How to control humidity accurately in a rapidly changing environment

Rapidly changing environments are always a challenge for humidity control. This is especially true in applications where relative humidity is the preferred control parameter. Relative humidity is the most commonly specified control parameter in many industries and standards. It is a good choice for example in drying, moistening, or storage applications such as food drying, where the equilibrium moisture content (EMC) of an end-product is closely related to the relative humidity. The downside of relative humidity as a control parameter is that it’s relative to temperature, which means that accurate measurement requires representative readings from both humidity and a temperature sensor.

Measuring in slowly changing environment vs. rapidly changing temperatures

A good-quality capacitive humidity sensor can accurately measure relative humidity in a stable or slowly changing environment, but as relative humidity depends on temperature, rapidly changing temperatures may be challenging for accurate relative humidity measurements. Humidity sensors measure from inside a filter, which is attached to a sensor head. In industrial applications, the sensor head is usually made out of metal and has a significant thermal mass, which becomes a problem if there is a rapid change in the surrounding environment.

Let’s imagine, that the temperature drops suddenly; it takes some time for the probe head mass to stabilize to the new temperature. Meanwhile, the conditions inside the probe filter are not the same as on the outside: it’s still warmer in the microclimate created by the filter and the probe body while it’s cooling down. The rapid change in the environment can’t be detected accurately because of the sluggishness of the thermal mass, and this sluggishness may be a challenge for responsive humidity control.

Solution: a lighter probe body can adapt to the changing environment faster, improving the response time

The solution to this problem is to reduce the thermal mass of the probe. A lighter probe body can adapt to the changing environment faster, improving the response time of the temperature measurement. The Vaisala HMP9 Compact Humidity and Temperature probe has a miniature probe head (5 mm in diameter) with a low thermal mass providing a superior response time. The probe contains a HUMICAP® sensor that provides industry-standard humidity measurement performance, and its small size enables installation in tight spaces. This allows measurements to be made in locations where traditional 12 mm diameter probes simply can’t fit.

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