measurement of low pressures (vacuum)
DESCRIPTION
Measurement of low pressures (vacuum). Vacuum tends to be used for pressures less than the atmospheric pressure, namely 1.013×10 5 Pa. A unit that is often used for such pressure is the torr ( 托） , this being the pressure equivalent to that given by a column of mercury of height 1 mm. - PowerPoint PPT PresentationTRANSCRIPT
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Measurement of low pressures (vacuum) Vacuum tends to be used for pressures less
than the atmospheric pressure, namely 1.013×105 Pa. A unit that is often used for such pressure is the torr ( 托） , this being the pressure equivalent to that given by a column of mercury of height 1 mm.
1mmHg=133.322Pa=1 torr
The lower the absolute pressure is, the higher the degree of vacuum is.
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Vacuum regimes( 真空划分 )(10-12~105)
Low vacuum( 粗真空 ) 105 ～ 3.3*103Pa Medium vacuum( 中真空 ) 3.3*103 ～ 100Pa Medium vacuum( 高真空 ) 100 ～ 10-3Pa High vacuum( 很高真空 )10-3 ～ 10-6Pa Ultrahigh( 超高真空 ) 10-6 ～ 10-9Pa Extreme ultrahigh ( 极高真空 ) <10-9Pa
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Vacuum gages
Absolute gages( 基于力的作用原理 ) ： U-
type ， Bourdon type ， diaphragm ；
Gas compression ( 基于压缩作用原理 ) ：Mcleod gage( 麦氏真空计 ) ；
Thermal conductivity ( 基于导热作用原理 ) ：Thermistor gage( 电阻真空计 ) ， Thermocouple gage( 热电偶真空计 ) ；
Ionization gage( 基于电离作用原理 ) ：Hot cathode ionization gages( 热阴极式 ) ， Cold cathode ionization gages( 冷阴极式 )
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（ 1) Elastic Vacuum 弹性式真空表 ( 粗真空）
（ 2 ） Capacitance gage( 电容式薄膜真空计 ) （ 10-2 ～ 105Pa ）
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（ 2 ） 压缩真空计 利用波义耳定律，将被测真空系统中一定的残余气体加以压缩，比较压缩前后体积、压力的变化，即能算出真空度。
玻义耳 - 马略特定律 它反映气体的体积随压强改变而改变的规律。对于一
定质量的气体，在其温度保持不变时，它的压强和体积成反比；或者说，其压强 P 与它的体积 V 的乘积为一常量，即
PV=C （常数）（ T 不变时）或
P1V1 ＝ P2V2=…=PnVn
实际气体只是在压强不太高、温度不太低的条件下才服从这一定律。
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The McLeod gauge is a pressure-measuring instrument and laboratory references standard used to establish gas pressures in the sub atmospheric range of 1mm Hg abs down to 0.1 Hg abs. One variation of this instrument is sketched next page in which the gauge is connected directly to the low-pressure source.
At the equilibrium and measuring position in FIGURE b, the capillary pressure,p2,is related to the unknown gas pressure to be determined,p1,by:
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Letting be the specific weight of the mercury, the difference in pressures is related by:
Such that the unknown gas pressure is just a function of y:
Where
In practice,a commercial McLeod gauge will have the capillary etched and calibrated to indicate pressure,p1,or equivalent head, ,directly.
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FIGURE 9.3 McLeod gauge
(a) Sensing position
Pressuresensing
port
(b) Indicating position.
