Dear Johnny Zhuang,

Adjustment and calibration
Calibration means comparing the instrument reading with a reference. Adjustment is correcting the instrument to minimize deviation from the calibration reference. It is important to understand calibration uncertainty when deciding whether or not to make adjustments to your measurement equipment. The amount of calibration uncertainty varies depending on multiple factors, including temperature differences, the thermal mass of the instrument, and the measurement technology used. For this reason, it may not be wise to make small adjustments, because you may end up adjusting the random error and adding noise to your measurements as a result

Single-point calibration vs. multi-point calibration:
In single-point calibration, the calibration is performed at one point against the working standard by placing the reference instrument as close to the unit under calibration (UUC) as possible. It is important to allow sufficient stabilization time so that there is a temperature equilibrium between the working standard and the UUC. Single-point calibration is an effective way to maintain a sensor’s performance in applications where the operating conditions do not vary greatly. Sensors that operate at a stable temperature, humidity, pressure, and so on are ideal candidates for single-point field calibration.
Using a working standard and generator or chamber, which is able to produce various points enables the user to perform multi-point field calibration. The difference between one-point field calibration and multi-point field calibration is that the UUC must be removed from the process.

Laboratory calibration vs. field calibration
Laboratory calibration is the most accurate method of calibrating measurement equipment. It offers lower uncertainties than field calibration, environmental effects are minimal, and the number of factors that can influence the calibration process is significantly reduced. Field calibration provides the opportunity to perform rapid checks and diagnostics without the need to remove measurement equipment.
Handheld devices are especially good for quick pass/fail checks, but with a bit of extra care and attention, they can also be used to perform single-point calibration and adjustments in the field.

The GMP regulations specify only that sensors should be calibrated, and do not provide any guidance as to the specific calibration method. It is up to you to decide which calibration method is suitable for your application and will provide you with confidence in your measurements. Auditors and clients may have preferences or requirements of their own that exceed the GMP regulations, so you should have a solid rationale, including a risk assessment, for any calibration method you choose. If you have questions about the benefits and risks of a given calibration method, your best course is to consult a metrologist for advice.

Best regards,
Sanna Lehtinen and Paul Daniel