冷镜式露点仪具有操作原理简单,测量范围广,精度高等特点,广泛应用于各种行业的测量和控制领域中。

 

露点比较通俗易理解的定义是当将气体等压冷却降温时,气体容纳水的能力减弱,气体中的水分由气相变为液相从气体中凝析出来时的温度定义为露点温度。若将一个光洁的金属表面放到相对湿度低于100%的空气中并使之冷却,当温度降到某一数值时,靠近该表面的相对湿度达到100%,这时将有露在表面上形成。因为在这个温度下空气中的水汽达到了饱和,冷表面附着的水膜和空气中的水份处于动态平衡,也就是说,在单位时间内离开和返回到表面上的水分子数相同,这就是Regnault原理。该原理可以叙述为:当一定体积的湿空气在恒定的总压力下被均匀降温,直到空气中的水汽达到饱和状态,该状态叫做露点;在冷却的过程中,气体和水汽两者的分压力保持不变。如果空气的温度是Ta,露生成的温度为Td,则湿空气的相对湿度可以通过下式算出:

U=[(在露点温度Td时的饱和水气压) /(在原来温度Ta时的饱和水气压)] *100%

式中饱和水汽压的数值可以通过查表得到。在0℃以下,水汽达到饱和时,水在镜面上结冰,此时的温度又叫做霜点。

一、冷镜式露点仪的结构和原理


 

冷镜式露点仪的结构

冷镜式露点仪的结构

冷镜法的仪器具有一个冷镜传感器,由镜面,半导体致冷系统、温度测量系统、光学测量系统、信号控制系统等部分组成。当镜面温度高于露点时,镜面上是光滑的,反射光较强,当致冷器工作后镜面温度下降,当镜面温度低于露点温度时,镜面上即有冷凝物形成,此时反射光减弱,此时光电检测器的信号发生变化,再反馈控制致冷器的工作电流,使得镜面上维持一定厚度的冷凝物,由埋在镜面下面的四线制铂电阻测得此时的温度即为露点温度。该类仪器的优点是测量准确度较高,稳定性好,可以作为标准器使用,当然经费充足的单位也可以将该类仪器作为工作计量器具。

 

上面是对仪器测量原理做的一个大约介绍。另外,有了合适的仪器,还不能保证测得准确可靠的测量结果。露点测量结果受许多因素的影响,比如取样管路的材质,阀门接头的密封,阀门的安装位置,比如安装有测量仪器之前,从测量仪器出来后即通大气,此时为常压测量。若安装在测量仪器之后,气体先进测量仪器,再经过阀门调节流量后排走,这就是带压测量。同时还需注意对进测量仪器之前的气体的净化处理,若气体中有颗粒状污染物,需选用除去颗粒状污染物的过滤器,若需除去液体污染物,则需选用聚结型过滤器。

总而言之,由于气体中的水分含量范围较宽,从几万个ppm,到ppb水平,并且水汽分压不能象气体总压一样的直接测量,又不能象温度一样的做非接触测量,因此比较难测,并且由于需溯源至温度或压力,或其它基本量,因此其准确度很难做到象温度测量的mk级。加之大气中含有大量的水分,对测量结果会带来影响,因此露点(湿度)的测量比较困难。

二、露点测量中应该注意的若干问题


1:镜面污染对露点测量的影响

在露点测量中,镜面污染是一个突出的问题,其影响主要表现在两个方面;一是拉乌尔效应,二是改变镜面本底放射水平。拉乌尔效应是由水溶性物质造成的。如果被测气体中携带这种物质(一般是可溶性盐类)则镜面提前结露,使测量结果产生正偏差。若污染物是不溶于水的微粒,如灰尘等,则会增加本底的散射水平,从而使光电露点仪发生零点漂移。此外,一些沸点比水低的容易冷凝的物质(例如有机物)的蒸气,不言而喻将对露点的测量产生干扰。因此,无论任何一种类型的露点仪都应防止污染镜面。一般说来,工业流程气体分析污染的影响是比较严重的。但即使是在纯气的测量中镜面的污染亦会随时间增加而积累。

