Bourdon Presssure Gauge

A Bourdon Pressure gauge could also be referring to a pressure measuring instrument, usually limited to measuring pressures near to atmospheric. The Bourdon pressure gauge uses the principle that a flattened tube tends to change to a more circular cross-section when pressurized. Although this change in cross-section may be hardly noticeable, and thus involving moderate stresses within the elastic range of easily workable materials, the strain of the material of the tube is magnified by forming the tube into a C shape or even a helix, such that the entire tube tends to straighten out or uncoil, elastically, as it is pressurized. Eugene Bourdon patented his gauge in France in 1849, and it was widely adopted because of its superior sensitivity, linearity, and accuracy; Edward Ashcroft purchased Bourdon's American patent rights in 1852 and became a major manufacturer of gauges. Also in 1849, Bernard Schaeffer in Magdeburg, Germany patented a successful diaphragm (see below) pressure gauge, which together with the Bourdon gauge, revolutionized pressure measurement in industry. But in 1875 after Bourdon's patents expired, his company Schaeffer and Budenberg also manufactured Bourdon tube gauges.

In practice, a flattened thin-wall, closed-end tube is connected at the hollow end to a fixed pipe containing the fluid pressure to be measured. As the pressure increases, the closed end moves in an arc, and this motion is converted into the rotation of a (segment of a) gear by a connecting link which is usually adjustable. A small diameter pinion gear is on the pointer shaft, so the motion is magnified further by the gear ratio. The positioning of the indicator card behind the pointer, the initial pointer shaft position, the linkage length and initial position, all provide means to calibrate the pointer to indicate the desired range of pressure for variations in the behaviour of the Bourdon tube itself. Differential pressure can be measured by gauges containing two different Bourdon tubes, with connecting linkages.

Bourdon tubes measure gauge pressure, relative to ambient atmospheric pressure, as opposed to absolute pressure; vacuum is sensed as a reverse motion. Some aneroid barometers use Bourdon tubes closed at both ends (but most use diaphragms or capsules, see below). When the measured pressure is rapidly pulsing, such as when the gauge is near a reprocating pump, an orfice restriction in the connecting pipe is frequently used to avoid unnecessary wear on the gears and provide an average reading; when the whole gauge is subject to mechanical vibration, the entire case including the pointer and indicator card can be filled with an oil or glycerin. Typical high-quality modern gauges provide an accuracy of ±2% of span, and a special high-precision gauge can be as accurate as 0.1% of full scale.

(Source: weihrauch-sport.de & http://en.wikipedia.org)

Rotameter

 

Rotameters are simple industrial flow meters that measure the flow rate of liquid or gas in a closed tube.  Rotameters are popular because they have linear scales, a relatively large measurement range, low pressure drop, and are simple to install and maintain.  Rotameters are a subset of meters called variable area flow meters that measure the flow rate by allowing the fluid to travel through a tapered tube where the cross sectional area of the tube gradually becomes greater as the fluid travels through the tube.  The flow rate inside the rotameter is measured using a float that is lifted by the fluid flow based on the buoyancy and velocity of the fluid opposing gravity pulling the float down.  For gasses the float responds to the velocity alone, buoyancy is negligible.

The float moves up and down inside the rotameter’s tapered tube proportionally to the flow rate of the fluid.  It reaches a constant position once the fluid and gravitational forces have equalized.  Changes in the flow rate cause rotameter’s float to change position inside the tube.  Since the float position is based on gravity it is important that all rotameters be mounted vertically and oriented with the widest end of the taper at the top.  It is also important to remember that if there is no flow the float will sink to the bottom of the rotameter due to its own weight.

The operator reads the flow from a graduated scale on the side of the rotameter, which has been calibrated to a specific fluid with a known specific gravity.  Specific gravity or the weight of the fluid has a great impact on the rotameter’s accuracy and reliability.  All of Global Water’s rotameters have been calibrated using water as the standard fluid with a specific gravity of 1.0.

Rotameters can be calibrated for other fluids by understanding the basic operating principles.  Rotameter accuracy is determined by the accuracy of the pressure, temperature, and flow control during the initial calibration.  Any change in the density and weight of the float will have impacts on the rotameter’s flow reading.  Additionally any changes that would affect the fluid such as pressure or temperature will also have an affect on the rotameter’s accuracy.  Given this, rotameters should be calibrated yearly to correct for any changes in the system that may have occurred.

There are several advantages to a rotameter over a more complicated flow meter including:

• Rotameters can be installed in areas with no power since they only require the properties of the fluid and gravity to measure flow, so you do not have to be concerned with ensuring that the instrument is explosion proof when installed in areas with flammable fluids or gases.
• Rotameters can be installed with standard pipe fittings to existing piping or through a panel.  You do not have to worry about straight runs of pipe as with a magnetic or turbine flow meter.
• Rotameters are simple devices that are mass manufactured out of inexpensive materials keeping investment costs low.
• A glance at a Rotameter acts as a sight glass telling the operator that a filter needs cleaning, that there is some other problem causing discoloration of the water, or that the fluid is actually flowing.  With a transparent rotameter they can instantly see if there is any build up on the float or tube walls.
• With a properly maintained rotameter the operator can expect sustained high repeatability.
• Rotameters offer wide flow measurement ranges or rangeability.  A typical ratio of 10:1 from maximum to minimum flow rate can be expected.  Operators will be able to measure minimum flow rates as low as 1/10 of the rotameter’s maximum flow rate without impairing the repeatability.
• The rotameter’s scale is linear because the measure of flow rate is based on area variation.  This means that the flow rate can be read with the same degree of accuracy throughout the full range.
• Pressure loss due to the rotameter is minimal and relatively constant because the area through the tapered tube increases with flow rate.  This results in reduced pumping costs.

There are also a few disadvantages to the use of rotameters that you should keep in mind:

• Because gravity plays a key roll in the flow measurement the rotameter must always be installed vertically with the fluid flowing up through it.
• The graduated scale on the side of the rotameter will only be valid for the specific fluid and conditions where it was calibrated.  The specific gravity of the fluid is primary property to consider, however the fluid’s viscosity and any temperature changes may also be significant.  Rotameter floats are generally designed to be insensitive to viscosity, but the operator should verify that any rotameters installed in their system are calibrated to their specific setup prior to relying on the flow measurements provided.
• It is difficult for rotameters to be adapted for machine reading, although a magnetic float may be used in some instances.
• Rotameters are typically made of transparent material, however all operators should check the chemical compatibility of the meter with their fluid prior to full installation.

(source: maharashtradirectory.com & http://www.globalw.com)