How to calibrate a leak detector
The calibration of a leak detector is to be understood as the adjustment of the display at a leak detector, to which a calibration leak (or test leak) is attached.
A calibration leak is a leak whose leak rate at a certain temperature and under specific pressure conditions is known precisely. This leak rate is provided on the calibration certificate of the calibration leak or documented on a label which is attached to the calibration leak.
In vacuum operations one must differentiate between two types of calibration:
- Internal calibration of a leak detector uses a calibration leak which is permanently built into the leak detector. Using this "internal calibration leak", only the leak detector, in which the calibration leak is built in, itself may be calibrated.
All Leybold leak detectors are equipped with an internal calibration leak. Via the automatically operated calibration routine, each leak detector may be "internally calibrated" within just a few seconds. - External calibration of a leak detector uses a calibration leak which is installed at the inlet of the leak detector. Using this „external calibration leak“, the leak detector may be calibrated in just a few minutes.
Furthermore, the external calibration leak may be used to check the proper functioning and operation of the internal calibration leak. In particular, external calibration leaks are always used if complete leak detector systems or partial flow arrangements must be calibrated.
Sniffer devices and sniffer arrangements must also usually be calibrated with external calibration leaks. In this case, it must be ensured that on the one hand the entire test gas from the calibration leak reaches the sniffer tip and on the other hand the gas flow through the sniffer unit is not hindered by the calibration process (see Fig. 9 below).
Fig. 9: Calibration connection of a sniffer calibration leak
When making measurements using the sniffer technique it is also necessary to take into account the distance from the probe tip to the surface of the specimen and the scanning speed; these must be included as a part of the calibration. In the special case where helium concentration is being measured, calibration can be made using the helium content in the air, which is a uniform 5 ppm worldwide.
Calibration leaks normally comprise a gas supply, a choke with a defined conductance value, and a valve. The configuration will be in accordance with the leak rate required.
Fig. 10: Examples for the composition of calibration leaks
a - Calibration leak without gas supply
b - Calibration leak for sniffer and vacuum applications
c - (Internal) capillary calibration leak
d - Permeation calibration leak (also called „diffusion calibration leak“)
e - Refrigerant calibration leak
Figure above shows different helium calibration leaks.
Permeation leaks are usually used in case of leak rates of 10–10 to 10–7 mbar·l/s. Capillaries are usually used for leak rates between 10–7 to 10–4 mbar·l/s and, for very large leak rates in a range from 10 to 1000 mbar·l/s, pipe sections or orifice plates with exactly defined conductance values.
Calibration leaks used with a refrigerant charge represent a special situation since the refrigerants are liquid at room temperature and usually feature a vapor pressure between 4 bar and 10 bar. One technological problem which is difficult to solve is the risk of clogging of the capillaries. All refrigerants are also very good solvents for oil and grease and thus are often seriously contaminated so that it is difficult to fill the calibration leaks with pure refrigerant. Decisive here is not only the chemical composition but above all dissolved particles which can repeatedly clog the fine capillaries.
Fundamentals of Leak Detection
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