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Temperature Calibration: All You Need to Know About Calibrating Sensors

Find out more about the process of a temperature calibration and
why it is necessary to conduct a sensor calibration.

Watch our Calibration Video Series

Temperature Calibration: All You Need to Know About Calibrating Sensors

Find out more about the process of a temperature calibration and
why it is necessary to conduct a sensor calibration.

What is a Temperature Calibration?

A temperature calibration is a reliable, reproducible and documented comparison of one piece of equipment under test (data logger, sensor) with another piece of equipment that has been temperature calibrated and referenced to a known set of parameters. The referenced equipment is a high-precision instrument which is checked regularly by an accredited laboratory (Switzerland: SAS/SCS, USA: NIST, Germany: DaaKs, UK: UKAS). A temperature calibration is always performed at one or several defined measurement points. To perform the calibration, a device/machine is needed to keep a stable environment. For example:

  • A Temperature Calibration requires a calibration bath or a calibration block keeping a constant temperature to let the two devices adjust for a defined time (e.g. 30 minutes) to the temperature point
  • A Humidity Calibration requires a humidity generator or a reference solution (salt water) generating a defined humidity at a given temperature to let the two devices adjust for a defined time (e.g. 2 hours) to the relative humidity

The temperature or humidity measurement rows of the tested device and the reference device are then compared. If they do not match, the tested device is adjusted or sometimes replaced. As soon as the tested device measures the temperature or humidity within the specification, the result is documented in a calibration certificate.

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Infosheet on GxP-compliant Temperature Calibration

Download our infosheet on everything you need to know about a GxP-compliant Temperature Calibration.

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Video Series on Temperature Calibration

The following short Video series explains you everything you need to know about calibration, re-calibration and ISO 17025.

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Why a Temperature Calibration is Necessary
From a Regulatory Perspective

Sensor Calibration - In-process calibration explained

Data loggers are typically used in the GDP environment where packaged pharmaceutical products are stored and transported. Regardless which guidance document in the GDP-environment you consult (FDA, EU, PDA, ISPE, WHO, USP or ICH) – most of them require to work with “calibrated sensors”. Some more extreme interpretations require an ISO 17025 calibration also for cold chain shipments.

Calibration certificates are popular reference documents asked for during FDA audits “Shipment 123 which took place on October 1st two years ago – can you show me the data from the temperature data logger used for the product release? And can you also show me the calibration certificate from this specific data logger?” The calibration certificate should be available from anywhere and at any time and therefore stored in a safe cloud database.

Why a Temperature Calibration is Necessary
From a Technical Perspective

The core of each Data Logger is its measurement sensor. Each sensor type has a specific measurement range and accuracy and different strengths and weaknesses. Before the sensors are installed into a data logger, each sensing element is calibrated, i.e. tested on its accuracy, documented and thus exactly within the defined specification. The change of the accuracy over time is called “drift”. After one year, the sensor will be recalibrated to verify if the sensor accuracy is still within specification.

What is an “in-process calibration”?

An in-process calibration is a faster, less reliable and cheaper way to calibrate. It is mostly used on temperature sensors and is typically performed on-site and without using a calibration bath or block. It simply compares the current reading of a device under test and the reference device. An in-process calibration is by definition only performed at the current process temperature. In-process calibrations don’t have a very high reputation since the theoretical risks of measurement errors is higher. However, they still deliver a strong indication of the accuracy of the tested device.

What causes "drift"?

Drift can occur due to physical influences and the ageing of components over time. The risk for drift varies depending on the sensor type, as it depends strongly on the measuring principle used and how well the sensing element is protected.

Temperature sensors have a minimal risk of drift. In all applications - rooms and equipment as well as transportation - they are usually stable over many years.The risk for drift of humidity sensors depends on the application and the sensing technology.

As a general rule the following applies:

  •  use capacitive humidity sensors for room temperature environments and transport
  • use electrolytic humidity sensors for hot cabinets, incubators and for high precision environments (e.g. clean room)

Sensor Calibration - Risk of Drift Overview

The Life-Cycle of a Data Logger

To understand the term calibration, it is vital to know about the physical and technological properties of a data logger and its life-cycle. Calibration as such is not making a sensor more accurate. Accuracy is a quality built by design. The right sensor is chosen already during the engineering of a data logger and carefully built into the device using the right algorithm. Each sensor is calibrated already during production before it is build into the data logger. Afterwards, several checks and sample tests are performed in order to verify that the product meets all requirements – including accuracy. So when a data logger leaves the factory it meets all requirements, which is often stated in a validation certificate, and it has a production calibration certificate.

Would you like to know more about the differences between wired and wireless sensors? Download our infosheet on everything you need to know about using wireless sensors for temperature monitoring.

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Why to Re-Calibrate Temperature and
Humidity Sensors After One Year?