p1Measuringcapillary
yp2
zero line
Referencecapillary
p1
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2 Thermal conductivity( 热导式真空计 )
热导式真空计是根据在低压强下气体热导率与其压力之间关系制成的一种测量低真空的相对真空计。
原理：基于大气体压强低于某一定值时，气体的导热系数 K 与 P 成正比，
即： K = b P b ：比例常数
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热导真空计的工作原理是假设灯丝由导热损失的热量与加热电流 I 所产生的热量平衡时，灯丝温度不变。其平衡方程为．
其中 R 为灯丝电阻； E1 为气体分子迁移热量；E2 为辐射迁移热量； E3 为引出导线的迁移热量。若由于压力减小而使 E1 减小，则当 I 不变时，平衡方程将失去平衡，使灯丝温度变化。由此可根据灯丝温度来衡量压力的变化。所以热导真空计是通过测量灯丝温度来决定压力大小的。
21 2 3I R E E E
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根据测定气体热传导方法的不同，热导式真空计可分为电阻真空计和热电偶真空计两种。
(1) Thermistor gage( 电阻真空计 )
它主要由电阻式规管和测量电路两部分组成。电阻式规管如图所示。在电阻规管内封装一只电阻温度系数较大的电阻丝，常用的有钨丝和铂丝。测量时规管与被测真空计系统相连。在较低的压力 ( 小于 13.3Pa) 时，热电阻丝的电阻值取决于周围气体的压强。
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热偶真空计测量真空度的元件是热偶规管，其结构见图。它主要由玻璃壳、铂丝、热电偶构成。铂丝用于加热热电偶，通以恒定电流，其温度为 100—200℃ 。热电偶是由镍铬—镍铝、或镍－康铜的丝制成的，其作用是在它的加热端与冷端 ( 非加热端）温度不同时，会产生温差电动势。我们利用真空度不同，气体传热性不同，进而使温差电动势不同的特性来测量真空度的。
(2) Thermocouple gage( 热电偶真空计或相对真空计）
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3 ionization gages( 电 离 真 空 计 )（量程范围为： 1×10-3 ～ 5×10-10 托）
电离真空计诞生于 1916 年，由巴克利首先研制成功。以后不断地改进，使其测量范围达到1.3×10 － 2 一 1.3×10 － 5 帕，变成了高真空区域广泛使用的真空计。这种真空计可以远距离测量，易于实现自动记录及控制，由于有炽热的灯丝，不适于测高氧气体。
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电离真空计是通过在稀薄气体中引起电离然后利用离子电流测量压力。
在气体中如果有动能足够大的电子与气体分子相碰撞，它可以从气体分子中击出一个或几个电子使气体分子成为正离子。把这种正离子收集到一个电极上使其产生离子电流，在稀薄气体中，它与气体压力有关。
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Ionization gages
热阴极电离真空计主要由圆筒式热阴极电离规管和测量电路两部分组成。
测量时规管与被测真空系统相连。通电后阴极加热所发射的电子在带正电的加速极作用下，以加速度运动，当电子的动能足够大，在飞向加速极的路途中与管内低压气体分子碰撞，即可使气体分子电离。电离产生的电子和正离子分别在加速极和收集极 (带负电位 )上形成电流 Ie 和离子流 Ii 。当压力足够低 ( 低于 10-1Pa) 时，离子电流Ii 与电子电流 Ie 之比正比于气体的压力 p ，即
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Fig.Schematic of ionization gage
The heated cathode emits electrons, which are accelerated by the positively charged grid. The electrons move toward the grid and produce ionization of the gas molecules through collisions. The plate is maintained at a negative potential, so that the positive ions are collected there, producing the plate current .
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Example1 ： For the situation shown in Figure 14, if the pipeline and manometer fluid densities are ρ = 1100kg/m3 (brine) and ρm = 13600kg/m3
respectively, x = 0.7m and the pressure at Y is – 30 kN/m2, find the difference in meniscus levels, h.
Figure Example 1
h819136007081911001030ghgxpp 3mYX ...
0h819136007081911001030 3 ...
m168081913600
7081911001030h
3
..
..
