11/44 6.3.2 Manometers Comparison with deadweight gages: >The manometer is self-balancing,is a deflection rather than a null instrument,and has continuous rather than stepwise output. The accuracies of deadweight gages and manometers of similar ranges are quite comparable. Accuracy improvements: >Temperature expansion of the engraved(雕刻的)scale when visual reading of the height h is employed; >The variation of p with temperature for the manometer fluid; Local value of g; >Non-verticality(不垂直度)of the tubes;; Fig.6.4 U-tube manometer >The difficulty in reading h because of the meniscus 液面)formed by capillarity(毛细管作用). 12/44 6.3.3 Practically Variations of Manometers l.The cistern(蓄水池)or well-type manometer: Widely utilized because of the convenience in requiring reading of only a single leg. 41,42 Areas The well area is made much larger than the tube;thus the zero level moves very little when pressure is applied. And even this small error is compensated by suitable distorting the length scale. >Different from U tube,this arrangement is sensitive to Fig.6.5a nonuniformity of the tube cross-sectional area,and thus is Well-type (single-leg) considered somewhat less accurate. manometer 一Evacuated 2.Barometer(气压计)： >A"single-leg"instrument; >Set the zero level of the well at the zero level of the scale before each reading is taken to achieve high accuracy; The pressure in the evacuated portion of the barometer is not really absolute zero but rather the vapor pressure of the filling fluid,mercury,at ambient temperature.And it is usually negligible as a correction. Fig.6.5b Barometer
6.3.2 Manometers Comparison with deadweight gages: ¾ The manometer is self-balancing, is a deflection rather than a null instrument, and has continuous rather than stepwise output. ¾ The accuracies of deadweight gages and manometers of similar ranges are quite comparable. Accuracy improvements: ¾ Temperature expansion of the engraved(雕刻的) scale when visual reading of the height h is employed; ¾ The variation of࣋with temperature for the manometer fluid; ¾ Local value ofࢍ; ¾ Non-verticality (不垂直度) of the tubes; ¾ The difficulty in reading h because of the meniscus (凹凸 液面) formed by capillarity(毛细管作用). Fig. 6.4 U-tube manometer 11/44 6.3.3 Practically Variations of Manometers 1. The cistern(蓄水池) or well-type manometer: ¾ Widely utilized because of the convenience in requiring reading of only a single leg. ¾ The well area is made much larger than the tube; thus the zero level moves very little when pressure is applied. And even this small error is compensated by suitable distorting the length scale. ¾ Different from U tube, this arrangement is sensitive to nonuniformity of the tube cross-sectional area, and thus is considered somewhat less accurate. 2. Barometer (气压计): ¾ A “single-leg” instrument; ¾ Set the zero level of the well at the zero level of the scale before each reading is taken to achieve high accuracy; ¾ The pressure in the evacuated portion of the barometer is not really absolute zero but rather the vapor pressure of the filling fluid, mercury, at ambient temperature. And it is usually negligible as a correction. Fig. 6.5b Fig. 6.5a 12/44
13/44 6.3.3 Practically Variations of Manometers 3.The inclined manometer: Fig.6.5C >To increase the sensitivity,the manometer maybe tilted with respect to gravity,thus giving a greater motion of liquid along the tube for a given vertical- Inclined Micrometer height change. >A single-leg device; >The calibrated scale is corrected for the slight changes in well level so that rezeroing of the scale for each reading is not required. 4.Micromanometer: Magnifier >A variation on the inclined-manometer principle for accurate measurement of extremely small pressure difference; Flexible tube >Optional Fluid:water,alcohol. Micromanometer Fig.6.5D 14/44 6.3.3 Practically Variations of Manometers 4.Micromanometer:(continued-how it works) >The instrument is initially adjusted so that when P,=P2,the meniscus in the inclined tube is located at a reference point given by a fixed hair line viewed through a magnifier. >Application of the unknown pressure difference cause the meniscus to move off the hairline, but it can be restored to its initial position by raising or lowering the well with the micrometer. The difference in initial and final micrometer readings gives the Micrometer height change h and thus the pressure. >In a different version,the inclined tube is moved,not the well. 5.Monometers with Automatic Readout: >Sonar Monometer using an ultrasonic transducer at each column a digital counter to measure the height difference. Magnifier 6.Two large mercury cisterns(水箱connected by flexible tubing to create a U-tube manometer. Flexible tube >One cistern fixed and one vertically moveable by an electromechanical servo-system; >The mercury surface,a metal plate and the air in 727 Fig.6.5D between them form a capacitance Micromonometer
