IM-108/PD Ion Chamber Survey Meter (ca. 1950s)
The IM-108 is a small high range survey meter employing an ionization chamber—it seems to have been the predecessor to the IM-174/PD.
The only references I have found to the IM-108/PD are in the descriptions of the IM-174/PD in the 1966 and 1987 editions of the List of Military and Civil Defense Radiac Devices. The pertinent text is as follows: "Note: IM-108/PD is obsolete. IM-108(A)/PD is depot modified to IM-174/PD."
Range: 0.1-10R/hr, 1-500 R/hr
Size: ca. 6½” x 4" x 4”
Batteries: Two 5.2 volt (1318/U) and one 1.3 volt (1288/U) housed in a removable cylindrical housing that is connected to the meter with a four-foot cable (photo to left). Normally the battery chamber is located in the bottom of the meter case.
Weight: 3.5 to 4 pounds
Manufacturer: Landsverk of Glendale, California
This particular unit has a calibration sticker on it dated 14 December, 1961.
The IM-108/PD has two controls. One is an "OFF"—"SET" knob used to turn the instrument on and off and to zero the meter. The other control is a lever switch that is used to check the battery condition and to activate the zero adjust function. When the lever switch is held in the "Check" position, the instrument's sensitivity is increased and the rectangular label on the top of the unit (titled "Increased sensitivity meter correction.") indicates the exposure rates that apply. For example, when the lever is in the check position and the reading on the meter scale is 50 R/hr, the actual intensity is 5 R/hr.
When I first saw this instrument, I assumed that the removable chamber (pictured above left) was an ion chamber and that it was connected to the meter via a long cable so as to permit remote readings in a high radiation environment. To my surprise, it turned out that it was the battery compartment and that the ion chamber was located in the back (flat) end of the meter case. Why the removable battery compartment? Under very low temperature conditions, the batteries will fail. To prevent this, the battery compartment can be removed and placed inside the user's clothing next to the body. Is that clever or what?
The orange version of the IM-108/PD shown above and to the right is a remote-controlled unit designed for training. It was manufactured in 1957 by Admiral Corporation and apparently distributed by the U.S. Naval Training Center in Port Washington, New York.
- Defense Atomic Support Agency, List of Military and Civil Defense Radiac Devices. DASA 1243 revised, 1966.
- Defense Nuclear Agency List of Military and Civil Defense Radiac Devices HQDNA (AR) 3M Revised, August 1987.