Indoor or Outdoor GM Tube location

5 years 6 months ago #2731 by Simomax
Very nice work on your counter oliverb! I love the mix of old style with new tech. :)

With regards to indoor/outdoor, I would go for outdoor over indoor if you are intending on using is as a general (municipal?) monitoring station. As to cable length I'm really not sure, but taking an educated guess I suspect the same laws apply as do for antennas and such. That is better shielding provides a less noisy signal and thicker copper provides a higher signal. There is also the impedance between cable and load. Its a complex subject and I know only a fraction about it but enough to 'do stuff;.

Anyway, I digress. in my experience of geiger counters/Ion chambers I have only ever seen short lengths of cable. Milimeters even for ion chambers. I would say an average for cable runs to SBM-20 or similar tubes are about 1-1.2m. I don't see any reason why you can't run your tube from a distance providing you use a correct coax. You will need to test with different lengths and types of coax maybe to reach your goal. Although I don't know of 20ft is possible. I have neveer seen any setup with a length like this, nor done it myself.

If I were to try this myself, I would try with something like a RG59 coax (CCTV coax, 75ohm). The way I would try this is by mounting my tube somewhere and applying a check source of somekind. Something simple would do, uranium glazed pottery, Thoreum gas mantle or uranium glass, something of that kind. Amerecium from a smoke alarm would also help a little. The kind of stuff available from ebay or DIY source. The idea of this is to leave the tube where it is and move the counter with regard to the length of cable and test different lengths to see if the coax is sufficient for the job. So mount your tube and start with 6" leads from your tube to your counter. Get the CPM or whatever value you are using and start with background counts. Record this is a settling down period, then thry a source. Note this down also after a settling in period. Once you have done this and are happy with your setup and results move on to say 3ft of coax. Measure the same results. If they are the same then progress onto a longer cable run. Myabe 9ft. Then 15ft etc. If you start to see the counts become smaller then the cable either is too long or isn't suitable for the job. RG59 might work a treat though it might not. I've never tried something like this, but I am very interested in this. Cable capacitance can also be an issue as this can hold the line high for longer than the counter can read and lead to lower counts than is actually being picked up.

It really is a trial and error experimentation (for me) using long cable lengths. If you do try then please post your results as I am very interested. As a side note, I ahve my counter 30ft away but I chose to put the counter with the tube and relay the results as data. I only did this because I can do this wirelessly, and because I have the general idea that the tube can;t be too far from the detector/counter. It was also so I didn't have to run a physical cable along my garden.

Have a play about and see if you can make something stick. If you do then please let us know as I think somethnig like running longer cables would be sage information.


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5 years 6 months ago #2732 by BroHogan
While I appreciate the answer Simomax gave, I need to speak up.

Don't use coax cable. The Geiger Kit uses "cathode sensing" which means the pulse would be sensed at the shield side of the cable.
I guess you can used 2 conductor shielded and put the shield to ground but I think it would be just as good to use a regular 2 conductor cable.
I did a test once with 10 feet of 2 conductor cable and didn't see any loss of counts.

The best thing you can do is move the anode resistor to right at the tube. (There is a jumper to bypass the onboard anode resistor on my kits.)

This is explained in more detail on this on my site on the GM Tube Info page under the section The Anode Resistor, Stray Capacitance, Cable:

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5 years 6 months ago #2733 by Gamma-Man

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5 years 6 months ago #2737 by oliverb
Thanks for all your replies. I have purchased an outdoor waterproof box and ss soon as I get my Geiger Counter completed I will try and run the cable outside with twin core mains flex.

I'll post back and let you know how I get on.

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5 years 5 months ago - 5 years 5 months ago #2789 by oliverb
I have now received my SBM-19 tube and connected it to my external case using twin cable rated at 1000v. When I connected it up my Geiger Counter CPM shot up to around 400 CPM. I have disconnected the tube from the external wire and wired it back indoors next to my geiger counter. CPM is now down to 110-120 I have set the conversion factor on my tube to 875 as recommended by the DIYGeigerCounter site and Dose is showing around 0.14╬╝Sv.
Last edit: 5 years 5 months ago by oliverb. Reason: change 1.4 to 0.14

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