The Load Cell Calibration Standard ISO 376

The international ISO 376:2011 standard for calibrating force transducers has been effective since January 1, 2012. What are the implications for you as a user? To make this conversion as convenient as possible for our customers we have summarized the main changes for you. Table 1 gives an overview.

Table 1: Overview of changes in the new ISO376:2011

There have been no changes to the basic procedure, time frame and ambient conditions as stipulated in ISO 376 (table 1). However as an alternative, decreasing measurement series and therefore determining the hysteresis can be dispensed with. Instead, creep is now determined and considered as a new classification criterion. This results in four potential classification cases; table 2 shows which property is taken into account in which case. Case D enables direct comparison with the data provided in the previous calibration certificate and is relevant to the later use of the force transducer for calibrating material testing machines.

Table 2: ISO 376 classification criteria for the 4 possible cases, green fields are taken into account

Measurement uncertainty

Unfortunately, it is no longer possible to directly compare the measurement uncertainty with previous calibrations. A completely new approach for calculating the measurement uncertainty is now described in Annex C to ISO 376:2011. The modeling of the measurement process has been changed and, in addition, the calculations used for the individual contributions and weightings.

What is important is that the hysteresis is taken into account no longer. Since the hysteresis is relevant to later use and needs to be specified to enable comparison with previous calibrations, HBM calibration certificates provide this information in an additional table although it is not required in the standard. Customers do not have to make this calculation themselves and can take the value directly from the calibration certificate as usual.

Furthermore, a new function is specified that enables an expanded measurement uncertainty to be determined for any force. The customer can use it for his application and further calculations.

Figure 1: Excerpt from a calibration certificate; the function for computing the expanded measurement uncertainty is marked blue.

Since the underlying best-fit function describes all measurement uncertainties over the entire force range, it may happen that the value obtained for the expanded measurement uncertainty is higher than the one obtained in previous calibrations. Particularly for application as a transfer transducer, the analysis, as well, is therefore only suitable for specific forces, without interpolation. In this case it must be kept in mind that the transducer may be used solely for the points specified in the calibration certificate.

Fig. 2: Force calibration in an HBM calibration machine

HBM's new calibration certificates

Our calibration certificates are intended to provide our customers with as much information as possible.

For this reason, we continue to carry out calibrations with increasing and decreasing force, with hysteresis (Fig. 2: Calibration in an HBM calibration machine) to cover every application. The customer can then decide for himself which analysis, class or measurement uncertainty he wants to use.

Opting for a calibration certificate without hysteresis customers would have the restriction that only use with increasing force would be permissible later.

The changes to the standard have also resulted in a new layout of the calibration certificate. Customers now benefit from a clear layout and extensive information in line with modern measurement technology. The calibration certificate continues to provide all information required by the international ISO 17025 standard (for accredited laboratories).

We have updated the interactive calibration certificate on our website to make it easier for you to change to ISO 376:2011. It provides detailed descriptions of the data and calculations available in the calibration certificate.

Force calibration at HBM

HBM offers a unique force range for German Calibration Service calibrations ranging from 2.5 N to 5 MN in both directions (tension and compression).

It is not only HBM’s own make of force transducers that are calibrated with maximum precision here, calibration certificates are also drawn up for other makes.

Accreditation for the measurand force has been available already since 1977, when HBM was granted German Calibration Service accreditation as the first calibration laboratory in Germany. Customers now also benefit from this experience when implementing the new ISO 376.

Force calibration at HBM

Interactive calibration certificate

The new ISO 376:2011 calibration certificate - explained step by step:

Go to the interactive calibration certificate!