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4.7 Serial number

Every sensor has a unique serial number, that is assigned by Sensirion during production. It is

stored in the one-time-programmable memory and cannot be manipulated after production. The

serial number is accessible via I2C and is transmitted as two 16-bit words, each followed by an 8-

bit CRC.

4.8 Reset

A reset of the sensor can be achieved in three ways:

• Soft reset: send the reset command described in Table 7.

• I2C general call: all devices on I2C bus are reset by sending the command 0x06 to the I2C

address 0x00.

• Power down (incl. pulling SCL and SDA low)

4.9 Heater Operation

The sensor incorporates an integrated on-chip heater which can be switched on by the set of

commands given in Table 7. There are three different heating powers and two different heating

times accessible to the user. After reception of a heater-on command, the sensor executes the

following procedure:

1. The heater is enabled, and the timer starts its count-down

2. On timer expiration a temperature and humidity measurement with the highest repeatability

is started, the heater remains enabled

3. After the measurement is finished the heater is turned off

4. Temperature and humidity values are now available

The maximum on-time of the heater commands is 1 second to prevent overheating of the sensor

by unintended usage of the heater. Thus, there is no dedicated command to turn off the heater.

For extended heating periods it is required to send periodic heater-on commands, keeping in mind

that the heater is designed for a maximal duty cycle of less than 10%. To obtain a fast increase in

temperature the idle time between consecutive heating pulses shall be kept minimal.

Possible Heater Use Cases

There will be dedicated Sensirion application notes elaborating on various use cases of the

heater. In general, the applications of the on-chip heater range around:

1. Removal of condensed / spray water on the sensor surface. Although condensed water is

not a reliability / quality problem to the sensor, it will however make the sensor non-

responsive to RH changes in the air as long as there is liquid water on the surface.

2. Creep-free operation in high humid environments. Periodic heating pulses allow for creep-

free high-humidity measurements for extended times.

Important notes for operating the heater:

1. The heater is designed for a maximum duty cycle of 10%, meaning the total heater-on-

time should not be longer than 10% of

the sensor’s lifetime.

2. During operation of the heater, sensor specifications are not valid.

3. The temperature sensor can additionally be affected by the thermally induced mechanical

stress, offsetting the temperature reading from the actual temperature.

4. The

sensor’s temperature (base temperature + temperature increase from heater) must

5. The heater draws a large amount of current once enabled (up to ~50mA in the highest

power setting). Although a dedicated circuitry draws this current smoothly, the power

supply must be strong enough to avoid large voltage drops that could provoke a sensor

reset.

6. If higher heating temperatures are desired, consecutive heating commands must be sent

to the sensor. To keep times between consecutive heating pulses minimal, polling of the