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35.6.8 Comparators

•Accuracies commonly below 1/10 thousandth of an inch

•These instruments try to reduce the friction that is such a problem for the dial indicators

•There are four common principles used to design these instruments,

-mechanical

-pneumatic

-electrical

-optical

•comparators have very limited ranges of motion, but very high sensitivities (and therefore accuracies). As a result the comparators are often calibrated against standards such as gauge blocks.

•The basic requirements of these instruments are,

-rigidity of the design

-linear magnification within the operation range

-coarse and fine offset adjustments

35.6.8.1 - Mechanical Comparators

• The Johansson Mikrokator used a twisted strip with a pointer attached. as the plunger is depressed, it causes the strip to stretch. As the twisted strip is stretched, it changes the angle of the pointer, and thus the indicated deflection.

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scale (side view)

pointer (moves in and out of page)

twisted strip

bell crank lever

plunger

• The Sigma Mechanical Comparator uses a partially wrapped band wrapped about a driving drum to turn a pointer needle.

page 192

	pointer	arm that is essentially
		a pivoting beam

drum

knife edge and saphire bearing block (knife edge

position is adjustable)

flexible driving band

plunger

35.6.8.2 - Mechanical and Optical Comparators

• The Eden-Rolt Reed system uses a pointer attached to the end of two reeds. One reed is pushed by a plunger, while the other is fixed. As one reed moves relative to the other, the pointer that they are commonly attached to will deflect.

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pointer

moving reed

fixed reed

plunger

35.6.8.3 - Optical Comparators

• These devices use a plunger to rotate a mirror. A light beam is reflected off that mirror, and simply by the virtue of distance, the small rotation of the mirror can be converted to a significant translation with little friction.

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35.6.8.4 - Pneumatic Comparators

•Flow type

-the float height is essentially proportional to the air that escapes from the gauge head

-master gauges are used to find calibration points on the scales

-the input pressure is regulated to allow magnification adjustment

page 194

-a pressure bleed off valve allows changes to the base level for offset

-The pressure is similar to that shown in the graph below,

flow through gauge tube

clearance at gauge head to let air escape

zero adjust

output to gauge

tapered glass tube

scale

float (with vanes to encourage rotation for ballistic stability)

input flow from regulator

• The Soloflex Back Pressure System uses an orifice with the venturi effect to measure air flow. If the gas is not moving, the pressure on both sides of the orifice will be equal. If the flow is moving quickly, the air pressure on the downstream side of the orifice will be at a lower pressure.