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Wednesday, December 21, 2011

LENA - Now featuring dose measurement

The DOSE feature discussed in the previous post is now done. Taking into account the GM tube characteristics, a realistic measure of radiation dose in uSv/hour is now being calculated, based on the single event rate. From the information obtained on the internet regarding the SI-39G tube, the value of 0.00049 uSv per pulse was taken into account for the calculation of the dose.

Additionally, another mode usually found in most Geiger counters and dosimeters is the CPM (Counts Per Minute) mode. It provides a relative measure (that is of course GM tube dependent) of the radiation exposure, based on the ammount of particles detected by the tube. In this case these can be either Beta particles with more penetrating energy, and Gamma photons. Alpha particles cannot be detected by this particular tube (the SI-39G).

All the calculations are performed on the User Interface module (the cream colored box). No firmware changes had to be performed on the probe module (where an Atmel ATTiny85 is used). Only on the PIC18F2455 responsible for controlling the UI module.

The video below shows the new set of features in operation:

To enrich the feature set of the device, battery voltage measurement was also added. As the PIC also has an ADC converter, monitoring the voltage was just a mather of adding a resistive voltage divider, and a 5.1 V Zener diode for protecting the ADC input in case of excess voltage (given the designed voltage divider, in excess of 18 Volts).

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