Z540-1 Calibration Services

 Brief history

For many years, customers doing US government contract work were required, as part of their contract, to calibrate their measurement and test equipment (M&TE), according to MIL-STD-45662A. In 1994, as a cost saving measure the Department of Defense prohibited the use of all military standards in favor of commercial performance standards. In response, NCSLI members quickly developed ANSI/NCSL Z540-1. Z540-1 was renewed several times until it was rescinded in 2007, after the newer ANSI/NCSL Z540.3 was approved.

 Measurement Uncertainty

The traditional quality figure of merit for calibration measurements used in MIL-STD-45662A was Test Accuracy Ratio (TAR). This is a simple ratio of the [accuracy] specification of the lab standard to the specification of the instrument submitted for calibration. A ratio of 4:1 was considered desirable and adequate. Even today TAR sometimes works well for simple comparisons such as length in gage blocks, or DC Volts where electronic standards generally require only a single connection.

However, when calibrating RF/MW instruments where calibration procedures often involve measurement paths with up or down conversion, attenuators, mismatch error, and frequency response corrections, TAR is insufficient to address multiple sources of error.

In the 1980s many metrologists began to experiment with using Test Uncertainty Ratio, TUR, where the denominator, U, was a combination of the measurement errors (uncertainties). The publication of the ISO Guide for Expression of Uncertainty of Measurements (aka ISO GUM), in 1995, provided a formal method to combine multiple uncertainties in the form of an international standard.

Hewlett Packard, which had been using TUR at the time switched over to the ISO GUM for new product introductions of most instruments by 1997. While Z540-1 does allow the use of TAR, later versions of the Z540-1 handbook recommend the use of the ISO GUM. ISO 17025 first published in 1999 references the ISO GUM as does ANSI Z540.3 (see Figure 1).

How to Order Z540-1 Calibration

If you speak to someone from the Keysight Englewood call center and request a “Z540 calibration”, they will ask if you mean Z540-1 or the newer Z540.3. Many customers still require Z540-1 and Keysight is happy to continue to provide that level of calibration, even on legacy instruments. Choose from these two services:

• For newer instruments, we recommend “Keysight calibration + uncertainties”. This calibration service meets both ISO 17025 and Z540-1, includes ISO GUM uncertainties, and an accreditation body symbol
• For older legacy instruments, generally introduced before 1995 (the 1st publication of the ISO GUM), Keysight offers “Z540-1 Cal service”, which includes TUR uncertainty, and complies with Z540-1. “Z540-1 Cal service” does not include the accreditation body symbol, as ILAC-P14 (published Dec 2010) only allows the accreditation symbol when the measured results are also provided along with ISO GUM uncertainties