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Model 3-IS & 3-IS-1 Technical Manual
Section 2
Ludlum Measurements, Inc. Page 2-6 October 2010
If the needle is pointing as indicated below and the range selection switch is on
the ×0.1 scale, then the reading is 0.1mR/hr.
The same needle indications on successive ranges would be:
×1 = 1.0 mR/hr (or 1,000 µR/hr)
×10 = 10 mR/hr (or 10,000 µR/hr)
×100 = 70 mR/hr (or 70,000 µR/hr)
The dial shown below has three arcs: a counts per minute scale (cpm), a
linear mR/hr scale, and a non-linear mR/hr scale for the ×100 range only.
The meter face also has a
BAT TEST
position.
The top cpm scale is valid for the ×0.1, ×1, ×10 and the ×100 ranges. The
linear (middle) mR/hr scale is valid for the ×0.1, ×1, and the ×10 ranges.
The non-linear mR/hr scale is commonly referred to as a “combo” meter
face, since it has both, count rate (cpm) and exposure rate (mR/hr) arcs.
Simpler meter faces may only have a count rate or an exposure rate arc(s)
like the previous meter faces shown.
A “combo” meter face is specifically designed for a particular detector. In
the example above, the 1.0 mR/hr mark on the middle arc lines up with
3.3kcpm on the upper arc. The meter face in this example works with a
detector that receives 3.3kcpm per mR/hr (the Ludlum Model 44-9
pancake detector.) Additional detectors may be used with this meter face,
but only the cpm dial is valid for these detectors. The mR/hr scale is not
valid for these additional detectors.
In the following picture, the needle is on the first tick mark past the 4kcpm
mark. Therefore, if the instrument selector switch is on the ×0.1 range, the
reading is 4.2kcpm (multiplied by) ×0.1 = 420cpm.