06 Dose
Temperature-pressure correction
Open-air chamber correction to 22 °C and 101.33 kPa.
Listen
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Simulation
Temperature-pressure correction — 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_TP
Temp/pressure factor
1
Explanation
What it means
Open-air ionisation chambers are calibrated at 22 °C and 101.33 kPa (TG-51). Density of the cavity gas scales as P/T, so k_TP = (273.2+T)/295.2 × 101.33/P. A hot, high-altitude clinic has k_TP > 1 (less gas, you multiply the reading up). 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
Measure T in the phantom (not the room) after equilibrium. P is station pressure, not corrected to sea level. A 5 °C error is 1.7% in dose — not negligible. Change one input and watch the curve and the simulation follow.
Symbols
- TTemperature22 °C
- PPressure101.33 kPa
Worked example
A typical case from the default values: T = 22 °C (Temperature); P = 101.33 kPa (Pressure). Substituting into the relation gives k_TP = 1. These are teaching numbers — align them with your machine.
Typical values give
- k_TP = 1
Where it comes from
The displayed formula is the working relation. Open-air chamber correction to 22 °C and 101.33 kPa. Usual reference: AAPM TG-51. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: AAPM TG-51
Assumptions & limits
Ideal-gas, dry air. Humidity correction is usually <0.3% and often omitted. Sealed chambers (some Farmer-type with a collector seal) should not receive k_TP blindly.
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. Ideal-gas, dry air. Humidity correction is usually <0.3% and often omitted. Sealed chambers (some Farmer-type with a collector seal) should not receive k_TP blindly.
Keep this
Trace every gray back to a protocol, a chamber, and a quality index. Ideal-gas, dry air. Humidity correction is usually <0.3% and often omitted. Sealed chambers (some Farmer-type with a collector seal) should not receive k_TP blindly.
In this specialty