Accuracy and Drift of Humidity Measurements

Last update: 15/03/2021
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When talking about drift and sensors, what is actually described, is the sensor’s ability to maintain precision over time. An example could be if you have a calibrated sensor that is accurate at e.g. 60 %RH and tests it again after two years of operation, you may find that it reads 61 %RH instead of 60%. This would indicate a drift of 1 %RH for the sensor.

All sensors drift over time – it’s just a question of how much. It is unavoidable – but often also not an issue since the drift of the sensor will often be less than your requirements for precision.The drift and rate at which it occurs are often related to the conditions the sensor is experiencing. Typically extreme conditions can cause a more rapid drift.

The CMP5000 has been designed with an Anti-Drift System (ADS) to drastically reduce the drift of the sensor and deliver a reliable measurement result in very challenging conditions.

Humidity sensors in general

Humidity sensors can use a wide range of technological approaches to determine the humidity content of e.g. air. One of the most common is a digital sensor based on circuitry. It is reliable, versatile, small in dimensions, and can measure most of the relative humidity range, which makes it a good choice as a sensor.

One of the drawbacks of this type of sensor is its ability to remain precise in high humidity conditions over a long period of time. If used in applications like weather monitoring, the humidity has a tendency to fluctuate causing the high humidity condition to be a temporary situation, and therefore the drift is not an issue. 

Humidity sensors in concrete and materials

When monitoring humidity in materials such as concrete, wood or sand, the challenges revolving around humidity increase. The main challenge still arises when the humidity is high, but due to the absorption of moisture in the material, the exposure can last for very long periods compared to e.g. moisture in the air.

A material like concrete can contain a lot of water, which also makes the drying process very slow compared to e.g. air. In addition to this, some materials may give off volatile components (such as gasses) that can accelerate the degradation of the sensor.

CMP5000 and its Anti-Drift System

Being aware of the issues around high humidity and long term exposure, we approached the development of the Anti-Drift System with a simple goal: Allow for long-term exposure in high humidity conditions without the risk of drift or damage to the sensor.

The CMP5000 has an Anti-Drift System (ADS) that actively protects the humidity sensor in extreme conditions. Because of this, the sensor handles high humidity conditions drastically better than any of its comparable rivals. The ADS uses a small amount of energy to protect the sensor when at high humidity, and is automatically disabled once the conditions have returned to a safe state for the sensor.

The CMP5000 has a long-term drift of maximum <0.5 %RH/year. 

Under normal conditions, long-term exposure to high humidity may temporarily offset the RH signal (e.g. +3%RH after 60h kept at >80%RH).

After returning to the normal temperature and humidity range the sensor will slowly come back to the calibration state by itself. Prolonged exposure to extreme conditions may accelerate ageing.

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