IBA KermaX-plus 120-131 E - Air Density Correction; 7.1. Air Density Fluctuation in the Ionization Chamber; 7.2. Correction Overview

KermaX-plus
®
Chamber | Air Density Correction | 37
7. Air Density Correction
This KermaX-plus
®
DAP meter types are not available with an optional built-in sensor (temperature and
air pressure) for automatically correction of air density fluctuation in the ionization chamber.
Which means that the DAP Values are not corrected for air density. For the best accuracy, the user has
to follow the procedure described in following Chapter.
7.1. Air Density Fluctuation in the Ionization Chamber
The ionization current is proportional to the density of air in the measuring volume of ionization chamber.
According to the thermal equation of state for ideal gases, the Air density  is proportional to the quotient
of pressure p and temperature T. It follows immediately, the correction for the influence of air density:
k

= (p
0
/ p) · (T / T
0
)
The reference values of air pressure and temperature are:
p
0
= 1013hPa
T
0
= 293,15K
If necessary, using k

to the DAP reading correction:
D
dap
= M
dap
· k

For example, at a temperature of T = 23
o
C (296,15K) and an air pressure of p = 990hPa is the correction
of the displayed DAP M
dap
by 3.4%.
7.2. Correction Overview
Due to variable reference sensors, air density correction for the chamber and / or the reference detector
may be indicated for best accurateness.
Reference detector
DAP Chamber
Semiconductor detector
correct DAP chamber value
1
Ionization chamber (air density uncorrected)
no action
Ionization chamber (air density corrected)
correct DAP chamber value
1
1
Temperature measurement should be done as close as possible to the corresponding chamber