How to Manage Measurement Instability when using an Insertion Chilled Mirror

Measurement instability can be challenging, especially when dealing with sophisticated instruments. If you have confronted this common issue, you're not alone. But fear not – by embracing a few essential best practices, you can mitigate this problem. This blog will guide you through practical tips and key considerations to ensure consistently stable and reliable measurements.

Why is measurement instability an issue? 

Measurement instability can lead to inaccurate and unreliable readings, which can have significant implications for various applications where a precise measurement of dew point, humidity, and temperature is critical, including: 

Given the potential disruption, it is crucial to address measurement instability proactively. 

Six tips for managing measurement instability

The measurement instabilities shown below were recorded using Michell Instruments Optidew 501 insertion chilled mirror.

1. Keep the mirror clean and calibrated

Ensuring stable measurements with your Optidew device starts with maintaining a clean and calibrated mirror. Dust and dirt can interfere with the optics, leading to unstable readings. Here’s how to keep your mirror in optimal condition: 

A clean and calibrated mirror is essential for consistent water or ice film formation, which is crucial for accurate measurements. By following these steps, you can significantly enhance the stability and reliability of your Optidew readings. 

2. Inspect the mirror for damage 

A regular check for damage or discoloration can prevent measurement instability before it has the chance to develop. If you notice either of these, and cleaning the mirror doesn’t resolve the issue, the sensor head may need to be returned for service. 

3. Watch for corrosion

Abnormal conditions leading to corrosion of the mirror and surrounding electrical components can also cause instability. Regular inspections can help catch and address this issue early.

Corrosion of the mirror and surrounding electrics 

4. Manage the flow rate

Flow rate across the sensor head is another critical factor. The recommended maximum flow rate is 2 l/min. Exceeding this can destabilize the water film on the mirror, leading to measurement instability. If you’re within the recommended flow rate and experiencing issues, consider using a sintered sensor guard as a buffer. 

OPT2 guard

5. Control lighting conditions 

Lighting will significantly impact your device’s performance. Bright sunlight or artificial lighting can disrupt the sensor optics, leading to unstable readings. For accurate measurements, it's important to manage lighting conditions effectively. 

Consider relocating the sensor head away from direct light sources. If moving the sensor isn't an option, try using shading to block out intense light or opt for a sintered sensor guard. These measures can help mitigate the effects of disruptive lighting and maintain the accuracy of your sensor, as shown in the graph below. 

Data demonstrating the instability of the dew-point measurement (orange) when the internal climatic chamber light was briefly switched on. 

6. Manage rapid environmental changes 

Rapid Increases: If humidity quickly becomes elevated, it can cause the Optidew to lose the dew point, resulting in periods of instability. In environments with high relative humidity (RH), a sudden rise in both temperature and humidity can push the dew point above the sensor body's temperature. This creates saturation conditions, leading to water flooding the mirror and causing the device to lose control of the dew point. 

Rapid Decreases: A fast drop in humidity can cause the Optidew to lose the dew point, leading to periods of instability. In high relative humidity (RH) environments, a sharp drop in temperature and humidity can cause saturation if the temperature falls faster than the humidity, a common occurrence in climatic chambers. When the temperature drops below the dew point, saturation occurs, and the Optidew quickly loses control of the measurement.  

The best way to address these issues is to slow down the rate of environmental changes. This helps keep the sensor head and ambient temperature above the dew point. If it's not possible to control the rate of change, you can schedule a Dynamic Contamination Correction (DCC) during the transition. This will prevent the mirror from flooding and maintain measurement stability. Once the DCC is complete, the sensor quickly adjusts to the new dew point. Also, positioning the sensor horizontally can help the condensate drain from the mirror, further stabilizing the readings. 

By following these best-practice tips, you can maintain measurement stability and ensure confidence in your insertion chilled mirror device. Regular maintenance and careful management of environmental conditions are key to preventing instability and achieving reliable measurements. 

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Author: James Coveney, Technical Support Engineer