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計測機器 Ashcroft a4aのPDF インストレーション＆メンテナンス・マニュアルをオンラインで閲覧またはダウンロードできます。Ashcroft a4a 20 ページ。 Precision pressure gauge

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Consult Figures 5.1 & 5.2

Move the quadrant slide outward, thereby increasing the

angle, or rotate the pointer and pinion shaft clockwise to

indicate about twice the amount of the error. Holding the

pointer in this position release the quadrant clamp screw

permitting the Bourdon tube to adjust its position, then re-

clamp. This also results in an increase in angle.

Step 4-

Reset the pointer to zero and repeat the linearity check to

evaluate the adjustment. Repeat procedure until linearity is

established within the desired tolerance.

5.3.3

Range Test

Step 1-

Load the gauge to full scale pressure by dead weight and

note the reading on the gauge. If the full scale reading is not

correct within the required tolerance proceed as follows:

Step 2-

If the pointer indicates greater pressure than the dead

weight, turn the Micro-Slide adjusting screw counter-clock-

wise. If the pointer indicates less pressure than the dead

weight, turn the Micro-Slide adjusting screw clockwise.

Should the range adjustment be excessive, loosen the two

Micro-Slide clamping screws and move the slide inward to

increase the range and outward to decrease the range.

Adjustment of the Micro-Slide screw changes the amount of

pointer travel between zero and full scale. It is necessary

after each adjustment to return the pressure to zero, re-

adjust the pointer to zero and repeat to determine the effec-

tiveness of the adjustment.

Adjustment of the Micro-Slide screw requires a movement

of approximately 180 degrees in order to have a pointer

show a change of ? of one percent of full scale reading on

most gauge dials. To correct some gauges an adjustment of

as little as 0.0005" or a turn of approximately 10º of the

Micro-Slide screw is required. The adjusting screw may be

operated with a small Allen wrench.

5.3.4

Final Adjustment

The range and linearity adjustments are interdependent.

With linearity established, fine readjustment may be made

in range and linearity re-checked to bring both adjustments

within the desired limits of accuracy.

5.3.5

Check For Hysteresis

Step 1-

Connect the gauge to the deadweight tester in a vertical

operating position.

Step 2-

If the gauge has been drained, fully or partially, air may be

entrapped in the Bourdon tube and this trapped air must be

removed through the bleeder valve (flushing connector) to

insure positive, accurate response.

If the gauge has been used with and contains mercury, care

should be taken to avoid contamination of brass tubing and

fittings on the tester.

Gauges used in pneumatic systems should, of course, be

cleaned of oil, water or similar entrapped matter before testing.

Step 3-

Set the pointer to zero using the dial adjustment.

Step 4-

Applying pressure slowly, load the gauge to one-half scale

pressure and record the reading.

Step 5-

Load to full scale pressure.

Step 6-

Release to one-half scale pressure and compare the down-

scale reading to the upscale reading.

If the second reading is higher than the first it may be

caused by either hysteresis or friction. With dead weight still

at one-half load, tap the gauge gently. If the pointer returns

to substantially the original one-half load reading, the differ-

ence in readings was caused by friction. If the amount of tap

is not excessive it might be considered satisfactory for use.

It is normal procedure to tap an instrument to observe its

sensitivity. If the tap is excessive it is usually remedied by

cleaning the movement mechanism with solvent such as

ether, carbon tetrachloride, etc.

If after tapping the gauge the pointer still reads higher than

the first reading, the cause is hysteresis. Increased hystere-

sis effect may be brought about by crystallization of the

Bourdon tube due to excessive cycling. A similar condition

may result from exposure to excessively high pressure

causing a partial fracture of the tube. In cases where the

Bourdon tube has been found to have unacceptable levels

of hysteresis a new tube is required. It is recommended that

gauges requiring a new Bourdon tube be returned to the

factory for repair.

Note: If a new Bourdon tube is required for a gauge that has

the optional bimetallic thermal compensation a new thermal

compensation will be required.

6.0

POINTER REPLACEMENT

6.1

GUIDE TO POINTER REPLACEMENT

A "perfect" gauge can be defined as a gauge which yields a

reading which is exactly equal to the actual pressure which

is being applied. Plotted on a graph, these points can be

connected to form a straight line, indicating perfect "lineari-

ty". If a point was plotted that did not fall along this line, it

can be said that this is a "non-linear" reading. Most pressure

instruments exhibit some inherent non-linearity characteris-

tics, due mainly to a disproportionate movement of the

sensing element (Bourdon tube) under different pressures.

Prior to correction, an Ashcroft A4A precision pressure

gauge will generally, in the first half of the scale, indicate a

higher pressure reading than the actual applied pressure

(called "running fast"), and will be the fastest at ? scale. The

gauge will usually read perfectly at midpoint, run "slow" in

the second half of the scale (slowest at ? scale) and read

perfectly at full scale.

In order to maintain high accuracy, some form of compen-

sation must be employed to overcome the additional inaccu-

racy caused by non-linearity. There are two methods of

compensation for non-linearity in Ashcroft A4A gauges:

1-Non-Linear Dials

These gauges are manufactured with balanced pointers

and dials on which the graduations have been plotted to

coincide with the position of the pointer at specific pressure

points. The placement of the graduations is determined by

the use of readings recorded during the preliminary calibra-

tion of each gauge. These readings will then be pro-

grammed into a computer linked to a photo-plotter, which

will produce a negative image of the dial. A positive dial is

then produced from this negative.