The Bauer Qualimeter system, measures the "hardness" of the X-rays derived from the tube voltage.
It is of great importance that the energy or "hardness" of the X-rays is adjusted in accordance with the irradiation depth specified in the treatment prescription. The early unstable X-ray ion tubes had to be "run in" with care to establish the desired high voltage and current.
In 1914, the voltage of the X-ray unit is stepped up carefully until it reaches a stable operation at about 80 kV anode voltage. A spark gap was used as an indicator of the high voltage. When the discharge occurs across a set gap width, the desired voltage has been reached. Using a "ripened" Helm X-ray tube which has been operating up to the point where a stable gas pressure was obtained, Dr. Gaarenstroom was able to reach a tube current of about 3 mA.
To monitor the energy of the X-ray beam Gaarenstroom also uses the Qualimeter based on the Bauer system and obtains a reading of 8.5 - 9.5 Bauer "degrees". The Bauer measuring system consists of an electrostatic voltmeter which is attached to the cathode and measures the high voltage. The Bauer scale indicates the lead thickness sufficient to stop the radiation. The units of the Bauer system are called "degrees". One degree in the Bauer System corresponds to 0.1 mm Pb. Hence, 8.5 to 9.5 Bauer degrees equal 0.85 to 0.95 mm Pb. Assuming that the beam intensity is reduced to less than 0.1%, the tube voltage will have been around 85 kV.
The radiologist had a range of X-ray tubes at his disposal. The shape of the tube and the voltage and power were specific for different applications. For each irradiation, the chosen X-ray tube was fixed in a stand above the area to be irradiated and connected to the high voltage supply with wires from the ceiling. Occasionally an X-ray tube would break down or overheat during treatment and it would then be quickly replaced and the irradiation continued. The water cooling of the Helm X-ray tube allowed irradiations of 20 to 30 minutes. Thereafter, the next patient would be prepared and the tube would have time to cool down. In this way, 7 to 8 patients could be treated consecutively during an afternoon with one X-ray tube, without a reduction of beam "hardness" or intensity. This was a great improvement as a new X-ray tube did not need to be used for each patient. However, the tubes were fragile and dr Gaarenstroom reported the use of on average 60 X-ray tubes per year.
The radiation from the X-ray tube is virtually unattenuated by the glass of the tube. The radiation is therefore a mixture of harder and much softer X-rays which causes skin damage rapidly. To counter this, Gaarenstroom applies a strong filtration: a primary filter of 5 mm of aluminium, followed by 5 mm of leather to reduce the scattered ("secondary") radiation from the filter. With the thick aluminium filter, the skin could be more protected than without it, but the irradiation time would then be increased to 20 - 30 minutes.