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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Oh, that explains the weird appearance of the pulses from your Geiger tube. Your 3.2 design was just plain wrong, the tube is supposed to run off well-regulated smooth DC.
As far as I can see from the 3.2 schematic, the problem has nothing to do with a lack of diodes. It looks like you forgot to add a smoothing capacitor to the output.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Unfortunately there is no filter capacitor in v3.2
Steve Conner wrote ...
As far as I can see from the 3.2 schematic, the problem has nothing to do with a lack of diodes. It looks like you forgot to add a smoothing capacitor to the output.
Well, good that I moved further to 3.3 then, but I'll correct the 3.2 as well. At the time I made the 3.2, I did not have a scope. EDIT: first thread modified. It also shows your suggestions, under 3.2.
Steve Conner wrote ...
Oh, that explains the weird appearance of the pulses from your Geiger tube.
What is the weird appearance you are referring to?
EDIT: let's better focus on 3.3, to avoid creating confusion with the old versions.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
radhoo wrote ...
Here is how the pulses look like:
See the series of little notches? That's the "weird appearance" I'm referring to. A proportional counter would interpret that as 6 or 7 pulses of different energies. I was puzzled, but now I see it's your unsmoothed power supply feeding through onto the signal.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
Steve, while your logic makes sense, I'm not sure that's the cause.
See the pulses in 3.3, looking similar to those in 3.2:
But as you already saw, 3.3 has a nice constant 400+ DC line.
If I zoom out they look like this: I'd say it's a perfectly normal shape. So why the saw-tooth shape in the zoomed version?
I thought it was a train of damped waves caused by shock excitation following the steep wavefront at the instant of GM tube conduction. Reducing Ra will shorten dead time, and may attenuate the effect. Try, say, 6M8 to start with, and see if that has a helpful effect. Don't go below 3M9 without serious thought. Changing Rk away from the classical Ra/Rk = 45:1 ratio should also be considered within reason, if it seems to help with the ragged pulse tail. Moving Va down to the lower end of the plateau could also be tried.
A simple discriminator or edge detector is often used to chop off or ignore the lower parts of the pulse to separate pulses that have run together in a bulge at the bottom - a problem that increases and becomes statistically significiant with rising count rate. Discriminator level control may be set either on the front panel, or by a pre-set on the PCB.
Registered Member #1938
Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701
Thanks Proud Mary, but this is exactly why I told Steve that the 3.2 remarks might be creating confusion. The pulses are counted correctly in 3.3, and the shape looks good. I will correct the missing capacitor in 3.2 ,and publish updated diagrams, but this is of minor importance.
Dr.Nicolae Zamfir, director of Horia Hulubei National Institute of Physics and Nuclear Engineering commented in regards to my uRADMonitor construction, when contacted by the local press. My reply is in the comments section of the page:
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
radhoo wrote ...
Thanks Proud Mary, but this is exactly why I told Steve that the 3.2 remarks might be creating confusion. The pulses are counted correctly in 3.3, and the shape looks good. I will correct the missing capacitor in 3.2 ,and publish updated diagrams, but this is of minor importance.
Dr.Nicolae Zamfir, director of Horia Hulubei National Institute of Physics and Nuclear Engineering commented in regards to my uRADMonitor construction, when contacted by the local press. My reply is in the comments section of the page:
Please use an online translator.
It's good to see that your project has aroused interest and is helping to raise awareness of radiaoctive pollution!
Here are a few ideas based on the criticism of your work:
You can improve the confidence level in your readings by increasing the aperture of your detector - in practice, by using multiple GM tubes.
You knew a direct conversion of CPM into dose rate would cause trouble!
You can improve your counting accuracy by dead-time compensation. The effect of this will be very very small at the low count rates of normal background, BUT it will enable you to put "Figures corrected for dead time." on your results. I've said it will be a tiny effect at low count rates, but might grow to mean something significant in measurments over very long periods of time - years, or until there is another big accident. You can calculate the real dead time of your system (that is, the GM tube as it is connected into a functioning circuit) by a simple practical test which I will paste up here if you don't know it.
Over a long period of continuous operation, it is likely that the GM tube's characteristics will change slightly. You must devise a simple test that can be repeated once a month to record the sensitivity of the tube, so you can detect the trend of any change and make corrections for it in the results.
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uRADMonitor – Online remote radiation monitoring station