Solution:Note that the right-hand meniscus is now lower than the left-hand meniscus. The pressure at Y is sub-atmospheric, i.e. a negative gauge pressureFor the right-hand limb pX = 0 (atmospheric pressure)
For the left-hand limb
Equating the two expressions for pX:
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5.10 Sound Measurement
The sound wave propagates through air at a velocity of 330 m/s, dependent upon characteristics of medium
Sound field-anywhere where sound is present Amplitude is measured in Pa Microphones are sensitive to variations in air
pressure and convert the pressure variations to electrical voltage, acosticalelectrical transducer
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Principles of acoustics Sound waves in gas or liquid
No shear forces → no transverse waves
→ purely longitudinal waves
Audible sound range 20 Hz – 20 kHz Fully described by 3 variables
Pressure Particle velocity Density
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Measurement of Sound
.00002 Pa lowest sound pressure human ear can hear 20µPa
Decibel scale devised to talk about these units dB.
Logarithmic scale (base 10)-quantifies the sound pressure level by taking the logarithm (base 10) of the ratio of two sound pressures and multiplies them by 20
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Measurement of Sound
Decibel scale calculates the sound level from a ratio of two sound pressures
(p1 and p2): 20 log10 (p1/p2) Max= 200,000,000 µPa Just audible =20 µPa 200,000,000/20 for p1/p2=10,000,000/1 Log10 of 10,000,000 (10)=7 Therefore 20x7=140 dB, max tolerable
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Measurement of Sound
Just audible 20 µPa, p1/p2 is 20/20, or 1 Log 1=0, 20x0=0dB Absolute measure using a reference sound
pressure level (SPL) scale is 2x10-5 Pa or 20 µPa
SPL in dB=20 log [p1/(2x10-5)] Pa 2x10-5/ 2x10-5=1, log 1=0, 20x1=0 dB 0dB does not mean absence of sound, rather
it corresponds to SPL at 2x10-5 Pa
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dB summary
Decibels (dB) are logarithmic units of pressure or intensity that are used to measure the amplitude of a sound.
It is a ratio scale use to describe the large dynamic range of the auditory system.
A decibel is a relative measure, not an absolute measure (dB expressed relative to 20 micropascals expressed in dB SPL)
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Measurement of Sound
Sound level meters-use microphones to change sound pressure variations into voltage variations
The RMS amplitude of these voltage variations are determine electronically and displayed in SPL
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Measurement of sound pressure filtered by frequency (A-weighting) time-domain (RMS)
Mimics response of human ear to noise
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A-weighting curveFor subjective responses in special cases there are B-, C- and D-weighting curvesvery high or low levelspecial noise, e.g., of aircraft
Human hearing frequency response
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IEC International Standard 651 ”Sound Level Meters”
Tolerances per frequency band defined for 4 classes of accuracy Type 0: precision laboratory use Type 1: general purpose Type 2: low price Type 3: not used in practice (too wide tolerances)
Sound level measurement
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When sound reflects of a wall, the reflected wave may encounter other waves from the source and destructive (or constructive) interference may occur.
P
Destructive Interference
Condensation
Rarefaction
Sound Source
ConstructiveInterference
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Any environment that contains sound is a sound field
A sound field without any reflections is called a free field-almost impossible to obtain
Anechoic room-environment in which every attempt is made to reduce the amount of reflection using materials that absorb rather than reflect sound
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Free field acoustics Sound propagates to
all directions without diffraction, reflection or absorption
Spherical waves In principle, infinite,
empty space without reflections
In practice, anechoic chamber, with near 100% absorptive walls
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Free field microphone Intended to measure the
sound pressure as it existed before the microphone was introduced
Microphone pointed to source
Microphone tip causes an increase in sound pressure
Taken care of by internal acoustical damping to achieve flat frequency response
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Dynamic Characteristics ofGas Filled Systems
The straight tube can oscillate like an organ pipe with f = C/4L(2n-1)
The cavity vibrates like a helmholtz resonator with
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Pressure Instrument CalibrationStatic-- Performed against dead weight tester for >5 psi
For lower ranges, calibrate against a manometer.
Dynamic--Most pressure sensors will behave as a system of at least order 1. As such, it is necessary to determine parameters like or n and . As you know, this requires a step function input. How do you make pressure step suddenly?
