Hi Jason

When I was new to the hobby I had my first CDV-700 exposure calibrated by KI4U.? I attach the Cal sheet just for reference.? I would not recommend it.? As Geo's paper explains, it marries the probe to the meter and is really only good for a uniform field of Cs137.? I attach the Cal sheet just for reference.? It does show the calibration levels.? Pulse calibration is probably best done at those same points.

Mike L.

All,

Does the voltage applied to the geiger tube have a significant effect on the calibration if it is within the operating range or 'plateau'?

For the LND720, if I were to correct 950V down to 900V is recalibration required?

Clint

Thanks for the replies. From my slim understanding of what I'm dealing with and what I'm reading here, the pulser is the only correct way to calibrate for counts per minute. Now if a person were to calibrate with a radioactive source would they go by the mR/hr scale? Seems like that could be an issue if you calibrate with one element and are reading a different radioactive element. CPM is CPM regardless of what beta emitter you're dealing with but mR/hr is tied to the energy of the particles right??

Excuse the mess.
pic 2579: notice the "+" mark, that is where the reed switch is.
pic 2580, little magnet on a string is placed over the reed switch, and the meter is indicating ~ 120 cps
pic 2581: close up of the pulser circuit.
pic 2582: schematic of the pulser.
??? The TL1431 is sort of optional. It helps regulate the voltage for stability but its not critical as the pulse rate is pretty much independent of the voltage.
The loading of the pulser is pretty low compared to the gm tube's pulse so it is connected all the time to the gm tube line.

If there are any questions please ask.
P

G8RPI (Robert) designed a simple pulser with fixed rates based on a watch crystal and a CMOS chip.? It is well documented in the files of GeigerCounterEnthusiasts IO group.? I built two of these and found them very useful, not only for calibration, but also for a signal generator for various Geiger and Scintillator projects.? The CMOS chip generates a square wave and a few components in the last section condition the signal to look like a Geiger Pulse.

If you have an Oscilloscope, you could purchase a cheap signal generator and use Robert's last section to create the Geiger pulse.? The O'scope would be needed to get the pulse to look right and measure the rate (frequency).? This would have the advantage of being able to select? the specific rates to calibrate to.

A third method that I have only though about, but should work if you are programmer (I am not), is that Arduino using CircuitPython has a library function that will generate a square wave in the background of the program that could be used with Robert's last section to produce a very
simple pulser that would also be able to select? rates to calibrate to.? I have been meaning to try this myself, but never get around to it.

Mike L.

I built a simple calibrator into my CDV700.
It uses a PUT as a fixed frequency pulser.Its switched via a magnetic reed sw, so there are no additional sw on the CDV700.
I just place a magent on the outside of the CDV700, and the pulser turns on.
If there is interest, I can post the circuit.( may need some time to dig it up)
P

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I was looking at the Victoreen CD V700 I just got back going and was wondering exactly how do you calibrate these? Check source seems to not be the right way to go because I don't have a clue how active it is compared to when it was made. I read something about pulsers. Is that what I'm supposed to do, connect a pulser to the probe anode lead? How does that work??

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Like others have said, it doesn't disassemble, it's epoxied to make it waterproof. The entire CDV was waterproof when new. You could throw it in a lake and it would float.

The only way I have non destructively disassembled a probe housing is to drill a 4mm hole into the place where the cable goes in and use a 4mm pin punch to tap the core out. Heat the core with a hair dryer first to expand the base, then tap the core out.

I still have a few sockets left. You can have one if you wish. The hard part in reassembly is to insulate where the wires are at the bottom so they don't short. The inside of the probe has a brass/bronze insert in many models. Pin #1 is always hot, and #2 and #3 should be bridged to guarantee tube compatibility. Some use 2 and some use 3 for ground.

Lenox,

Good to know about the rubber washer. ?I found a post online that claims a soak in mineral spirits and then heating with a hair dryer will help. ?If I can't get it without destroying it and can't find a socket I'll see about drilling the case and putting a Type C connector in it. ?I prefer keeping the original pickle probe on it if I can.

Clint

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I believe the socket was phenolic and there is a black buna rubber spacer between socket and probe 3/16” thick to prevent arcing and modest shock absorption but they do work fine without it.?


AlanJ

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ah, getting apart is hard.? I had to punch out the old socket, broke it, then replaced with new socket

Thanks, Mike. ?The only thing left is to get it apart.

David71,

Thank you for your generous offer. ?The cable was damaged right at the probe so I plan to re-use it.

Clint