On-Site Dew Point Sensor Calibration
Why do we Need Dew Point Sensor Calibration?
Some manufacturers claim that dew point sensors (DPS) do not need calibration or include self-calibration features. However, calibration is essential when:
- Measurements must be traceable for quality systems or to meet regulatory compliance
- Two instruments give different measurements under the same conditions
- An industrial drying process depends on accurate humidity-moisture measurement
In these situations, reliable measurements are critical for product quality and process efficiency, which traditionally involves regularly sending sensors back to manufacturers for calibration.
Bringing Multi-point Calibration On-site:
The FPG, the world’s first transportable dew-frost point calibrator, is designed to enable users to calibrate sensors, probes and transmitters without removing equipment from site or sending it to the manufacturer or a calibration laboratory.
Until now, on-site testing of dew point sensors, moisture analysers, and hygrometers operating between 0.1 and 10,000 ppmv (-100 to +10 °C frost/dew point) was usually limited to a single-point check. Full multi-point calibration across the instrument’s operating range was normally only available from the manufacturer or a specialist calibration laboratory. The FPG now makes multi-point, on-site calibration possible.
The Benefits of On-site Calibration with the FPG
The FPG puts you in control on-site, offering numerous benefits, including:
- The ability to calibrate the complete measurement loop or monitoring system for full system validation
- Regular, traceable calibration that supports ISO 9001 and other quality management requirements
- Reduction in ongoing costs of maintaining and calibrating dew point sensors
- Cutting out the need to stock multiple sensors
- Removal of shipping and administration costs
- Removing the labour and downtime associated with changing sensors
- Reduction in documentation and change-control costs
- Universal compatibility – the FPG has been developed to be easily configurable to suit all types of DPS
Simple Calibration with the FPG
The FPG is easy to set up and only requires an AC power supply, making it suitable for use wherever calibration is needed.
Fit the correct FPG PA adapter for the instrument under test (IUT), install the sensor, and make sure the system is sealed. Set the first (lowest) required dew/frost point and allow the system to stabilise. Record the result, then repeat the process for each calibration point until the test is complete.
The initial data provide the As Found calibration results. If the instrument is outside specification and needs adjustment, follow the manufacturer’s adjustment procedure. Repeat the calibration to produce the As Left results. These results can then be recorded in a calibration report or certificate if required.
The profile shown illustrates a typical FPG calibration run, comparing the FPG saturator temperature, a reference dew point mirror, and the instrument under test.
Calibration or Adjustment?
Calibration and adjustment are often confused; they are different processes:
Calibration compares an instrument with a reference standard to determine its precision. The instrument is not changed in this process.
Adjustment changes the instrument to reduce or eliminate any measurement error.
The FPG performs calibration only. If adjustment is required, refer to the instrument manufacturer’s instructions. Once the adjustment has been completed, the FPG can be used again to confirm the instrument’s performance.
Managing the Effect of Dew Point Sensor Filters
Most dew point probes use filters to protect the sensing element from damage and contamination. However, these filters can significantly affect measurement and calibration results, particularly at very dry conditions below -50 °Cdp.
Although a filter has a small external surface area, its porous structure creates a much larger internal surface where moisture is retained. Water vapour trapped in the filter can take many hours to reach equilibrium with the gas flow, increasing stabilisation time and results. Removing the filter before calibration reduces stabilisation time and lowers the risk of contaminating the FPG.