Mcleod Gaues

The McLeod gauges is used for measuring very low pressures down to one hundered-thousandth of an inch of mercury (or 10m bar down to 10^-3m bar). The pressure can be calculated by using the following equations:

P=KH H_{o ­­­}(1-KH)

P= measured pressure

K= a constant, determined by the geometry of the gauge

H= difference in hights of the two mercury columns

H_o =hight of the top of the closed capillary tube above the line marked on the tube.

The McLeod gauge is a very accurate pressure-measuring device and often serves as a standered for calibrating other low-pressure gauges. If can be designed to measure pressure as low as 0.05 microns(0.00005 torr).

DISADVANTAGES

I) lack of good vibration and shock resistance
II) difficult to repair
III) limited to relatively low pressures

ADVANTAGES

i) their moderate cost

ii) high over range characteristics

iii) adaptability to absolute and differential pressure measurement

iv) good linearity

v) availability in several materials for good corrosion resistance

vi) small size

vii) adaptability to slurry services

SLACK DIAPHRAGM GAUGE

It is more difficult to measure pressure below the atmospheric pressure because the changes are small. The full range from atmospheric pressure to a perfect vacuum is only 14.7 psi (1 kg/cm²)

A slack diaphragm gauge with a weak spring and a large area can be used over pressure ranges as low as 0.01- 0.40 mm Hg (torr). It is possible to achieve accuracies of 1-2%.

METALLIC DIAPHRAGM GAUGE

It consists of a thin flexible diaphragm made of materials such as brass or bronze. The motion of the diaphragm operates an indicating or a recording type of instrument.

This type of gauge is capable of working in any position and is portable, and therefore well adapted for use or for installation in moving equipments such as aircrafts.

DIAPHRAGM PRESSURE TRANSDUCERS

Diaphragms are widely used for pressure (gauge pressure) and draft measurements, particularly in very low ranges. They can detect a pressure differential even in the range of 0 to 4 mm.

The diaphragms can be in the form of flat , corrugated or dished plates and the choice depends on the strength and amount of deflection desired.

DISADVANTAGES

Following are the advantages of Bourdon tubes :

i) low spring gradient (i.e. below 50 psig)

ii) susceptibility to shock and vibration

iii) susceptibility to hysteresis

ADVANTAGES

Following are the advantages of Bourdon tubes :

i) low cost

ii) simple construction

iii) time tested in applications

iv) availability in a wide variety of ranges , including very high ranges

v) adaptability to transducer designs for electronic instruments

vi) high accuracy, especially in relation to cost

Adjustments

Basically there are two types of adjustments of the Bourdon tube :

a) Multiplication Adjustment

b) Angularity

Construction And Working

A C-type Bourdon tube consists of a long thin-walled cylinder of non-circular cross-section, sealed at one end, made from materials such as phosphor bronze,steel and beryllium copper and attached by a light line work to the mechanism which operates the pointer. The other end of the tub is fixed and is open for the application of the pressure which is to be measured. The tube is soldered or welded to a socket at the base, through which pressure connection is made. Figures 5.8 (a) & (b) show the schematic arrangement of a complete Bourdon tube gauge.

As the fluid under pressure enters the Bourdon tube, it tries to change the section of the tube from oval to circular and this tends to straighten out the tube. The resulting movement of the free end of the tube causes the pointer to move over the scale. The tip of the Bourdone tube is connected to a segmental lever through an adjustable lever link. The lever length also may be adjustable. The semental lever end on the segment side is provider with a rack which meshes to a suitable pinion mounted on a spindle. The segmental lever is suitably pivoted and the spindle holds the pointer, as shown in fig 5.8 (b). A hairspring is provide the necessary tension for proper meshing of the gear teeth, thereby freeing the system from backlash. Any error due to friction in the spindle bearing is known as “lost moion”.

Bourdon tubes are made of a number of materials, depending upon the fluid and the pressure for which they are used, such as phosphor bronze, alloy steel, stainless stel, “Monel”, metal, and beryllium copper. For adequate reliability, the materials for Bourdon tubes must have good elastic or spring characteristics. Bourdon tubes are generally made in three shapes:

ELASTIC PRESSURE TRANSDUCERS

This type of pressure transducers use elastic primary sensing elements such as the Bourdon tube, bellows and diaphragm.
The Bourdon tube is the most frequently used pressure gauge because of its simplicity and rugged constuction. It covers ranges from 0-15 psig to 0-100,000 psig, as well as vacua from 0 to 30 inches of mercury.

Disadvantages of Manometers

i) large and bulky

ii) need for leveling

iii) lack of portability

iv) measured fluid must be compatible with manometer fluid

v) no over-range protection

vi) condensation may present problems

ADVANTAGES AND DISADVANTAGES

Following are the advantages of manometeters –

i) Simple and time proven

ii) High accuracy and sensitivity

iii) The availability of a wide range of filling fluids of varying specific gravities

iv) Reasonable cost

v) Their suitability for low pressure and low differential pressure applications.

FLUIDS FOR MANOMETERS

The most common fluids for manometers are water, red oil and mercury. Water is the best fluid to use if air is the fluid to be measured and the glass length is within reasonable limits. To minimize the effects of freezing and evaporation, kerosene or antifreeze may be used.

Mercury is used when the measured fluid mixes with the manometer luid or a very long tube length is necessary, while red oil is used as the indicating fluid when it is required to read a manometer from a distant place. It is easier to read and it does not combine chemically with some fluid in a dry system.

ERRORS IN MANOMERS

a) Effect of Temperature

b) Capillary Effect

c) Effect of Variale Meniscus