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Checking the Geiger tube

A project log for Global radiation monitoring network

The uRADMonitor is a plug-and-play, low power, self contained radiation monitoring device, connected to a centralised server component.

Radu MotisanRadu Motisan 02/18/2015 at 00:160 Comments

uradmonitor_oscilloscope_geiger_pulse

The Geiger tubes used in uRADMonitor model A units are mostly the SBM-20 or the SI-29BG, both of Russian provenience, manufactured at military grade specs. Therefore they were meant to resist fluctuations over a wide interval of temperature or pressure. If a tube failure is suspected for a particular uRADMonitor unit, for example when the number of counts per minute reported shows zero, checking the Geiger tube might be necessary, and the following indicators should be verified:

Voltage on tube

Assuming the uRADMonitor unit is otherwise functional, it should first be connected to the LAN Network to get a valid IP, so that it can be accessed locally by opening the IP in the web browser:
uradmonitor_web
The voltage on tube is displayed in the web interface, together with several other parameters. The voltage on tube should read 380V +- 5V. The duty cycle must be between 25% and 45%.
This method of checking the tube voltage is preferred over directly measuring high voltage inverter output for a several reasons:
– it is safe: no electric shocks can accidentally occur by touching the high voltage sections of the circuit
– a high impedance voltmeter would otherwise be needed to directly measure the voltage on the board. A 10M impedance voltmeter (or higher) would be needed to measure the voltage directly, to avoid the voltage drop and so erroneous readings to a maximum extent. The high voltage inverter is a low current high voltage supply.
If both the "radiation" and the "average" fields show zero while the "voltage" is correctly measured close to 380V, it could indicate a tube failure. For certainty in this case, direct tube verification is needed, and the following approaches should be used:

Geiger tube pulses

The purpose of the Geiger tube is to count radiation induced pulses. When the tube internal environment is ionised by intersecting radiation, it becomes conductive for a very short amount of time. The anode (for example 10M) and cathode (for example 220K) resistors form a resistive divider. If the signal is collected from the cathode, then the maximum pulse amplitude for 380V across the entire assembly will be Vout = 8V according to the following calculations:
voltage_divider
To test the tube is functioning properly, we need to check it is able to count radiation pulses. In the absence of an active radiation source, the background radiation can be used as a source, which for the SBM-20 should be about 20CPM (counts per minute), depending on the particular region. So a first test would be to see if the tube registers any pulses, and to count them to see if we get the expected number, that is approximatively 20 pulses in a minute. Due to the random nature of radiation, we will get more or less, but repeated tests should slowly tend to reach this value. On the other hand absolutely no pulses at all, would clearly be an indication of tube failure.

The following tube tests can be used: http://www.uradmonitor.com/checking-the-geiger-tube/

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