| 摘要 |
An apparatus for measuring the surface density of a web has a portable and generally U-shaped housing in one of whose arms there is provided a radiation source. The other arm has across from the radiation source a first radiation detector, and a second radiation detector is provided in the one arm next to the radiation source. A shield operable by an adjustment knob on the housing can be displaced to vary the relative quantities of radiation passing from the source to each of the detectors. The detectors are connected to an electronic comparator whose output is read on a meter. In use the shield is adjusted until the meter reads zero indicating that both detectors are receiving radiation at the same level. Then a web is inserted between the two arms, thereby coming between the radiation source and one of the detectors, so that the meter will detect the relative change in the two radiation levels. This change is indicative of the surface density of the web inserted between the two arms. The meter can be calibrated in surface density, as in g/m2, or can be of the zero-center type, with indicia on the adjustment element for the shield alignable with a marking. Thus the surface density is read directly off the meter or the meter is adjusted to zero and the surface density is read off the knob.
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