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 2.GQ Geiger Muller Counter
 GC-500 tube J321 not M4011 / CPM bug in GC-500		  New Topic  Reply to Topic
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duncho

Germany
1 Posts
Posted - 06/18/2024 :  07:34:29  Show Profile  Reply with Quote
Hi All
I bought GC-500 last year and yesterday also Radiacode 103G which was about 5x more expensive than GC-500.

I noticed that GC-500 readings in CPM are significantly different (lower) compare to Radiacode 103G which shows about 60x higher! I assume that GC-500 has a bug in calculation of CPM as it actually shows CPS (Counts Per Second) and not CPM (Counts Per Minute). Attached comparison photo. At the beginning I suspected that something is wrong with my GC-500 as it says that it has M4011 tube, but mine has J321? Has anyone noticed this, is my GC-500 I bought on Amazon.de counterfeit product? I attached photos.

Last point, I was suspecting HV out of range, my GC-500 shows 411V supplied tube J321. Is this correct voltage?

Thank you a lot, I appreciate your time for replying to my post!

CPM comparison


uSv comparison


J321 Tube (not M4011 as noted by manufacturer)
Reply #1

Kaninbjerget

Denmark
18 Posts
Posted - 07/26/2024 :  13:03:05  Show Profile  Reply with Quote
Here's a surprize: Both your Radiacode and GMC-500 are both 100% correct about that.
That's right. Count Per Minute respectively CPS - Counts Per Second - is nothing more than interactions per given time in the sensor.
You can't compare anything unless they're two identical sensors and even then they might have slightly different sensitivity because of manufacturing tolerances.
The GMC-500 is a classic geiger tube with a low pressure gas inside it. Chances of an interaction is therefore quite low as there's not much material to interact with.
Then there's you Radiacode - a whole other beast. It has a very(!) large scintillation crystal. Being solid it has a lot more matter that radiation risks to interacts with so it has a lot higher count for the same giving amount of radiation.

And another surprize: Being a classical geiger-müller tube your GMC-500 really can't give an accurate dose rate. Here you need to know what you're measuring. GQ usually (if not always) adjust for Cs-137 so only for that source the dose rates are accurate as all you can do with a geiger tube is a simple CPM/CPS to µS/h conversion rate. Another benefit of a scintillation sensor is that it is able to measure not only has there been an interaction but also what energy level. So your Radiacode actually can give you the correct dose for just about all energy levels of gamma and x-rays. Hence with a scintilation sensor you should always just focus on the dose rate as that is correct (within it's given tolerances).
And while at it. CPM/CPS is meaningless unless you know which tube. It's like giving tachometer readings for how fast a vehicle is going without bothering whether it's a small petrol, large diesel, electric, electric or fuel RC car. For 60 km/h you can have readings between 1000 and 30000 RPM depending on the vehicle - absolutely correct readings but also a pretty useless information by itself.
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