Practical work
Goal of the work: studying spring pressure gauges MBM type (device, principle of operation, operation).
Spring pressure gauge type OBM
Manometer (from the Greek manos - rare, loose and metreo - measure) - a device for measuring excess pressure (pressure above atmospheric) of vapors, gases or liquids enclosed in a closed space. A type of pressure gauge is a vacuum gauge - a device for measuring pressure close to zero and a pressure-vacuum gauge - a device for measuring vacuum and excess pressure.
The most popular among consumers are pressure gauges with a Bourdon tube or deformation pressure gauges, the design of which was invented by E. Bourdon in 1849.
The Bourdon tube is the main structural element of the pressure gauge, its sensitive element, which is the primary pressure transducer.
The Bourdon tube is usually made of brass or phosphor bronze, has a semicircular shape for low pressures, and a coil shape for medium and high pressures. One end of the tube is connected to the inlet fitting of the pressure gauge, which is the connecting element to the medium being measured, and the other end is sealed and located in a cantilever. By using tubes of a more complex shape (spiral, helical), it is possible to obtain devices with greater sensitivity, but a smaller measurement limit.
Operating principle of deformation pressure gauges.
Under the pressure of the medium, the cantilevered end of the Bourdon tube moves - the tube tries to straighten. The magnitude of this movement is proportional to the magnitude of the pressure.
A simple gear-lever drives a pointer that indicates the pressure value on the instrument scale. Most pressure gauges of the domestic brands MP, MTP, DM TM, M 3/1, OBM, MTI, MPTI, MO, German pressure gauges Wika 111.10, 111.12, 213.53, RCh, RСhg, RChgG and pressure gauges from other manufacturers have such a device.
A general view of the OBM type spring pressure gauge is shown in Fig. 1.
Figure 1 – Spring pressure gauge type OBM
Figure 2 - Diagram of a pressure gauge with a Bourdon tube
1-Bourdon tube, 2-rod transmission mechanism, 3-tooth sector, 4-pointer, 5-fitting
Tubular springs are used as sensitive elements in pressure gauges. As can be seen from Fig. 3, one end of the tubular spring 3 goes into the fitting 7 for receiving the measured pressure. Under the influence of pressure, the free end of the pressure tube 5 will be deformed (bent), and the amount of elastic deformation is proportional to the measured pressure. Due to this relationship, measuring needle 1, due to the movement of the kinematic unit (tribe 2 - sector 4 - driver 6), shows the true value of the measured pressure relative to the instrument scale.
Figure 3 – Kinematic diagram of a pressure gauge with a Bourdon tube
1-arrow, 2-tube, 3-spring, 4-tooth sector, 5-pressure sensor (manometric tube), 6-lead, 7-fitting
Spring indicating and recording pressure gauges are repaired by the repair services of the metrology department. To do this, in a special area, workplaces must be equipped with backup glasses of a standard range with a diameter of 60, 100, 160 and 250 mm, standard scales, and special pullers for removing measuring needles from the instrument axes; clamps for fastening parts of pressure gauges, a set of ladders for restoring clogged threads of fittings M 20X1.4, devices for drawing scales, sets of tweezers and watch magnifiers, sets of small gas burners for soldering sensitive elements (springs).
The most labor-intensive operations are replacing the sensitive element (tube) of the pressure gauge and adjusting the kinematic link “sector - tube” (see Fig. 3).
The sensing element of the device is replaced after it is used to measure pressure exceeding the maximum. As a result, the tube stretches, causing residual deformation that cannot be repaired. To repair such a device, it is completely disassembled, the fitting 7 secure in a vice and dismantle the tube using a gas torch 5 from the board. After the solder has melted, the faulty tube is removed with pliers, and in its place, after cleaning the surface, a similar gauge spring is installed (at a given pressure measurement limit). The soldering area is treated with a solvent - rosin with acetone (alcohol) or hydrochloric acid.
The main part in instruments that measure pressure is the so-called Bourdon spring, which is a hollow tube bent in a circular arc with an oval or some other elongated cross-section (Fig. 147).
Such a tube straightens somewhat, and the movement of the end of the tube is transmitted through the multiplying mechanism to the pressure gauge needle (Fig. 148).
Based on the deviation of the arrow, the value of the measured pressure is judged.
