ventilation. Can ASC despite this work?

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

Your graphs in the datasheet show typical accuracy; how is typical interpreted?

The measured deviation against reference may be characterized by an average value and a coverage factor k (k=1 is equivalent to standard distribution σ in case of normal distribution). For typical tolerances of accuracy 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 limit.

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

​​How do I understand the RH response time of 8s? Is that the time I have to wait before I take my first measurement after powering the sensor?

It is the time taken to achieve 63% of a step function, in this case given by a humidity step from 10% RH to 90% RH. The value of 8s is valid at 25°C and 1m/s airflow. At lower temperatures, the response time will be slower; at increased temperatures, the response time will be faster. The sensor adapts to ambient conditions even without power.

My LSB of the received data always shows the same value 0xFF. The MSB shows different values when I change the humidity. What could be wrong?

Most likely the ACK is missing after transmission of the MSB. The master has to acknowledge the successful reception of the first 8bits by pulling the SDA low on the 9th clock and then releasing the SDA again.

What is the reconditioning procedure and when should I apply it?

Extreme conditions – e.g. very low or very high humidity or exposure to solvents – can offset the sensor. The reconditioning procedure may bring the sensor back to the calibration state. 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, right after soldering and without rehydration, the sensor usually shows an offset of about -2%/-3%.

The reconditioning procedure comprises two steps:

Baking: 100–105°C at < 5%RH for 10 hrs

Rehydration: 20–30°C at ~ 75%RH for 12 hrs

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

It looks like the sensor is in clock stretching mode. Should I decrease the clock (SCL) frequency?

What you are observing is most likely the hold master mode. During measurement, the sensor will pull the SCL low until it is finished. It can be changed by sending a different command for SHT2x, SHT3x, SHTC1 and SHTW1.

I experience problems with communication and would like to reduce the clock frequency on the interface. What is the minimum frequency?

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

I measure a deviation of temperature from a test sensor to my reference. What could be the cause?

If a temperature deviation is observed, be aware that an out-of-specification deviation must be larger than the sum of the specified accuracy tolerances of the tested sensor and reference sensor. Make sure the reference is working well.

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

Possible solutions might include disconnection of the sensor from the heating element by adding slits into the PCB, or connecting the sensor only with narrow bridges to the rest of the PCB. Make sure the sensor is not mounted directly on to heat sources or heat sinks. Reduce sampling. Shorten the cables and/or use a decoupling capacitor (typ. 100nF) between VDD and GND pins of the sensor as close as possible.

Guidelines for sensor implementation can be found in the Design Guide.

Can I measure soil humidity with your sensors?

The sensors are designed for measuring air humidity, not soil humidity. The translation from values measured close to solids to the actual humidity inside the solid is theoretically possible but complex.

How can I verify that my received values have been transmitted without errors?

You can implement a CRC check as described in the datasheet or application note for the relevant sensor. The documents can be found in the Download Center.

When is the ‘End of battery’ flag updated, after a T or RH measurement or both, and is the flag latching or is it updated at every T/RH conversion?

On the SHT2x and STS21 sensors, the battery flag is updated after each measurement. The flag is not latching and is therefore reset with each power on, soft reset or OTP reload.

All information was provided by Sensirion Download Center and FAQ .
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