06 Dose
Humidity correction k_h
k_h ≈ 0.997 at 50% RH, 20 °C, 101.3 kPa — a 0.3% air-density / W-value mix.
Listen
Listen · English
Simulation
Humidity correction k_h — Change the numbers; the scene follows.
Where it works
Water phantom
Ion chamber
In the water tank under the linac, at the ion chamber — reference dosimetry happens here.
Open this machineFormula
Variables
Results
k_h
Humidity correction
0.995
Explanation
What it means
Calibration coefficients are specified for dry air, but the chamber is used in humid air. Water vapour changes both the mass of the cavity gas (density) and W/e (a bit). The net correction is only ~0.3% at typical 50% relative humidity and is often absorbed into the stated N_{D,w}. You will meet k_h in a careful TRS-398 uncertainty budget. This is a working relation in Dosimetry.
Where it is used
Clinically it sits on the Water phantom — Ion chamber. In the water tank under the linac, at the ion chamber — reference dosimetry happens here. Dosimetry is the chamber in water under the linac, or the well counter in the hot lab: TG-51, TRS-398, kerma, and recombination. These numbers are the calibration the rest of the department borrows.
Water phantom · Open this machineHow to use it
Enter relative humidity in %. The teaching slope −0.0001 per % RH around 20 °C matches the usual 0.997 at 50%. For a reference-class measurement use the BIPM / TRS-398 humidity table instead. Change one input and watch the curve and the simulation follow.
Symbols
- hRelative humidity50 %
Worked example
A typical case from the default values: h = 50 % (Relative humidity). Substituting into the relation gives k_h = 0.995. These are teaching numbers — align them with your machine.
Typical values give
- k_h = 0.995
Where it comes from
The displayed formula is the working relation. k_h ≈ 0.997 at 50% RH, 20 °C, 101.3 kPa — a 0.3% air-density / W-value mix. Usual reference: IAEA TRS-398 / ICRU 31. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: IAEA TRS-398 / ICRU 31
Assumptions & limits
Linearised, 20 °C, 101.3 kPa. Real k_h is a weak function of T and P as well. Do not stack this on top of a certificate that already states ‘humid air’.
Pitfalls
kQ is for that chamber and that beam quality — not a neighbour's value. Polarity and recombination are measured, not copied. A ⁶⁰Co N_D,w is not an MV calibration until kQ is applied. Linearised, 20 °C, 101.3 kPa. Real k_h is a weak function of T and P as well. Do not stack this on top of a certificate that already states ‘humid air’.
Keep this
Trace every gray back to a protocol, a chamber, and a quality index. Linearised, 20 °C, 101.3 kPa. Real k_h is a weak function of T and P as well. Do not stack this on top of a certificate that already states ‘humid air’.
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