AN607

Rev. 1.7

7

Figure 7. General Thermal Model for Sensor Placement

If the system has a large thermal mass (C1) or other internal heat sources the thermal resistance from the sensor

to the system (R2) should be much larger than the thermal resistance of the sensor to ambient (R1). For rapid

response, the thermal mass connected to the sensor (C2) should be minimized. A practical example of this along

with some specific numbers is discussed in "Appendix E—Thermal Model for a Sensor on a Paddle" on page 38.

3.2.3. Optimizing Humidity Response

The humidity sensor element of the Si7006, Si7013, Si7020, Si7021, and Si7034 devices have a response time of

17–18 seconds at room temperature. For the Si7007, Si7022, and Si7023 devices, the humidity sensor response

time is six seconds at room temperature. If the humidity sensing element is placed within a cavity, this will be

affected by the following factors.



As discussed in "2.1.Humidity Sensor Technology" on page 4, as air is heated or cooled, the air humidity will

decrease or increase by 5% per degree of heating or cooling. The sensor and surrounding air must track the

ambient temperature in order to accurately measure humidity.



Generally, the opening should be comparable in size of the cavity in order to avoid slowing of the response.

Organic vapors, such as those from glue, can also produce a response. These effects are worse if the

cavity opening is small compared to the cavity size. The cavity should be made of materials, such as hard

plastic, metal, or hard rubber. Any sealants should be fully cured.

3.2.4. Protection Against Liquids and Dust

Like most other IC type humidity sensors, the Si70xx humidity sensors measure humidity by the change in

dielectric constant of a porous polyimide film. Conductive dust particles will cause errors in the readings,

particularly if they become lodged in the humidity sensing film. Organic solvents can cause a semi-permanent shift

in humidity readings. Even liquid water can have a permanent effect on humidity response if it leaves a mineral

residue after evaporation.

Generally, a porous cover is used to protect against dust and liquids. Typically, the cover is made of expanded

The effectiveness of the cover is expressed by the ingress protection or IP rating. A cover with a rating of IP67 is

recommended, which means the cover is completely dustproof and can withstand water immersion to a depth of at

least one meter.

Tambient

Sensor

Tambient

System

R3

R2

R1

C1

C2