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压力检测系统的动态校准首先需要解决标准动态压力信号源问题。
产生标准动态压力信号的装置有多种形式，根据其所提供的标准动态压力信号可分为两类：一类是稳态周期性压力信号源，如机械正弦压力发生器、凸轮控制喷嘴、电磁谐振器等；另一类是非稳态压力信号源，如激波管、闭式爆炸器、快速卸载阀及落锤液压动校装置等。
Pressure Instrument Dynamic Calibration
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激波管 (Shock tube) 是测定压力传感器频率响应特性的最常用的方便而简单的设备。目前，激波管已成为国际计量部门用来校准压力传感器动态性能的标准装置。
激波管法校准压力传感器动态特性系统包括激波管、气源、测量和记录部分。 A1 ， A2 ， A3 为压电式压力传感器，装在激波管的侧面，其中 A1 和 A2 特性相同，用于测量激波速度； Ax 和 Ay 为被校压力传感器； B1 ， B2 ， B3 为电荷放大器。
激波管校准系统图
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压力检测仪表的正确选择、安装和校准是保证其在生产过程中发挥应有作用及保证测量结果安全可靠的重要环节。
5.11 Pressure Instrument Application
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压力仪表的选择应本着经济合理的原则综合考虑仪表类型、测量范围和精度等方面。
仪表类型应根据被测介质情况、现场环境及生产过程对仪表的要求，如信号是否需要远传、控制、记录或报警等，结合各类压力仪表的特点选择。
仪表的量程要根据被测压力的大小及其在测量过程中变化的情况来选取。在测量稳定压力、脉动压力和高压时，最大工作压力应分别不超过仪表测量上限值的 2/3 、 1/2 和 3/5 ；被测压力的最小值不应低于仪表测量上限值的 1/3 ，以保证仪表的线性和测量结果的准确性。具体选择应在国家规定的标准系列中选取。
仪表的精度应根据工艺生产的要求在规定的精度等级中选择确定。所选精度等级应小于或至少等于工艺要求的仪表允许最大引用误差。
压力仪表的选择
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压力测量系统 (包括测取压力的取压口、传递压力的引压管路和测量仪表 ) 安装的正确与否直接影响测量结果的准确性。应根据具体被测介质、管路和环境条件，选取适当的取压口位置、正确安装引压管路和测量仪表。
Pressure Instrument Application (cont.)
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表：一些常用压力仪表的性能和用途 : 仪表型式 常用测量范围（ Pa ） 精度等级 用途与特点
液柱式压力计
U形管压力计
0 ～ 105 或压差、负压 高 基准器、标准器、工程测量仪表
单管压力计 0 ～ 105 或压差、负压 高 基准器、标准器、工程测量仪表
斜管压力计 0 ～ 2×103 或压差、负压
高 基准器、标准器、工程测量仪表
弹性压力
计
弹簧管压力计
0 ～ 109 较高 工程测量仪表、精密测量仪表
膜片式压力计
0 ～ 2×106 或压差、负压
一般 工程测量仪表、精密测量仪表
膜盒式压力计
0 ～ 4×104 或压差、负压
一般 工程测量仪表、精密测量仪表
波纹管压力计
0 ～ 4×156 或压差、负压
一般 工程测量仪表、精密测量仪表
Pressure Instrument Application (cont.)
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活塞式压力计 0 ～ 2.5×108 或负压 很高 基准器、标准器
电位计式压力传感器 0 ～ 6×107 一般 工程测量仪表
电容式压力传感器 0 ～ 107 或压差 较高 工程测量仪表
电感式压力传感器 0 ～ 6×107 较高 工程测量仪表
霍尔式压力传感器 0 ～ 6×107 一般 工程测量仪表
振频式压力传感器 0 ～ 107 或压差、负压 较高 工程测量仪表
应变式压力传感器 0 ～ 108 或压差、负压 较高 工程测量仪表
压电式压力传感器 0 ～ 107 或压差、负压 较高 工程测量仪表
Pressure Instrument Application (cont.)
表：一些常用压力仪表的性能和用途（续） :
