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12

TH08 datasheet

4.5. Protecting the Sensor

Because the sensor operates on the principal of measuring a change in capacitance, any changes to the dielectric

constant of the polymer film will be detected as a change in relative humidity. Therefore, it is important to minimize

the probability of contaminants coming into contact with the sensor. Dust and other particles as well as liquids can

affect the RH reading. It is recommended that a cover is employed in the end system that blocks contaminants but

allows water vapor to pass through. Depending on the needs of the application, this can be as simple as plastic or

metallic gauze for basic protection against particulates or something more sophisticated such as a hydrophobic

membrane providing up to IP67 compliant protection.

The TH08 may be ordered with a factory-fitted, solder-resistant cover that can be left in place for the lifetime of

the product. It is very low-profile, hydrophobic and oleophobic, and excludes particulates down to 0.35 microns in

size. See section “8. Ordering Guide ” for a list of ordering part numbers that include the cover. A dimensioned

drawing of the IC with the cover is included in section “9. Package Outline ” . Other characteristics of the cover are

listed in Table8.

Table 8. Specifications of Protective Cover

Parameter

Value

Material

ePTFE

Water Entry Pressure

2.7 bar

Pore Size

0.35 µ

Operating Temperature

–40 to 125 °C

Maximum Reflow Temperature

260 °C

Oleophobicity (AATCC 118-1992)

7

IP Rating (per IEC 529)

IP67

4.6. Bake/Hydrate Procedure

After exposure to extremes of temperature and/or humidity for prolonged periods, the polymer sensor film can

become either very dry or very wet, in each case the result is either high or low relative humidity readings. Under

normal operating conditions, the induced error will diminish over time. From a very dry condition, such as after

shipment and soldering, the error will diminish over a few days at typical controlled ambient conditions, e.g., 48

hours

of 45 ≤ %RH ≤ 55. However, from a very wet condition, recovery may take significantly longer. To accelerate

recovery from a wet condition, a bake and hydrate cycle can be implemented. This operation consists of the

following steps:



Baking the sensor at 125 °C for

≥ 12 hours



Hydration at 30 °C in 75% RH f

or ≥ 10 hours

Following this cycle, the sensor will return to normal operation in typical ambient conditions after a few days.

4.7. Long Term Drift/Aging

Over long periods of time, the sensor readings may drift due to aging of the device. Standard accelerated life

testing of the TH08 has resulted in the specifications for long-term drift shown in Table 4 and Table 5. This

contribution to the overall sensor accuracy accounts only for the long-term aging of the device in an otherwise

benign operating environment and does not include the effects of damage, contamination, or exposure to extreme

environmental conditions.