Dial gauge

Magnehelic dial gauges A measuring device recording the static pressure of a fluid. 

The specimen can be square or round to fit a dilatometer test tube in a free sliding manner. The length is governed by the sensitivity of dial gauge, the expected expansion, and the... 

Both the DIAL GAUGE (b) and TELESCOPE WITH MICROMETER (c) methods are varicmts of the microscope-method (a). A temperature-controlled furnace is needed and a physical set-up where the DIAL-GAUGE or TELEISCOPE WITH... 

Facility is provided to bring the equipment up to the desired absolute pressure without subjecting the test unit to excessive pressure difference between its interior and exterior. This is accomplished by first filling the test cell with water by means of a hand operated hydraulic pump(17 in Figure 1) to a suitable value as indicated on the Bourdon dial gauges(P in Figure 1). The pressure thus developed is used also to control the appropriate back-pressure relief valve... 

Whilst all aspects of a laboratory s operation require systematic control, it is the calibration of test equipment which gives rise to most problems and which is also the most expensive. All test equipment and every parameter of each instrument requires formal calibration. For example, it is not good enough to calibrate the force scale of a tensile machine, there are also requirements for speed of traverse, etc. plus associated cutting dies and dial gauges. 

Dimensions such as the width of a dumb-bell or the depth of a nick in a tear specimen will be less than 30 mm but could not be measured with a dial gauge. Because of the virtual impossibility of applying a known pressure, such measurements must be made in an essentially contactless manner. For low precision, calipers or a rule may suffice but for readings to 0.01 mm a travelling microscope or projection microscope is most suitable, and this is specified in ISO 23529 Method D and applies also to dimensions over 30mm. Projection microscopes also find use in examining profiles and for rapid swelling tests (see Chapter 16, Section 2.1). 

The test piece assembly is strained in a tensile machine at 5 1 mm/min until a maximum shear strain of 30 % is reached. Mechanical conditioning is optional but, if used, five conditioning cycles are applied before the measuring cycle. No details of apparatus to measure the strain are given but this could be a dial gauge or, with a stiff tensile machine, the crosshead movement. 

Luminous dials, gauges, wrist watches Luminous paint... 

It is very important to be able to measure the pressure in a vacuum system, particularly when carrying out a distillation. For low vacuum measurement a simple manometer, such as that shown in Fig. 8.3a, is commonly used and the pressure is taken by subtracting the heights of the mercury levels. Dial gauges are also useful for in-line measurements and they are particularly valuable when used with rotary evaporators. For high vacuum... 

A Bourdon type dial gauge may be used to indicate pressures within the range 20 toiT to 1 toiT with an accuracy of about 3%. A bellows-operated gauge magnifies the movement of a metal bellows by an optical method and indicates pressure difference over the range 50 torr to 10 toiT. Neither bellows gauges nor Bourdon gauges are in common use in laboratory work. 

The measurement of dimensional or volume changes can be used in appropriate instances to observe the progress of a solid state reaction. Line2ur expansion can be measured by dial gauges, micrometers, interferometer, telescopes, linear differential transformers and from X-ray patterns. Except for the X-ray techniques, the reaction can be studied in situ. Non-isotropic materials probably require measurements in several orientations. 

Figure (2) shows the load-deformation diagram in the short-term experiments. The vertical axis of this diagram represents the controlled pressure changes in the balloon the horizontal axis represents the lateral deformation which is recorded by the dial-gauge readings. 

For theoretical correlation, the displacement of the sub-panel relative to its edges would be needed. The measured total deflection of the mid-point of the sub panel would be the sum of two dial gauge readings one at the edge another at the midpoint this would give... 

The permeation experiments were carried out on 2 cm thick clay specimens placed into an aluminium cylinder of 5.38 cm diameter. The testing equipment allows the generation of a constant flow rate using a piston moving at a constant velocity. The chamber of the piston is connected to the fluid chamber in the permeameter cell located above the clay specimen, which is toped by a porous stone. A pressure transducer is connected to the hydraulic circuit. The fluid outlet in the lower aluminium plate is kept at atmospheric pressure. An external vertical load is applied to the specimen by means of a spring-loaded cap prior to permeation. Vertical settlements are monitored using a dial gauge and a settlement rod mounted on top of the specimen. 

It is fairly obvious that the use of a different foot pressure would, with a soft and deformable material such as rubber, produce a different result. Hence, it is not permissible to use a dial gauge with a return spring, calipers, or a travelling microscope when this standard method is specified. The errors resultant on using different pressures have been reported by Clamroth and Dobrosehke [29]. 

By contrast, there is no advice on which method to use for a given purpose, and the standard oversteps its scope by discussing the effect of the force on the presser foot of a dial gauge on soft elastomers. One suspects that the authors have never read ISO 4648. and this perhaps illustrates that even in closely related industries test standards can diverge. 

There are currently no standards tests for measuring the thermal expansion of plastics in ISO or in BS 2782. In ASTM, test method D696 [124] uses a relatively thick test piece, which is placed in a chamber whose temperature can be controlled. This is shown schematically in Fig. 23. The expansion of the sample is transmitted to a remote dial gauge via a quartz rod that has a very low expansion coefficient. This same technique is applied in modern thermomechanical analyzers (TMA). but the dial gauge is replaced by linear displacement transducers or other electronic devices capable of detecting smaller dimensional changes. In turn this allows thinner specimens to be tested and permits wider temperature ranges to be examined. There are developments wnthin the ISO to provide a standard for these types of instruments. 

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