Introduction: Homemade High Range Tube for CDV-700 Geiger Counter

The original geiger tubes for the CDV-700 civilian gieger counter were the OCD-D-103 and the later issued high range OCD-D-101. The original tube is more commonly called the 114/6993 and the high range the Eon 5115. Both operate at a nominal 900 volts. The higher range tube was made to be exactly 1/10th as sensitive as the original and came in a box with a peel and stick label that corrected the range scale. No further modification of the CDV-700 was required to make the switch. Just plug and play.

All geiger tubes have a maximum useful range. If the radiation is ionizing the gas inside the tube faster than it can be recycled by the quench gas then the number of pulses will drop off and the tube may even quit working. The maximum useful range of the 6993 is about 100 millirem/hr. The CDV-700 meter will max out at 1/2 of that as a safety feature but it is still possible to be in a field so intense that a CDV-700 simply quits working. The companion instrument, the ion chamber series 715 (and others) will still work in a field that is very quickly lethal.

The lowest range of the ion chamber series is 500 millirem/hr. full scale (5 r/hr x 0.1x). The highest CDV-700 full scale is 50 millirem/hr. ( x 100x). Using the high range tube in a CDV-700 makes both instruments overlap.

Step 1: Making Your Own High Range Tube

There are few 5115 tubes for sale. FEMA ordered plenty, but they have remained in government hands because the CDV-700 can be used for personal decontamination and checking living space and food stores much easier than the ion chamber series can. Civil defense officials decided that the original range of the CDV-700 was too low.

There is a Soviet geiger tube called the Si3BG that is plentiful and cheap and made to function up to 300 r/hr and will easily fit into a CDV-700 pickle probe. less than $10 each shipped. The Soviet tube is made to run from 300v to 460v, so there has to be something added to the probe to drop about 500v.

10 pieces for $7 shipped on eBay. These 100v zeners turned backwards inside the probe will drop the voltage 100 for each one in series. 5 in series backwards makes the voltage at the end to be within the range on the Si3BG.

The socket is an Amphenol 91-PC3M. I cut up a bad 6993 to get the socket, but if you search Octopart for Amphenol 91-PC3M you will find sellers. The supply is short so I won't post it here. You can also find the socket on eBay, but the prices are much higher.

Pin #1 on the socket is + , 2 and 3 are - The string of diodes are soldered into the #1 pin and the + end of the tube is behind that. The negative end of the tube goes to both pins 2 and 3.

The band on the diodes face toward the #1 pin, this forces a voltage drop across each diode.

Step 2: The Completed Tube Is 15 to 20 Times Less Sensitive Than the 6993

I used heat shrink tubing to insulate the assembly.

I put a final layer of shrink tubing over the assembly and tested it against a 6993 with a radium vacuum tube, both times with the pickle probe closed and got 15 to 20 times the counts per minute with the original 6993 tube. I got the same results putting a Si3BG inside a homemade probe with a BNC and comparing it with a 6993 in a BNC pickle probe on a CDV-700 modded with a BNC connector.

The exact ratio doesn't bother me. The Si3BG won't quit in a field except one so high that it probably wouldn't matter. If it maxes out a CDV-700 on 100x you are in a field at least between .75rem and 1rem per hour and maybe more. That's high enough to justify getting out immediately.


DavidN71 made it! (author)2016-05-05

One person who did this said the pins on the Amphenol socket are too long.

If the pins are too long adding back the original 6993 gasket is an easy fix. I remove and discard most of the gaskets on my CDV-700's because they get gooey with age. If they are still too long, they can be cut down and smoothed with a Dremel with a cutoff wheel.

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