Frequently Asked Questions (English only)
What do you suggest the time interval should be to test that the device is still calibrated and whether adjustments need to be made?
Sensirion does not suggest a time interval to test the device as it depends on too many external factors. If any regulations for your product demand verification on a certain time base, it might be advisable to make the sensor exchangeable in case it requires replacement.
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.
It looks like the sensor is in clock stretching mode. Should I decrease the clock (SCL) frequency?
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. 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, 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 our website.
What is the maximum length of the cables between the sensor and the microcontroller?
We recommend that cables are no longer than 10 cm to avoid communication problems. With increasing bus length (cable length), the likelihood of capacitive crosstalk and insufficient Electromagnetic Compatibility (EMC) immunity increases.
Longer cables are possible, but often require additional measures. These measures might improve communication if you are experiencing problems:
- 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 value of the pull-up resistor to, for example, 3kΩ
- Use shielded cables
- Use the CRC check feature; see also application note 'CRC Check' in the download center.
Do you have a chemical sensitivity chart for our reference?
As with all polymer-based capacitive humidity sensors, SHTxx sensors are sensitive to chemicals. Sensirion does not have a chemical sensitivity chart. Compliance with the handling instructions is recommended to ensure correct functioning of the sensor.
Beside the substance itself, the duration of exposure, the concentration of the contaminating substance and the temperature are critical to contamination. As a rule of thumb, the air one can breathe over a long period without any harm to health is not expected to contaminate the SHTxx.
I measure a deviation of temperature from a test sensor to my reference. What could be the cause?
If a temperature deviation is observed, please 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. As we have not really looked into this kind of application, we are unable to offer support for this topic.
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 on our website.
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.
Can I use your humidity sensors to measure absolute humidity?
I have read that the sensors are RoHS and REACH-compliant. Where can I get the certificates?
You can find the certificates for the humidity and temperature sensors in the Download Center.
What is the best way to verify your humidity and temperature sensors?
Please refer to our Testing Guide Humidity to learn how the sensor's performance can be verified. The document can be found in the Download Center on our website.
I need to implement a SHT2x into a 5V environment. Can you give me any advice on how to do this?
An application note, available in the Download Center, describes a possible solution on how to embed the sensor in a 5V application.
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.
I cannot find a socket/connector for SHT7x. Do you have a reference for an available product?
Spring-loaded connectors with a pitch of 1.27 mm (0.05”) will work. They are available from various manufacturers, such as Preci-Dip or Mill-Max; e.g. R851-83-004-20-001.
Is a protective cap for SHT7x available?
Sensirion does not offer a protective cap for SHT7x. Nonetheless, you can design your own filter according to your needs. For some applications, a compressed air silencer has been successfully used as a protective cap.
Evaluation Kit EK-Hx
How can I change the default path of the log file on the EK-H4 and EK-H5 viewer software?
There is no configuration file for the viewer software. The default path C:\Temp\DataLog.csv cannot be changed as it is part of the program source code.
What is the purpose of the filter cap?
In combination with the casing, the filter cap provides protection against water immersion and against dust and particles forming on the sensor.