Humidity affects the results of the free-air chamber measurements in a number of ways.
Davis used the UW Attix free-air chamber , with an attenuation length, L, of 19 cm.
For the x-ray measurements, the air kerma rates were determined directly using the Wyckoff-Attix free-air chamber.
At the NIST each x-ray calibration was made by alternating between the transfer chambers and the standard free-air chamber, through the translation of the chambers to one beam center line.
The correction factors applied to each free-air chamber and the associated uncertainties are listed in Tables 4 to 6c.
All measured ionization currents using the free-air chamber standards are corrected for ion recombination, [k.
The Wide-Angle Free-Air Chamber (WAFAC) is specially designed to realize air kerma from a single-seed source emitting photons with energies up to about 40 keV, and is now used to measure the wide variety of seeds used in prostate-cancer therapy that has appeared in the last few years.
125]I brachytherapy seeds then available, Loftus  performed measurements with the national primary x-ray standard Ritz free-air chamber, and transferred the results to a spherical aluminum re-entrant ionization chamber which then served as the secondary standard for routine calibrations.
The estimated uncertainties for the standard free-air chamber
and re-entrant chamber are 1.
Williams, 50 kV Primary Standard of Exposure--1978 Design Of Free-Air Chamber, RS (EXT)54, National Physical Laboratory Report, Teddington, Middlesex, UK, April 1982.
Although free-air chambers are designed to keep all corrections to the mass ionization current to a minimum, some corrections must be applied.
All calibrations of the transfer chambers were preceded by measurements using the standard free-air chambers