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A 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 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 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:
The 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 within the specification, the result is documented in a calibration certificate.
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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.
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.
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)
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.
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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 recalibrate 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.
During production, temperature sensors are tested 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.
During production, humidity sensors are tested 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.
ISO 17025 is the international standard for calibrations. The general requirement to issue ISO 17025 certificates are:
Technically, the two calibrations are identical, except two additional criteria for ISO 17025:
Else wise the exact same processes and devices are used, the same tolerances applied and the certificate almost looks the same.
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