Shaw dew point meters

 Shaw dew point meters utilize a sensor based on a precise, hygroscopic material that saturates with water vapor at its maximum operating range, enabling automatic calibration (AutoCal) and ensuring stable, accurate readings. The system incorporates a desiccant dry-down chamber to keep the sensor dry between measurements, allowing for rapid spot checks and minimizing air or gas usage. 

Here's a more detailed look at the principles:

1. AutoCal (Automatic Calibration):

Saturating Sensor:

The SHAW dew point sensor is designed to saturate with water vapor at its maximum operating range, meaning it will reach a point where further moisture increase doesn't cause a further increase in its reading.

In-situ Calibration:

By exposing the sensor to an atmosphere wetter than its operating range, and adjusting the calibration control, the sensor can be calibrated in-situ without special equipment or skilled personnel.

Advantages:

This feature reduces downtime, minimizes calibration costs, and provides immediate verification of the system. 

2. Desiccant Dry-Down Technology:

Keeping the Sensor Dry:

The unique desiccant head and sensor assembly ensures the sensor is kept dry between tests, making it immediately available for rapid spot checks. 

Quick Response:

This technology allows for quick response from the dew point sensor, eliminating the need for cooling or servicing required by other methods. 

Minimizing Air/Gas Usage:

The dry-down feature also minimizes the amount of air or gas needed for testing. 

3. Sensor Technology:

Hygroscopic Material:

The SHAW dew point sensor is constructed with a hygroscopic material (usually ultra-high purity aluminum wire coated with a hygroscopic layer and porous gold) that absorbs and releases water vapor.

Base Layer Importance:

The construction of the base layer is crucial for reliable, stable, and drift-free performance, as it affects the sensor's capacitance and sensitivity.

Wide Range:

The sensor typically covers a dew point range from -100 °C to +20 °C, corresponding to 0.0015 ppm to 0.2 percent by volume. 

4. Operation:

Sample Gas/Air:

The gas or air sample is passed through the desiccant chamber, which dries it before it reaches the sensor. 

Reading:

The sensor's response to the moisture in the sample is then measured and displayed on the meter. 

Calibration:

The AutoCal feature allows for easy calibration and verification of the sensor. 

In essence, the Shaw dew point meter leverages the principles of moisture absorption and saturation, combined with a unique dry-down system and automatic calibration, to provide accurate and convenient trace moisture measurements.