Latest loop calibration techniques and digital hot-swap probes to save costs and reduce downtime

In modern advanced manufacturing, product storage, and development applications, any interruption to measurement data used for environmental control or monitoring presents substantial risks, leading to significant direct costs.

How can modern instrumentation help you mitigate these risks and reduce costs?

These risks can be mitigated through planned maintenance, servicing and where critical, the use of redundant instruments. Critical maintenance includes regular calibration and filter replacements. Redundancy can be achieved by using additional measurement locations, separate monitoring platforms, or fully duplicated building management systems.

Calibration whilst vital for reducing these risks is a significant cost and typically results in significant downtime for the instruments during calibration. This blog will explore traditional and modern methods used to perform loop calibrations in the field which are a requirement in many of the most critical applications.

Problem Definition: Risks Associated with Humidity Instrumentation for Control and Monitoring Problem Definition: Risks Associated with Humidity Instrumentation for Control and Monitoring

Humidity measurement and control are often described as a dark-art due to the complex relationship between temperature and humidity. In nearly all scenarios, excessive humidity leading to condensation or ice formation is unacceptable and can severely impact products, processes, and people.

Even without condensation, poor humidity control can still affect products and processes in several ways, such as spoiling food products, caking of powders, changing product viability and shelflife, causing static discharge on critical components, and reducing human comfort.

Quality measurements of humidity are essential, but all humidity measurement technologies face several challenges:

• Measurement Accuracy: Even the very best industrial humidity instruments typically achieve at best only 1 %RH accuracy.
• Speed of Response: Humidity sensors interact with air, causing them to react slower than sensors for other parameters like pressure.
• Measurement Drift: All humidity sensors drift over time, leading to less reliable data without regular recalibration and adjustment. Drift is amplified typically in challenging or contaminated applications.
• Installation Location & Homogeneity: The placement of humidity instruments in a process or room can significantly affects their readings due to the high impact of temperature on humidity.
• Sensor Damage: Humidity sensors are relatively fragile and susceptible to damage from vibrations or contaminants.

Technical Insight: Loop Calibration

Calibration confirms the performance of a measurement instrument. It is now common in modern operations or even regulated that complete ‘system’ loop calibrations must be performed.

What is a Loop Calibration?

A complete measurement loop includes the measurement instrument, associated wiring, indicators, control system input cards, and the final display values on the PLC or system control panel.

Why are Loop Calibrations Needed?

Full loop calibration is essential as it verifies the accuracy of the entire measurement loop, ensuring the output used for control decisions or monitoring accurately reflects the actual process conditions. This holistic approach is crucial for maintaining operational safety, quality, and efficiency. While it’s often assumed that only the measurement instrument needs calibration once correctly commissioned, factors like cabling damage, input card drift, and PLC configuration offsets can all affect the final readings.

Two traditional approaches for performing loop calibrations:

The instrument is calibrated in situ, allowing data to be read from the PLC and compared to a reference. For instance, a humidity transmitter in a portable calibration generator might be used to compare the PLC reading to the generator reference.

Advantages:
+ Full loop calibration
+Instrument calibrated as installed
Disadvantages:
-Live data is interrupted during calibration  
-Typically, only one instrument can be calibrated at a time 
-Access to instruments is often impractical or disruptive (e.g., AHUs, Cleanrooms, Hazardous Areas) 

The instrument is removed for separate calibration, and analog signals are measured. In situ analog signals are injected into the system using a calibrated field calibration device capable of generating analog signals accurately.

Advantages:
+ Full loop calibration
+ Instrument can be calibrated offline (often to a higher standard)  
+ Reduced downtime
Disadvantages:
-Instrument must be removed and replaced (likely with a spare)   
-Electrical/mechanical installation needed   
-New instrument should be commissioned  
-Advanced field calibration device required  

Modern Digital Probe based Loop Calibration

Rotronic has pioneered a third approach involving digital probe hot-swap and digital loop calibration over the last 20 years. In this method, the instrument remains fully installed, and the digital measurement probe is removed manually. Calibrated digital probe simulators are connected, sending digital measured values to the transmitter, which are converted to analog signals through to the final PLC interface. Typically, three different simulators are used to provide a three-point calibration. This approach eliminates the need for mechanical or electrical installation, as probes can be swapped by hand. Once the loop calibration is completed, a replacement or spare digital probe with fresh calibration is connected. Removed probes can be calibrated ex-situ to a high standard in a local calibration lab or off-site.

Advantages:
+ Full loop calibration
+Transmitter calibrated as installed  
+Very low process downtime
+No electrical or mechanical changes
+ Can be performed by less technically trained staff
+No instruments or field calibration devices needed at the measurement location
+Removed probes can be calibrated to the best standard (not driven by process downtime)
Disadvantages:
-Transmitters with digital probe interface present a higher initial investment

Conclusion: The advantages of Digital Probes and Simulators

Digital probes allow field probes to be swapped without affecting compliance and traceability. Removed probes can then be calibrated away from production or installation areas, typically to a higher standard with less time pressure. The hot-swap process is quick, resulting in no or very brief interruptions to measured values. In the field, digital probe simulators inject fixed values into transmitters, facilitating rapid and efficient calibration and significantly reducing downtime. For accredited applications, simulators can even be calibrated with accredited certification.

By using digital probes from Rotronic and the updated calibration and measurement techniques then enable, organizations like yours can significantly improve the reliability of their processes, reduce maintenance costs, and enhance overall operational efficiency. This approach ensures that humidity control is no longer a "dark art" but a precise and manageable process. These benefits also apply to our range of digital temperature differential pressure and CO2 probes, contact us to discuss further.

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