6.3.3 Practically Variations of Manometers 3. The inclined manometer: ¾ To increase the sensitivity, the manometer maybe tilted with respect to gravity, thus giving a greater motion of liquid along the tube for a given verticalheight change. ¾ A single-leg device; Fig. 6.5C Fig. 6.5D ¾ The calibrated scale is corrected for the slight changes in well level so that rezeroing of the scale for each reading is not required. 4. Micromanometer: ¾ A variation on the inclined-manometer principle for accurate measurement of extremely small pressure difference; ¾ Optional Fluid: water, alcohol. 13/44 6.3.3 Practically Variations of Manometers Fig. 6.5D ¾ The difference in initial and final micrometer readings gives the height change h and thus the pressure. ¾ In a different version, the inclined tube is moved, not the well. 4. Micromanometer: (continued – how it works) ¾ The instrument is initially adjusted so that when P1=P2, the meniscus in the inclined tube is located at a reference point given by a fixed hair line viewed through a magnifier. ¾ Application of the unknown pressure difference cause the meniscus to move off the hairline, but it can be restored to its initial position by raising or lowering the well with the micrometer. 5. Monometers with Automatic Readout: ¾ Sonar Monometer using an ultrasonic transducer at each column + a digital counter to measure the height difference. 6. Two large mercury cisterns (水箱) connected by flexible tubing to create a U-tube manometer. ¾ One cistern fixed and one vertically moveable by an electromechanical servo-system; ¾ The mercury surface, a metal plate and the air in between them form a capacitance 14/44
15/44 6.4 Elastic Transducers ◆Introduction: >The majority of practical pressure transducers utilize Bourdon tube( 管)，diaphragm(波纹管)，or bellows(膜片)as their sensitive element. >The gross deflection of these elements may directly actuate a pointer/scale readout through suitable linkages or gears,or the motion may be transduced to an electrical signal by one means or another. >Strain gages bonded directly to diaphragms or to diaphragm-actuated beams are widely used to measure local strains that are directly related to pressure. 16/44 6.4 Elastic Transducers-Bourdon Tubes Bourdon tube is the basis of many mechanical pressure gages. Tube cross section The basic element is a tube of noncircular cross section( 圆横截面).A pressure difference between the inside and outside of the tube causes the tube to attempt to attain a circular cross section. C-type Resulted distortion lead to >A curvilinear(曲线的translation of the free end,in the: ●C type; Spiral ·Spiral types; .Helical type. >An angular rotation (the output)in the in the: ●Twisted type. The construction of a higher accuracy C-type Bourdon test Twisted tube gage with an optional bimetal temperature compensator.It corrects for both thermal zero shift and span shift. The C-type Bourdon tube has been utilized up to about 1000lb/in2.The spiral and helical configurations have been 77m元 Helicol used mainly blow 1000 lb/in2. Fig.6.8a Bourdon Tubes
6.4 Elastic Transducers Introduction: ¾ The majority of practical pressure transducers utilize Bourdon tube(布尔登 管), diaphragm(波纹管), or bellows(膜片) as their sensitive element. ¾ The gross deflection of these elements may directly actuate a pointer/scale readout through suitable linkages or gears, or the motion may be transduced to an electrical signal by one means or another. ¾ Strain gages bonded directly to diaphragms or to diaphragm-actuated beams are widely used to measure local strains that are directly related to pressure. 15/44 6.4 Elastic Transducers - Bourdon Tubes Bourdon tube is the basis of many mechanical pressure gages. The basic element is a tube of noncircular cross section (非 圆横截面). A pressure difference between the inside and outside of the tube causes the tube to attempt to attain a circular cross section. Resulted distortion lead to ¾ A curvilinear(曲线的)translation of the free end, in the : z C type; z Spiral types; z Helical type. ¾ An angular rotation (the output) in the in the : z Twisted type. The construction of a higher accuracy C-type Bourdon test gage with an optional bimetal temperature compensator. It corrects for both thermal zero shift and span shift. The C-type Bourdon tube has been utilized up to about 1000lb/in2. The spiral and helical configurations have been used mainly blow 1000 lb/in2. Fig. 6.8a Bourdon Tubes 16/44