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uRADMonitor â€“ Online remote radiation monitoring station

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radhoo

Thu Feb 14 2013, 09:54AM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Hm, Stella, I was hoping it didn't come to that. Here are the numbers:

For 350V the tube counts a background radiation dose of (average) 66 counts/minute. For 400V it is 80 counts/minute.
The difference of 14 units , that represents 17.5% out of the total of 80 cpm, or 21% out of the total of 66cpm, and this is for a voltage interval of the 50V only.

If I was more careful at the beginning of the project, I would have probably chosen the minimum voltage. Now changing the voltage would create a big discrepancy between the previous values (obtained at 400V) and the new values (at 350V).

Do you have any docs regarding tube's plateau variations related to tube damage?

Proud Mary

Thu Feb 14 2013, 11:03AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

radhoo wrote ...

Do you have any docs regarding tube's plateau variations related to tube damage?

"An investigation into the causes of short lifetimes of geiger-muller tubes used in aircraft oil gauging systems"

This very useful paper covers wide ground not covered elsewhere, and focuses particularly on halogen (Br) depletion as tubes age.

It also mentions low frequency spurious oscillation under depleted halogen conditions, which might possibly explain your tube's sudden burst of counts.

If your problem is oscillation, it might be possible to stop it by altering LCR in a way that does not affect count rate.

Here is another helpful paper looking at some of the fundamentals we need to think about right now: Test Procedure for Geiger-Mueller Radiation Detectors

This paper proposes a 15% slope as the cut off between good and bad tubes: "In general, the value of the slope must be less than 15 % to consider that the detector is in good conditions." This seems to me to be a very large figure when you think of really good new GM tubes having plateau slopes of less than 3%.

This paper also offers some useful insights:

radhoo

Sat Feb 16 2013, 05:13PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

Thanks! Love the articles, great find!

Some bit of additional info, even if redundant, should help clarify things.
The 10M anode resistor is a quality component, mounted correctly without fingerprints, dust or humidity. As well pointed above, should this resistor malfunction in any way, one result could be avalanche discharges in the Geiger tube. This resistor has been checked recently and it meets the requirements.

Not related to the current discussion, there has been an issue that I would like to present at this point. Have a look at the circuit diagram:

Resistor R5 is used in a voltage divider , used by the microcontroller to continuously measure the voltage across the Geiger tube, using one of its ADC (analog to digital) ports. In just a few words, for 400V set on the tube, the voltage divider will retun 3.96V ( 400V x 47/(4700+47) ). The microcontroller is set to a relative voltage of 5V (Vref). The ADC port used to measure the voltage, has a resolution of 10bits, so for a maximum of 5V on PC3/ADC3, the software would read the max value of 2^10 = 1024. For 4V we would get the proportional value. By doing so, the measurement of the anode voltage is extremely precise.
Should the voltage on tube be lower than the preset threshold (400V), the duty factor of the inverter's PWM is increased, in small steps, until we reach the target voltage (in a given tolerance, initially set to 5V and now changed to 2V). If the voltage on the tube is too high, we do the opposite and decrease the duty factor. In a few words, this is a known regularization mechanism that works well, fact confirmed by the logs, showing several months of constant voltage tube values.
The issue I mentioned, is regarding R5. After some use, it "burned" and got interrupted. As a result, the voltage measurement could no longer be done, and the software saw the voltage on tube as being 0. As a result, the duty cycle began to rise, resulting in uncontrolled high voltage generation, way above the tube's safety limits. Luckily, this high voltage also resulted in microcontroller temporarily failure, and so the PWM generation stopped . This in turn, also stopped the dangerous voltage from being applied to the tube (except for the first few miliseconds).

Closing the parenthesis and moving further

In regards to the papers kindly indicated by Proud Mary, I have run a few measurements with the current setup, also in regards to the "Test Procedure for Geiger-Mueller Radiation Detectors" document. SBM-19's operation interval is 350-475V. For my particular tube, I got unsatisfactory readings for 350V, but good performance in the 375-450V interval, despite its age.
The measurements has been performed during several hours, to acquire sufficient data for computing average values.
Voltage on tube / CPM
349.33 / 65.88
375.64 / 78.62
399.65 / 79.98
424.71 / 80.18
449.51 / 79.68

Complete measurement details in attached PDF document: ]sbm_19.pdf[/file]

To test the tube's performance indicator as presented in "Test Procedure for Geiger-Mueller Radiation Detectors",
I set N1 = 78.62 , V1 = 375.64V , N2 = 79.68 and V2 = 449.51V

For P = 100 * ((N2 - N1)/(V2-V1)) * (100 / ( (N1+N2) / 2)) we get: 1.81060853 % .

LE: The device has been set to 375V, as compared to 400V used from October 2012 until now.

Proud Mary

Sat Feb 16 2013, 07:15PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Did you set R7 at 10MÎ© to reduce power draw and/or dissipation in the tube, extend tube life etc, or some other reason?

How does the circuit behave if you reduce R7/R8 to 4.7MÎ©/110 kÎ© , or even 3.9MÎ©/82kÎ© ? Reducing R7 will reduce dead time,
and create stronger, sharper, pulses with less possibility for instability.

radhoo

Sun Feb 17 2013, 05:33PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

I set it to a maximum to preserve tube's life (considering the purpose of this device) , also according to Centronic's "Geiger MÃ¼ller Tubes" document recommendations.

radhoo

Sat Nov 02 2013, 09:45PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

One year after the original construction was completed, I finally had the time for a major upgrade. The test board PCB was dropped in favor of a brand new PCB, nicely done via the toner transfer method:

Much better than the previous, don't you think? :)

The ferrite transformer was reused, as it demonstrated very good efficiency. After all, rewinding 600 secondary turns is not an easy task. But I almost had to do that, because for some reason I had issues with the inverter. It turned out the driving transistor was misplaced due to an error in the eagle symbol used (it was mirrored!) . But that was an easy fix once I located where the problem was.

So I added 3 additional sensors for detecting rain and for air quality ( MQ-9B for combustible gases and Sharp GP2Y1010 for counting the dust particles). The MQ9 was producing a lot of heat so I had to move it some distance away from my temperature sensors:

I also had to do some considerable work at the software side, first to support all the new sensors and the new PCB (mainly because of pin changes, new 8MHz crystal instead of the older 16MHz, new VCC of only 3.3V etc), but also on the server side to fit the mySQL database for the new readings and to draw the additional charts. These can be see here, as real time charts:

Rain detection:

MQ9 Combustible gases sensor:

Dust sensor:

This is my way of saying "happy birthday" to this project :)

radhoo

Sun Dec 01 2013, 08:20PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 699

After a few days of inactivity, I was finally able to investigate a few sever stability problems with the new circuit presented in the last update.

Uradmonitor Radiation Monitoring Station

I was able to locate the problem, and I fixed it, but besides that I improved the software and finalized the part for the last three sensors added, rain, dust and cobustible gases. So now the station is back online, and the real time data is available here:

Today it December the first, Romania's national day. We had a beautiful sunny day, seen the military parade, a few concerts and the winter fair, that is now open here.

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