Monitoring the transport conditions of travelling standards during interlaboratory comparisons by means of data loggers

For the success of international interlaboratory comparisons, it is of essential importance that the measurement artifacts forwarded (and their properties) do not change during the comparison. A simple data logger, which is sent together with the artifact, allows extreme conditions to be identified during the transports. From the data obtained with such a data logger within the scope of the BIPM interlaboratory comparison CCAUV.U-K3.1, clear recommendations could be derived on the use of different means of transport (airplane, rail,…) and of different types of transport (courier service,  personal accompaniment,…).

Interlaboratory comparisons are an important method to validate the calibration and measurement capabilities of National Metrology Institutes (NMI) independently – in the "Mutual Recognition Arrangement (MRA)" of the BIPM it is, for example, stipulated that the NMIs which participate in the MRA recognize the equivalence of the results of other NMIs, provided that they have been validated by corresponding interlaboratory comparisons. In practice, a transportable reference standard (a measurement artifact) with stable and known properties is usually forwarded between the NMIs involved in the interlaboratory comparison and measured there. For the comparability of the results it is, of course, of utmost importance that the properties of the measurement artifact have not changed during the comparison. While it can be assumed that the measurement artifact is handled correctly and with care in the participating NMIs, the transport between the participating NMIs is a critical process.

Within the scope of the BIPM interlaboratory comparison CCAUV.U-K3.1, transport of the measurement artifacts has been monitored with a data logger which continuously records the temperature, the air pressure, the air humidity and the acceleration. Fig. 1 shows the corresponding values which have been recorded during a transport by courier from INMETRO (Brazil) to PTB. One clearly sees the day/night temperature fluctuations caused by storage in non-temperature-controlled rooms in Brazil, the flight phases with clear air pressure drops, the storage in Germany in temperature-controlled rooms or outdoors, as well as the moments when the parcel was moved.

Figure 1. Values recorded with the data logger for temperature and air humidity (top) and air pressure and acceleration (bottom) during transport by courier from INMETRO (Brazil) to PTB.

For comparison purposes, Fig. 2 shows the data which were recorded during a transport from NMIJ (Japan) to PTB. As – in this case – the parcel was transported during the flight in the passenger cabin, no clear temperature drop can be seen during the flight (in contrast to the transport in the cargo hold in Fig. 1) – the air pressure drop in the passenger cabin in Fig. 2 is, however, very similar to the drop during the transport in the cargo hold in Fig. 1.

Figure 2. Values recorded with the data logger for temperature and air humidity (top) and air pressure and acceleration (bottom) during the manual transport from NMIJ (Japan) to PTB.

From the data shown here and the data of further transports, conclusions can be drawn and recommendations be derived for future interlaboratory comparisons:
– Use of a data logger for monitoring of the transport conditions during an interlaboratory comparison can be recommended without any restrictions. The simple and low-priced model used in this work is probably sufficient for most cases.
– In particular for temperature-sensitive measurement artifacts, only a transport with personal accompaniment and as cabin luggage during possible flights is recommended.
– For pressure-sensitive instruments, air-tight packaging is indispensable, independent of the transport type.
– In any case, careful packaging as protection against mechanical shocks is to be recommended; how well the selected packaging absorbs shocks can also be checked with the aid of the data logger by comparing the measured acceleration at identical test conditions once with and once without packaging.

Literature:

[1] Haller J, Koch C, Monitoring transport conditions of key comparison travelling standards using a data logger. Experiences from CCAUV.U-K3.1, Metrologia 52(6), 764-774 (2015).

Contact persons:

Christian Koch, FB 1. 6, E-Mail: christian.koch(at)ptb.de

Julian Haller, FB 1.6, E-Mail: julian.haller(at)ptb.de