Meter, Contamination No. 1 - 5CG0012

These contamination meter sets were issued to British Military and Civil Defence organisations from 1953 onward. The dates on this example indicate that it was made about 1955. These meters and the later Mark 2 version, with rubber connectors, remained in service until the 1980s.

This is a radiation meter using a Geiger-Müller tube. The set consists of:

• an indicating unit - 5CG0034;
• a power unit, vibrator - 6665-110029;
• a probe - 5CG0033;
• a connecting lead - 5CG0035;
• and a G-M tube - CV2246 or CV2247 (this one has a CV2247).
• Missing from this one is the haversack, which was eaten by mice.

This one was very generously given to me by Terry Blake. The G-M tube is unfortunately suspect. The set uses a battery pack to generate HT using a vibrator. This is currently filled with AA MnO₂ cells giving ~6 volts. I believe the original equipment was mercuric oxide cells, 1.35V each, for 5.4V.

There is no heater supply. The valve line-up consists of 2 CV2255 four-element cold-cathode trigger tubes (one in the probe), a CV2236 'cold cathode valve' and, on one of the PCBs, an unidentified three element miniature valve, presumably another trigger tube of some sort. It is held by a clip which is closed by lacquered string. The indicator is working, so I haven't removed it for identification.

Construction is typical of British military equipment of the 1950's with very high quality components and robust build. The usual pink-buff wiring is used throughout and neatly laced.

The Indicating Unit 5CG0034

At the left is a two-pin connector for headphones, probably high-impedance 3-5000Ω ones. To it's right is the on/off switch. When switched on the vibrator can be both felt and heard. Below this, under the rotatable plate marked 'A', is an adjustment switch for the probe voltage.

The centre meter indicates dose rate in milli-Röentgens/hour. It has a logarithmic scale, with a maximum value of 10mR/hr. Up to 1.5 mR/hr is considered green, over 4 mR/hr is condition red. Normal background radiation is about 0.02 mR/hr.

Top right of the meter is a momentary (non-latching) test switch for battery state. Reasonably new AA cells give a reading just above the 'TEST' zone in the meter. This can be adjusted by the switch under the 'B' rotatable plate below the test switch.

Further right is a typical British military Plessey 6-pin socket, which mates with the right-angled plug on the probe lead. Below is a moisture indicator. This one is unrestored and pink indicating the ingress of moisture. A good indicator should be blue.

At the left of the image on the right is a glass encapsulated high value resistor of 145MΩ. Nicht mit der finger gepoken! - these must be kept spotlessly clean or their value will fall over time. Wash with iso-propyl alcohol if in doubt.

Also on this PCB there is a diode-capacitor voltage multiplier.

The Probe 5GC0033 and the Geiger-Müller tube CV2247

Here is the probe unit with the black rubber coated CV2247 Geiger-Müller tube fitted. The white 'bump' covers a CV2255 four-element cold-cathode trigger tube. The circuitry below has three 10MΩ conventional resistors and a glass encapsulated 75MΩ one. All the circuitry of the meter and probe operates at very high impedance, presumably as a method of reducing overall power consumption to extend battery life. In the '50s battery life was a major problem for portable equipment designers. The use of cold cathode tubes and very high impedance circuitry was perhaps the best solution available at the time.

The connecting lead 5GC0035

The connectors are Plessey type, typical of British military equipment of the 1950s. The later Mark 2 version of this set had rubber connectors which were presumably more weather-proof. The right hand plug is a six-pin right-angled one and connects to the indicating unit. The left-hand one is a straight four-pin one to connect to the probe.

Power Unit, Vibrator - 6665-110029

It is believed that the original power source was two 150V? batteries in series. These were connected to terminals 1&2 and to 3&4. Terminal 4 connects to the system chassis. The power switch on the indicator unit then connects 2 to 3 and completes the circuit.

The power unit is polarised by the silver pillar screw on the right hand side of the unit in the picture at left.

There are four terminals on the vibrator unit, and presumably also on the mains and original battery supplies. 1 & 3 are standard 4mm sockets, 2 & 4 are 3mm. In the vibrator unit terminals 2 and three are used to switch on and off, the HT supply of ???V appears across 1 (positive) and 4 (ground). The numbering on terminals was added by me. The original has no numbering.

Connecting terminals 2 & 3 switches on the vibrator pack which produces ~420V off-load.

Links

Wikipedia entry