In one of the books on measuring instruments, we happened to see the following explanation of the principle of operation of a Bourdon tube: “The action of a Bourdon spring is based on the fact that the pressure inside the tube on the upper surface of the spring will be greater than the pressure on its inner surface. Indeed, if the tube has a rectangular cross-section and if we denote the outer and inner radii of the tube by R 1 and R 2, then the outer (S 1) and inner (S 2) surfaces of the tube will be equal, respectively 
, where φ is the central angle of the spring, and is the size in the plane perpendicular to the drawing plane, R 1 and R 2 are radii.
Under pressure p kg/cm 2
total pressure on the outer surface 
and to the internal 
, and the force P 1 will be greater than the force P 2 and will tend to straighten the spring"
Is this explanation correct?
Explanation wrong. According to the above reasoning a tube, regardless of the cross-sectional shape, under internal pressure should always reduce its curvature - straighten. Experience, however, shows that tube with round cross section does not respond at all to internal pressure, and a tube having a cross-section with reverse arrangement of major and minor axes, under internal pressure does not reduce, but increases its curvature.
The author of the above explanation did not take into account that, except for the forces P 1 and P 2, acting on surfaces S 1 And S 2, there is also force acting on the bottom of the tube. This power gives moment, precisely equal to the difference between the moments of force P 1 and P 2, So the bending moment in any section of the tube is zero. In this case, there is no need to calculate the magnitude of these forces to verify what has been said. Tube surface on right from an arbitrarily taken section AA(Fig. 404) is closed surface, and the pressure will give in this section only normal a force equal to the product of pressure and the cross-sectional area “in the clear”.
At any tube shape Pressure forces will not produce a bending moment at all. A necessary condition for the operation of the tube is deformation of the cross-sectional contour. Whatever non-circular shape the cross-section of the tube may have, under the influence of internal excess pressure the contour of this section tends to take the shape of a circle. Wherein minor axis sections will increase slightly A the big one will decrease, and the entire contour will take approximately the same shape as shown by the dashed line in Fig. 404. At the same time each longitudinal fiber tube will get some movement in a direction parallel to the minor axis sections. In Fig. 404 is a move for mn fibers indicated by w.
When fiber mn will move by amount w, it will go to the arc larger radius and it will appear stretching voltage. In the fibers lying below neutral axis, will appear compressive voltage. The tube will be straighten up.
In light of what has been said, it becomes clear why round tube does not respond to internal pressure. In this case the section contour only stretches, and magnitude w will negligible. That's why the change in curvature of a round tube is very small and in the usual setting of the experiment not detected.
A Bourdon tube is an elastic element in instrumentation that allows you to control pressures of all levels used in industry. It senses changes in pressure and converts these changes into mechanical movement. A Bourdon tube is usually connected to a pressure gauge, which displays the change in pressure on a graduated scale.
The Bourdon tube is not an independent measuring device, but an auxiliary element that is installed in the measuring device. It allows you to create the pressure difference necessary to measure the flow rate of a liquid, gas or steam. Bourdon tube pressure gauges are the most common measuring instruments due to their low cost, versatility and high reliability.
Made from a variety of metals, including bronze, brass, and stainless steel. The choice of material is determined by the application environment and the level of measured pressure: the higher the pressure, the stronger the material.
Working principle of a Bourdon tube
One end of the C-shaped Bourdon tube is open, the other, called the tip, is closed. The open end is connected to a coupling having an inlet hole inside the tube. The pressure source is connected to the coupling, so the pressure flows from the source through the inlet and into the tube.
When pressure is applied, the Bourdon tube begins to move. Depending on the design of the element and the type of pressure applied, the tube tends to either straighten or curl. True, the displacement of the tip when pressure is applied is insignificant, in most cases it is no more than one centimeter. In this case, the amount of tip displacement is proportional to the amount of applied pressure. The pressure gauge to which the tip is connected converts this small movement of the tip into a needle movement that can be read.
Types of Bourdon tubes
In addition to the C-shaped Bourdon tube, there is a spiral Bourdon tube, basic device which is the same as the C-shaped one, except that the tube in this case has the shape of a spiral.
This winding makes it possible to straighten the tube to a greater extent than the C-shaped one. Ultimately, the displacement of the tube tip when pressure is applied is greater than that of a C-tube. Since some instruments require greater displacement than a C-tube, this increase using a spiral tube is considered an advantage.
