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Demystifying the Chilled-Mirror: A Powerful Tool for Dew Point Measurement

This blog post explores the concept of chilled-mirrors and their role in accurately determining dew point, a crucial parameter in various applications.


Table of contents


Understanding Dew Point:

  • Phase Transition: Gases transition from vapor (gas) to liquid (dew) upon reaching a specific temperature called the dew point. This point is influenced by the gas composition and pressure.

The Chilled-Mirror Technique:

  • Core Principle: By forcing condensation onto a cooled surface (the mirror), the chilled-mirror method directly measures dew point. This approach provides a highly accurate and fundamental measurement.

Widespread Applications:

  • Over a Century of Use: Chilled-mirrors have been employed for over 100 years, primarily for moisture determination.

  • Beyond Moisture: The principle can also be applied to measure the dew point of any gas or gas mixture.

Chilled-Mirrors in Natural Gas Analysis:

  • Two Key Dew Points: In natural gas analysis, two dew points hold significance:

  • Water (Moisture) Dew Point: The temperature at which moisture condenses out.

  • Hydrocarbon Dew Point (HCDP): The temperature at which heavier hydrocarbon components liquefy.

  • Chilled-Mirrors for Both Measurements: With the exception of ZEGAZ Instruments CEIRS(™) Technology, there are no other automated chilled-mirrors that are capable of measuring both dew points successfully and accurately.These instruments have been used for a long time to determine both dew points.

Essential Components of a Chilled-Mirror:

  1. Cooled Surface: A mirror that can be precisely chilled.

  2. Temperature Sensor: Measures the mirror's temperature.

  3. Condensation Detection: A mechanism to identify the onset of condensation (dew formation).

Essential Components of a Chilled-Mirror, chilled mirorr infographic
Essential Components of a Chilled-Mirror infographic by ZEGAZ

Evolution of Chilled-Mirrors:

  • Early Manual Systems: Early chilled-mirrors relied on an operator visually observing the metal mirror surface for condensation signs during cooling with refrigerant gas.

  • Limitations: Subjectivity based on the operator's experience impacted the accuracy of this method.

Automated Chilled-Mirrors:

  • Light Scattering Method (Pre-2009): This method used a light beam directed at the mirror during cooling. A change in the reflected light profile upon dew formation signalled the dew point, allowing for temperature recording.

  • Issues with Light Scattering:

  • Inability to Differentiate: It cannot distinguish between water and hydrocarbon dew points, leading to inaccurate readings.

  • Contamination Sensitivity: Dirt on windows or in the gas can cause false readings.

  • Mirror Degradation: Metallic mirrors used in these systems tend to corrode over time, requiring replacements.

Introducing CEIRS™ Technology:

  • Developed by ZEGAZ Instruments: This technology addresses the limitations of the light-scattering method.

  • Focus of Next Blog: A future blog post will delve deeper into CEIRS™, specifically its infrared aspects, explaining how it overcomes these challenges in dew point measurement.

In essence, chilled-mirrors offer a reliable and direct method for dew point determination. While the light-scattering technique has limitations, advancements like CEIRS™ technology address these issues, providing a more accurate and robust solution for various applications.

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