AN607

8

Rev. 1.7

Most of the Si70xx humidity and temperature devices are offered with a factory installed cover that is rated IP67.

The cover is solder resistant (it can withstand a peak temperature of 260 °C) and has a pore size of 0.25 µm, so it

blocks all dust while passing water vapor. The cover will block liquid water at pressure equivalent to over one meter

depth.

As a practical test of the effectiveness of the cover, Si7021 devices were subjected to a cigarette smoke test. In this

test, Si7021 devices were placed in a 3 liter jar with 20 lit cigarettes. During the time the cigarettes were burning, a

small air gap was allowed so the cigarettes would not extinguish. After burning was complete, the air gap was

removed, and the parts were allowed to sit in the smoke for 24 hours. Use of the cover reduced the maximum shift

from this extreme exposure from almost 40% RH to about 6% RH. In another test in which 100 cycles of

condensation were allowed to form on the parts, the cover reduced the shift in reading from an average of 2.0% to

1.57%. The cover has a minimal effect on response time.

3.2.5. Compensation for Heat Sources and Optimizing Transient Response

In some cases, it is not possible to completely isolate the sensor from the system for aesthetic reasons, and it is

desirable to still obtain a fast and accurate measurement of the ambient temperature and humidity by

compensating for the effects of the system.

In these cases, it is best to optimize the sensor location as much as possible using the above guidelines. To

compensate for the system effects, either the system must have a known effect, or an additional temperature

sensor (typically a thermistor) is needed. The thermistor should be placed to measure the temperature of the

system in the place where it is having the most influence on the humidity and temperature sensor.

In these cases, the Si7013 sensor is a good choice because it has an auxiliary A/D for digitizing the thermistor

voltage and a linearization engine for converting this to temperature.

3.3. Dealing with Condensation and High Humidity

Prolonged exposure to high humidity will cause gradual drift of the humidity sensor readings. All members of the

Si70xx family have on-chip heaters that can be used to heat the chip to counter local high humidity and reduce this

drift.

Condensation will also cause erroneous readings. If the condensation forms on the polyimide film, there can be

permanent shifts in sensor accuracy due to residue left after evaporation. The hydrophobic filter prevents liquid

water from penetrating, so condensation on the outside of the part will generally not result in condensation on the

polyimide sensing film of the Si70xx. However, if condensation forms on top of the filter, readings will be high until it

evaporates. Also condensation on the PCB can affect reliability and signal integrity. Turning on the heater will

reduce the chance of condensation forming and will also evaporate condensation.

However, turning on the heater will affect the local relative humidity (see also “Estimating RH with Heating” on page

28. For example, turning on the heater with the control setting 0x3 heats the sensor about 5 °C (depending on PCB

design and airflow), which results in a ~30% drop in local RH. However, due to variability in air flow and heater

current, the Si70xx heating can vary ±2 °C making RH readings with the heater on unreliable. Depending on the

nature of the application, there are several ways of dealing with this:

times, and use the on-chip temperature sensor to measure the amount of heating.

1 °C low) with the heater on. If dew point is the only concern, it can be calculated from the humidity and

temperature and then subtract 1 °C.

The Si7006, Si7013, Si7020, Si7021, and Si7034 have options for increasing the heater current up to 94 mA with

VDD = 3.3 V. Depending on the PCB layout and thermal design, it is possible to get junction temperatures well in

excess of 100 °C. Shifts in sensor readings from previous exposure to high humidity can be reversed by turning on

the heater with a sufficiently high setting to get the chip temperature over 100 °C for approximately 24 hours.