There is also a Bourdon screw tube, the design of which is very similar to the design of the C-shaped and spiral tubes. One main difference is this: in a helical tube, the turns are wound in a helical manner close to each other. This makes the tube design much more compact than others and can be used in confined spaces. Just like the helix, the helical tube has a greater tip offset compared to the C-tube.
Pressure gauge(from the Greek manos - rare, loose and metreo - measure) - a device for measuring excess pressure (pressure above atmospheric) of vapors, gases or liquids enclosed in a confined space. A type of pressure gauge is vacuum gauge– a device for measuring pressure close to zero and pressure-vacuum gauge device for measuring vacuum and excess pressure.
The most popular among consumers are Bourdon tube pressure gauges or deformation pressure gauges, the design of which was invented by E. Bourdon in 1849.
Bourdon tube
– the main structural element of the pressure gauge, its sensitive element, which is the primary pressure transducer.
Bourdon tube It is usually made of brass or phosphor bronze, has a semicircular shape for low pressures, and a coil shape for medium and high pressures. One end of the tube is connected to the inlet fitting of the pressure gauge, which is the connecting element to the medium being measured, and the other end is sealed and located in a cantilever. By using tubes of a more complex shape (spiral, helical), it is possible to obtain devices with greater sensitivity, but a smaller measurement limit.
Operating principle of deformation pressure gauges.
Under the pressure of the medium, the cantilevered end of the Bourdon tube moves - the tube tries to straighten. The magnitude of this movement is proportional to the magnitude of the pressure.
A simple gear-lever drives a pointer that indicates the pressure value on the instrument scale. Most pressure gauges of the domestic brands MP, MTP, DM TM, M 3/1, OBM, MTI, MPTI, MO, German pressure gauges Wika 111.10, 111.12, 213.53, RCh, RСhg, RChgG and pressure gauges from other manufacturers have such a device.
Diagram of a pressure gauge with a Bourdon tube
1-Bourdon tube, 2-rod transmission mechanism, 3-tooth sector, 4-pointer, 5-fitting
In addition to pointer pressure gauges, scaleless pressure gauges (having a similar device design) DER with unified electrical output signals are widely used, used in control systems, automatic regulation and control of various technological processes.
A significant disadvantage of deformation pressure gauges is hysteresis.
The essence of the phenomenon: the deformable element of the Bourdon tube, subjected to high pressure, will give slightly inflated readings in subsequent measurements. The same applies to the vacuum gauge, which, after pumping to a deep vacuum, will, on the contrary, underestimate the readings. Considering that the vacuum pump system operates in a pressure range from atmospheric to 0.133 Pa (10 V -3 mm Hg), such differences will adversely affect the accuracy of the strain gauge.
To prevent damage to strain gauges due to significant pressure drops in the measuring systems, a tap or valve is provided that turns off the device between measurements.
A Bourdon tube pressure gauge is used to measure gauge pressure from 0.6 to 70 bar. It is a mechanical pressure measuring device and operates without power supply.
A Bourdon tube is a ring-shaped tube with an oval cross-section. The pressure of the measured medium acts on the inner surface of the tube and causes movement of the loose end of the tube. This movement is a measurement of pressure and is displayed through a mechanism. This movement is a measure of the amount of pressure and is displayed through a mechanism.
One end of the C-shaped Bourdon tube is open, the other, called the tip, is closed. The open end is connected to a coupling having an inlet hole inside the tube. The pressure source is connected to the coupling, so the pressure flows from the source through the inlet and into the tube.
When pressure is applied, the Bourdon tube begins to move. Depending on the design of the element and the type of pressure applied, the tube tends to either straighten or curl. True, the displacement of the tip when pressure is applied is insignificant, in most cases it is no more than one centimeter. In this case, the amount of tip displacement is proportional to the amount of applied pressure. The pressure gauge to which the tip is connected converts this small movement of the tip into a needle movement that can be read.
In addition to the C-shaped Bourdon tube, there is a spiral Bourdon tube, the basic structure of which is the same as that of the C-shaped one, except that the tube in this case has the shape of a spiral.
This winding makes it possible to straighten the tube to a greater extent than the C-shaped one. Ultimately, the displacement of the tube tip when pressure is applied is greater than that of a C-tube. Since some instruments require greater displacement than a C-tube, this increase using a spiral tube is considered an advantage.
Loading...