From a technical point of view, recalibration of temperature sensors makes little sense, as the risk of drift is minimal and there are indications that these sensors will be stable for many years. So why replace or calibrate temperature sensors after one year? Due to regulatory requirements. Regardless which guidance document in the GxP-environment one consults (FDA, EU, PDA, ISPE, WHO, USP or ICH) – in general they all require “calibrated sensors”. While some guidance documents are more general and just write “calibrated sensors must be used”, others are more specific requesting “regularly calibrated sensors” or even “yearly calibrated sensors”. Some auditors even tend to do an extreme interpretation of the regulations requiring "ISO 17025 calibrated single-use sensors" for transportation. There is no general rule of what regulation requires but most auditors appreciate a risk based approach considering the measurement range of the application as well as the type of sensor used.

Sensor Calibration - Types of Re-Calibration

 

How to Calibrate Temperature Sensors

Sensor Calibration - Re-Calibration of temperature sensors

 

During production, temperature sensors are tested and calibrated and the results are documented with the name "as found calibration". Then the sensors are adjusted to match the expected temperature value and are tested once more. The new results are called "as left calibration". Since resistance thermometers (NTC, PT100, and Digital temperature sensors) have a very low drift, they typically don't need to be re-adjusted during re-calibrations. If a temperature sensor is outside tolerance it is very likely that it has been physically damaged. Therefore, it is typically exchanged.

Want to know more about Re-Calibration? Watch our free video series!

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How to Calibrate Humidity Sensors

Sensor Calibration - Re-Calibration of Humidity Sensors

 

During production, humidity sensors are tested and calibrated and the results are documented with the name "as found calibration". Then the sensors are adjusted to match the expected humidity value and are tested once more. The new results are called "as left calibration". In comparison to temperature sensors, humidity sensors have a higher risk of drift. Therefore they are typically adjusted during re-calibration. The first calibration is called “as found calibration” and documents the status before adjustment. The sensor is then adjusted by the measured deviation. Afterwards the so called “as left calibration” is performed to check if the adjustment has been successful. If a temperature sensor is outside tolerance it is very likely that it has been it has been polluted too much and/or is physically damaged. Therefore, it is typically exchanged.

What Are the Right Calibration Points
for Cold Chain Loggers?

What are the right points to calibrate a cold chain data logger? Regulation is not clear on the number of points and where to position them. But if there is something like common ground between EU GDP and WHO, the following requirements can be found:

  • The data logger should be calibrated in 3 points. If fewer calibration points are chosen, a detailed technical rationale should be delivered including evidence that the sensor is stable over its operating range. For example for single-use and low-cost electronic indicators it is industry standard to only calibrate the sensors in one temperature point.
  • The calibration points should cover the entire operating range of the specific application. For example would the calibration points -30°C, 0°C and +50°C cover all applications in frozen, refrigerated and room temperature environment.

What is ISO 17025?

ISO 17025 is the international standard for calibrations. The general requirement to issue ISO 17025 certificates are:

  • The company (and in particular the laboratory) must have an ISO 9001 certified quality management system, which means that all processes must be documented.
  • The laboratory as such must be mapped and accredited by local authorities (e.g. Switzerland: SAS/SCS, USA: NIST, Germany: DaaKs, UK: UKAS)
  • The calibration method used, including measurement uncertainties, must be documented and accredited by the local authorities.

What is the difference between a factory calibration and an ISO 17025 calibration?

Technically, the two calibrations are identical, except two additional criteria for ISO 17025:

  1. Not 2-eye but 4-eye approval
  2. The entire process is not just GAMP5 validated but also approved by the accreditation body.

Although the main difference is the price (the exact same processes and devices are used, the same tolerances applied and the certificate almost looks the same) - why is the ISO 17025 certificate to popular with auditors? Usually auditory have few specialty knowledge on calibrations. ISO 17025 is a known standard: "I can rely on this certificate, since the process has been approved by an official regulatory body".

Be Careful What Suppliers Sell You as “Calibration”

The term “Calibration Certificate” has many different interpretations. As said above, the ISO 17025 is the safest bet since it forms a standard with clear requirements. Many suppliers use the term “Calibration” but in fact don’t fulfil the requirements. Here are a few examples of “face calibrations”:

  • A batch sensing elements is produced and tested (without storing the individual test result of each single sensing element). A sample is taken and tested. Based on the acceptance of this sample, the entire batch is released with a certificate. This should not be considered as calibration but only as “Validation Certificate”.
  • A sensor (the device under test) is tested against a reference device. But the result of the test is not documented. Instead the calibration certificate of the reference device is provided to the customer. This should be considered as “mistaken in process calibration”
  • Many devices are exposed to various temperatures in an oven with given tolerances. At the end of the process, there is a test if all devices have been within the tolerance and a certificate issued saying “all devices have been within tolerance” without naming the exact deviation measured of each device. This should rather be considered as a “Validation and Test Certificate”.

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