2:露点仪测量条件的选择

在露点仪的设计中要着重考虑直接影响结露过程热质交换的各种因素,这个原则同样适用于自动化程度不太高的露点仪器操作条件的选择。这里主要讨论镜面降温速度和样气流速问题。

被测气体的温度通常都是室温。因此当气流通过露点室时必然要影响体系的传热和传质过程。当其它条件固定时,加大流速将有利于气流和镜面之间的传质。特别是在进行低霜点测量时,流速应适当提高,以加快露层形成速度,但是流速不能太大,否则会造成过热问题。这对制冷功率比较小的热电制冷露点仪尤为明显。流速太大还会导致露点室压力降低而流速的改变又将影响体系的热平衡。所以在露点测量中选择适当的流速是必要的,流速的选择应视制冷方法和露点室的结构而定。一般的流速范围在0.4~0.7L·min-1之间。为了减小传热的影响,可考虑在被测气体进入露点室之前进行预冷处理。

在露点测量中镜面降温速度的控制是一个重要问题,对于自动光电露点仪是由设计决定的,而对于手控制冷量的露点仪则是操作中的问题。因为冷源的冷却点、测温点和镜面间的热传导有一个过程并存在一定的温度梯度。所以热惯性将影响结露(霜)的过程和速度,给测量结果带来误差。这种情况又随使用的测温元件不同而异,例如由于结构关系,铂电阻感温元件的测量点与镜面之间的温度梯度比较大,热传导速度也比较慢,从而使测温和结露不能同步进行。而且导致露层的厚度无法控制。这对目视检露来说将产生负误差。

另一个问题是降温速度太快可能造成“过冷”。我们知道,在一定条件下,水汽达到饱和状态时,液相仍然不出现,或者水在零度以下时仍不结冰,这种现象称为过饱和或“过冷”。对于结露 (或霜)过程来说,这种现象往往是由于被测气体和镜面非常干净,乃至缺少足够数量的凝结核心而引起的。Suomi在实验中发现,如果一个高度抛光的镜面并且其干净程度合乎化学要求,则露的形成温度要比真实的露点温度低几度。

过冷现象是短暂的,共时间长短和露点或霜点温度有关。这种现象可以通过显微镜观察出来。解决的办法之一是重复加热和冷却镜面的操作,直到这种现象消除为止。另一个解决办法是直接利用过冷水的水汽压数据。并且这样作恰恰与气象系统低于零度时的相对湿度定义相吻合。

由上可见,无论是从热惯性或过冷现象来考虑,降温速度都不宜太快,如果超过合理范围,则降温速度愈快,热惯性也愈大,露点测量的误差就愈大,也越容易出现过冷。最佳降温速度一般通过实验来确定。

三、冷镜式露点仪适用于哪些领域


Metrology. A properly designed and maintained CMH provides measurements with uncertainties several orders of magnitude less than those of other popular humidity sensors. The CMHs inherent precision, especially when equipped with a 4-wire platinum resistance thermometer to measure the mirror temperature and a medium-power microscope to monitor the mirror condition, makes it an ideal NIST-traceable transfer standard. (NIST provides a calibration service for field instruments maintained as transfer standards.)

Environmental Testing. The CMH is ideal for measuring absolute humidity in the environmental test laboratory. It is often used as a NIST-traceable standard to monitor the accuracy of other instruments such as the relative humidity sensors used to control environmental test chambers.

Furnace Atmospheres and Highly Contaminated Environments. The inert nature of the CMH renders it impervious to most contaminating substances and allows it to be repeatedly cleansed, resulting in very long-term sensor life without loss of calibration. These inert characteristics make it well suited for use in gas streams where high levels of contaminants in the sample gas would render less inert types of humidity sensors inoperable. For example, CMHs are widely used to monitor dew points of carbuerizing atmospheres employed in the heat treating of metals. In such applications, easy access to the mirror for cleaning is especially desirable

Moisture-Sensitive Manufacturing. The specialized packaging environments required in the manufacture of pharmaceuticals, films, coatings, and other products are often monitored with CMHs. Again, their long-term precision and NIST traceability make them the instruments of choice. Furthermore, because these processes are usually less sensitive to instrumentation costs, the higher price of CMHs is less a factor in selecting a humidity-monitoring scheme.

