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 GMC-300 reading ?		  New Topic  Reply to Topic
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singlag

4 Posts
Posted - 06/29/2012 :  02:52:30  Show Profile  Reply with Quote
Just received my new GMC-300, i got the background reading is about 120-160 CPM and 0.35-0.55uSv/Hr

Is it something wrong, reading look like a bit higher than expected
Reply #1

ZLM

1261 Posts
Posted - 06/29/2012 :  07:39:46  Show Profile  Reply with Quote
I think the unit tube voltage is shifted. Open the cover and adjust the tube voltage lower untill the background count below the 40 CPM.

If you are unable to do that OR it does not change anything, then you need to send it back for replacement.
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Reply #2

singlag

4 Posts
Posted - 06/29/2012 :  10:08:17  Show Profile  Reply with Quote
Is it R25 to change the voltage value ?
my DMM show 41.5V DC and around 0.23uSv/hr & CPM 50, is it normal, i'm at Hong Kong ?
Edited by - singlag on 06/29/2012 10:48:20
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Reply #3

ZLM

1261 Posts
Posted - 06/30/2012 :  22:57:22  Show Profile  Reply with Quote
Yes. It is R25.

Your DMM internal resistance is too low. Change to a better DMM. From a 10Mohm internal resistance DMM, normally the voltage you can read at >150V

If you are in HongKong, then the background reading may be higher. Take your GMC-300 to different places, so that to check if your location has a higher background reading.
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