How to Control Humidity Accurately in a Rapidly Changing Environment

How to control humidity accurately in a rapidly changing environment
Joni Partanen
Product Engineer
Jun 4, 2019
Industrial Measurements

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.

Read more of Compact Humidity and Temperature Probe HMP9 for Rapidly Changing Environments

Comment

Hugo Cancio

Jun 6, 2019
You can also speed up the response by moving air at the sensor.

Joni Partanen

Jun 18, 2019
Thanks for your comment. You are absolutely right. The air flow at the sensor will reduce the response time of a temperature measurement. Even though that the probe response improves in the moving air, a measurement probe with a lower thermal mass will still be relatively faster in the same environment.

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