Why does my calibration fail? Sharing Experiences

Since 2018, when I had to enter the fascinating field of equipment and process qualification in the temperature area (depyrogenation tunnels, ovens, autoclaves, cold chambers, incubation rooms, etc.), we have had to carry out countless calibrations of our E VAL PRO data acquisition and recording system. Each study involved an independent calibration, linked to the temperature range to be used depending on the equipment or process.

The most difficult task has always been the verification of the thermocouples used in the validation studies of the sterilization processes by moist heat in autoclaves, this is because mechanically they can be subject to damage caused by the carts used to place the articles to be sterilized. However, we were surprised by how difficult it was to calibrate a set of thermocouples that were to be used in the qualification of a depyrogenation tunnel, which in practice do not suffer mechanical damage, but are subjected to high temperatures (>300℃).

After spending almost two days calibrating an ELLAB E VAL PRO measurement and recording system, and checking for any possible damage to the Kapton thermocouples to be used in the validation of the depyrogenation process in a tunnel, there was nothing left to do but go into detail.

Thermocouples are sensors used to measure temperature; they are made from two metals joined at one end. This union constitutes the measurement point (hot junction), the other end is called cold junction. The heating of the measuring joint causes an electrical voltage, approximately at the temperature (Seebeck effect). This E.M.F. is due to the electron density of the materials and the temperature difference between the hot spot and the cold spot.

The type of thermocouple used in depyrogenation studies are Kapton thermocouple type T, they are made of Constantan Copper (55% Copper and 45% Nickel).

?One of the elements that cause inaccuracy measurements is oxidation, and this phenomenon appeared in some of the thermocouples, which were linked to those that were reported as failed in the calibration process. The results were not long in coming.

Once this anomaly was detected, the parts with the presence of oxidation were removed and cuts of the thermocouples of no less than 50 mm were made, removing the outer Kapton jacket to expose the two wires, and make the corresponding contact welding.

The results were satisfactory

Conclusions

Calibration is a key element to guarantee the quality of the measurements to be carried out. Any detail that threatens this must be resolved in order to guarantee adequate metrological assurance.