Mechanical measuring machines for concrete materials have not been very extensively developed. One difficulty is that they require the constant attention of an attendant, unless the materials are perfectly uniform. If the machine is adjusted for sand with a certain percentage of moisture, and then is suddenly supplied with sand having greater or less moisture, the adjustment must be changed or the mixture will not be uniform. If the attendant does not watch the condition of the materials very closely, the proportions of the ingredients will vary greatly from what they should.

Fig. 129. Paddle Mixer.

Fig. 129. Paddle Mixer.

The Trump measuring device, shown in Fig. 130, consists of a horizontal revolving table on which rests the material to be measured, and a stationary knife set above the table and pivoted on a vertical shaft outside the circumference. The knife can be adjusted to extend a proper distance into the material, and to peel off, at each revolution of the table, a certain amount, which falls into the chute. The material peeled off is replaced from the supply contained in a bottomless storage cylinder somewhat smaller in diameter than the table and revolving with it. The depth of the cut of the knife is adjusted by swinging the knife around on its pivot so that it extends a greater or less distance into the material. The swing is controlled by a screw attached to an arm cast as part of the knife. A micrometer scale, with pointer, indicates the position of the knife. When it is desired to measure off and mix three materials, the machines are made with three tables set one above the other and mounted on the same spindle so that they revolve together. Each table has its own storage cylinder above it, the cylinders being placed one within the other, as shown in Fig. 131.

Fig. 130. Trump Measuring Device.

Fig. 130. Trump Measuring Device.