Best Practice for Sensor Calibration

The reference equipment should be a high precision device that is checked regularly by an accredited laboratory (Switzerland: SAS/SCS, USA: NIST, Germany: DaaKs, UK: UKAS). One should perform a temperature calibration either at one or several defined measurement points. For a successful calibration, the device should keep a stable environment.

Calibration is required to ensure consistency and product quality. Here are some of the reasons to conduct temperature calibration:

• Patient safety

• Regulatory compliance

• Product quality

While calibrating, never forget to test the complete system and one unit: meaning data logger and sensors.

What exactly is a temperature sensor, and how does it function?

A temperature sensor is a device, usually an RTD (resistance temperature detector) or a thermocouple, that collects the data about temperature from a particular source and converts the data into understandable form for a device or an observer.

Since a temperature sensor is always the first constituent in every temperature measurement loop, it is safe to say that it all begins with a sensor. It plays a critical role in the precision of the temperature measurement loop.

To have a complete temperature calibration, one should always test the sensor. By testing both the sensor and transmitter simultaneously, you verify the electronics. Adding a test of the control system input achieves a full loop calibration.

Temperature and sensor calibration ease the generation of the required papers needed as evidence of a successful calibration. The processes assist you to quickly and reliably calibrate your temperature equipment, keeping the operations on track.

Self-Calibration

Temperature changes slowly, making the calibration process very slow as one needs to wait for stabilization. Using a self-calibrating sensor to automate calibrations can come in handy.

That doesn’t mean the process will be shorter. It will still take the same time, but at least one will not have to wait for stabilization when opting for a self-calibrating sensor. The self-calibration procedure does not shorten the process duration, however, actively waiting for the stabilization completion is not required.

Self-calibration entails calibration carried out without any external connections with the process involving routing a known channel of reference to every other channel of the board.

The resulting voltage is then read against various gain settings and compared to a known set of parameters. As a means to compensate for any temperature change, the reference voltage is temperature protected.

A self-calibration process operates on-site and provides the documentation in compliance with the set requirements of regulatory agencies. Verifying that the temperature sensor is functioning as intended in between manual calibration cycles reduces the risk of undetected drifts and the need for frequent calibrations.

Grounded on the temperature at which its ferromagnetic properties suddenly change, the temperature sensor uses a high precision ceramic reference.

For applications using Steam in Place (SIP) operations, the device performs automated inline self-calibration at 118°C. It issues an alarm and displays an error message if there are any deviations to the distributed control system via a local LED.

Below are the advantages of using a self-calibrating sensor:

• It has an asset management software that produces printable and uploadable audit-proof calibration certificates

• Fully automated storage of up to 350 calibrations that can be used for verification

• Early detection of sensor failure, drifts and any measurement errors

• Reduces the cost of calibration

• Reduced risk of incorrect temperature measurements due to its short calibration intervals

• Straightforward visual monitoring via LED
  
  

Replacement Sensors

Most temperature sensors don’t have factory recommended replacement intervals. They technically work until they fail however, depending on the area of use, they may have a temperature drift which only a calibration can detect. Instead of a calibration, the more practical thing is to replace the sensor before the calibration certificate runs out.

If done regularly, replacing sensors is less costly as compared to repairing them. Repairing sensors might cost a lot attempting to identify the problem and ways to handle it.

A sensor can have incorrect readings due to the following reasons:

• Overheating Twisted cables

• Melted cables

• The unit might not be waterproof
  
  

On-Site Calibration

On-site calibration is sensor calibration carried out in the company’s lab by a third party professional. The equipment might be too large or fragile to be moved hence the need for on-site calibration.

Off-site calibration may be inconvenient for some companies as it entails sending equipment to an external calibration laboratory, which may result in equipment downtime.

That’s why on-site calibration is more beneficial compared to off-site calibration. However, calibrations happening outside a conditioned laboratory call for a high level of knowledge and experience.

On-site calibration has several advantages:

• Minimize equipment downtime

• Eliminate shipping costs

• Minimize process disruptions

• Eliminate equipment exposure to damage

Conclusion

Talking about warehouse monitoring it is crucial to have a system in place that complies with the regulations. Having a fully qualified system also requires a regular check of your system. Moreover, this check has to be duly documented according to GxP guidelines.

This check also includes a recurring system calibration where possible inaccuracies or sensor failures are detected. ELPRO recommends a complete annual service every year (or a sensor exchange) when the calibration certificate expires.