How to read a scope of accreditation

Accreditation bodies publish each laboratory's scope as a table. For temperature, a typical row reads: 'Platinum resistance thermometers, by comparison in stirred liquid baths, −80 °C to 250 °C, CMC 0.01 °C to 0.02 °C.' Each element constrains what the laboratory may claim.

Reading each column

• Measured quantity / instrument type: what can be calibrated (thermometers, loggers, chambers, indicators).
• Range: the temperatures covered; look for gaps between ranges.
• CMC: the expanded uncertainty (k = 2) the laboratory can achieve for a near-ideal device. ILAC-P14 forbids reporting less than it.
• Method or remarks: comparison, fixed point, simulation; bath or block; immersion conditions.
• Location: permanent laboratory or site; the site CMC is usually larger.

If your logger has 0.1 °C resolution, a laboratory with 0.005 °C CMC gives you no better certificate than one with 0.02 °C — resolution dominates either way.

Four things to notice in that table

First, the rows are the unit of accreditation, not the laboratory. Second, the medium changes everything: rows one and four are both temperature, and the CMC differs by more than a factor of ten because one is a probe in a bath and the other is a logger in air. Third, look for gaps between ranges — a scope reading −80 to 250 and then 250 to 660 is continuous, but one reading −80 to 200 and 250 to 660 has a hole in it that will not be mentioned when you send an instrument for 220 °C. Fourth, on-site rows carry their own, larger CMCs.

Matching the row to your job

• Identify what your instrument is in the scope's language. A data logger is an indicating instrument, not a thermometer, and it belongs in the chamber row.
• Confirm your points sit inside the range, including any extreme the instrument has to cover — an excursion to 8.4 °C needs coverage at 8.4 °C.
• Read the method column. A scope that lists only comparison in stirred liquid baths cannot calibrate a logger that measures air, however good the CMC looks.
• Check whether on-site work is listed separately, if that is what you need.
• Expect your certificate's uncertainty to exceed the CMC. If it does not, ask how the budget was built.

The check that saves money

Work out your instrument's own floor before comparing CMCs. A logger with 0.1 °C resolution contributes 0.029 °C from resolution alone, so its expanded uncertainty cannot fall much below 0.06 °C whatever the laboratory does. Against that, a laboratory with a 0.005 °C CMC gives you a certificate essentially identical to one with 0.02 °C — and usually charges more for it. Choose on scope coverage, certificate quality and turnaround instead, because on uncertainty they are the same supplier.