Making Sure to Compare Apples to Apples

A common conflict between operations and maintenance occurs when grab samples measured with lab analyzers vary from inline field analyzers. The main culprit is typically when grab sample temperature changes in the time between sampling and measurement. However, painful discrepancies between sampling and inline process measurements can also occur when variations are found in the:

• Sensor construction
• Analyzer settings
• Calibration practices
• Quality of calibration standards
• Sensor maintenance

Getting the Most Reliable Measurements in the Field and the Lab

In the case of pH, inline process sensors are selected based on process requirements. At a minimum, proper sensor selection must consider the glass type, junction type, reference material type, sensor shape, and ground loop potentials. In contrast, typical lab sensors are often less specific, general-purpose devices that vary in construction from the process sensor. Calibrations of the analyzers should accurately correlate the measuring voltage to pH equally in both field and lab meters. However, experience shows that different technologies produce different results.

Best Practices

Assuming analyzer settings are equivalent in both devices, and the sensors are cleaned, maintained, and calibrated using best practices, to compare apples to apples:

Measure grab samples as close to the process temperature as possible

Use a portable handheld meter with data logging features. Measure the grab sample in the field and close to the process sensor.

• • pH is temperature-dependent. If you take a sample and the temperature changes, the pH also changes. Remember that open samples may also change pH due to exposure to atmospheric gases such as CO2.
  • When extracting samples near the process sensor, use an analyzer with suitable ratings when measuring in electrically hazardous classified areas.

Utilize the same sensor design and construction with both field and lab analyzers

• • Using the same sensor type ensures equal accuracy, repeatability, and durability.
  • This can also streamline the supply chain through standardization and eliminate the need to keep a variety of sensors on hand.

Implement smart digital sensors

• • Use smart digital field sensors that you can calibrate and maintain offline in a shop or laboratory environment. This eliminates the challenges of calibration and maintenance in the field and will enhance sensor reliability.
  • Smart digital sensors with inductive connections to mating cables provide perfect galvanic isolation and eliminate troublesome ground-loop potentials.
  • Smart digital sensors have memory and retain calibration so that you can disconnect and reconnect the sensors from laboratory analyzers without re-calibration.

Use digital communication cables for both laboratory and field analyzers

This eliminates the effects of ambient noise with coaxial cabling. Coax cabling is subject to outside interference from electromechanical devices and rotating equipment. Induced noise can affect measurement accuracy. Digital communication cables, such as RS-485, are not subject to external interference and ensure a clean signal.

Conclusion

In summary, you can eliminate much of the variability in measurement values between laboratory and field pH analyzers by using the same (or similar) sensor technology. Rapid developments in digital sensing technology, especially for electrically classified areas, provide a platform for multipurpose usage (lab and field). Digital sensors can also eliminate inaccuracies due to ambient noise, grounding issues, and calibration errors. Finally, it is essential to recognize the impact of temperature change on pH readings. You should measure pH with a portable meter at the point of installation of the field device and directly after the sample is taken.

These steps will help resolve the conflict between maintenance and operations and ensure the most accurate process pH measurements.