High-Temperature Gases and Dew Points. CMH sensors are frequently chosen for the measurement of dew point temperatures above ambient temperature. Air- and water-chilled sensors were built as far back as 1966 to monitor early Apollo hydrogen fuel cells operating at 250 °C and 700 psig. With todays thermoelectric cooling technology, dew points up to 100°C(and higher, assuming above-atmospheric pressures are present) are readily handled. In such applications, all surfaces in contact with the sample gas must be held at a temperature higher than the highest dew point anticipated; otherwise, condensation will occur on these surfaces and the measurement will be in error. Sample lines must also be heated.

In sensors intended for high-temperature dew point applications, it is common practice to use electrical cartridge heaters with thermostatic control to maintain the sensing cavity walls above the highest dew points anticipated. Solid-state optical components such as LEDs and detectors are therefore held at a temperature below their rated operating temperatures (typically 85°C) to prevent their deterioration and ultimate failure. This can be accomplished by thermally isolating these components from the heated sensing compartment.

四、冷镜式露点仪不适用于哪些领域


Some applications are unsuitable for CMH devices. Here are a few tips on where not to use them.

Gases That React with Water. Reactive gases such as chlorine, oxides of sulfur, and ammonia tend to gradually react with the water dew deposit on the mirror, forming an acid or caustic that causes incorrect (i.e., higher than normal) equilibrium dew point readings. Of course, some applications such as the measurement of condensables in stack gases require measurement of acid dew points. In general, measurements of humidity in water-reactive gases have long challenged the industrial process engineer and, short of laboratory analytical methods, they remain unsolved.

Gas Mixtures Where the Gas Vapor Dew Point Is Higher Than the Water Vapor Dew Point. A CMH measures absolute humidity in terms of dew point temperature. As the mirror cools to the dew point, it will tend to control on the first condensate that occurs. For example, in the case of methane/water vapor mixtures, especially at the well head, the methane dew point is often higher than that of the water vapor dew point, and the instrument will control on and read out the methane dew point rather than the water vapor dew point. In the manually operated "Dew Point Tester," the U.S. Bureau of Mines standard instrument for water vapor measurements in methane, the user can override this condition by simply ignoring the buildup of the methane deposit and continue chilling the mirror until the water vapor dew point is reached. This is not possible in the automatic CMH, and such applications should be avoided.

Compromises Required at Very Low Dew/Frost Point Measurements. The degree to which the mirror can be chilled, i.e., depressed below the heat sink temperature, and hence the lowest dew or frost point that can be measured, is determined by the capabilities of the cooling method. Thermoelectric (TE) devices (Peltier coolers) are available as staged arrays, but devices with more than three stages offer only marginal improvement in cooling capability and are increasingly fragile with a tendency to failure. The amount of mirror surface cooling that can be reliably achieved with a two-stage TE device is limited to ~65°Cbelow the heat sink temperature at ~25 °C cThat is, from room temperatures of 25°C, frost points measurements are limited to about –40 °C A finned heat sink design with forced-air cooling as shown in Figure 2 ensures that the heat sink does not rise above room temperature, thereby reducing the sensors depression capability.

A third stage of TE cooling can provide a few more degrees of cooling capability. Use of an integrated auxiliary cooling loop for chilled water or glycol to assist in removing heat from the hot side of the TE heat pump allows lower frost point temperatures to be measured. As a generalization, however, chilled mirror hygrometers become unwieldy and expensive for measuring frost points below about –70 °C Most industrial gases at low water vapor levels are typically "clean," i.e., they have minimal contaminants, and the inert advantages of the chilled mirror sensor are not so evident. Other types of humidity sensors are often the better choice for very low humidity measurements, especially if the gas is free of contaminants and where the precision offered by the CMH is not required.


2017年03月17日

色谱仪
六氟化硫常识

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