露點濕度量測技術資料及參考文件
/ Technical Articles and Documentation
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濕度量測概念文件
/ Humidity Overview Notes |
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濕度量測應用技術資料
/ Moisture Application Notes |
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AN INTRODUCTION TO THE THEORY AND OPERATION OF
CONDENSATION
CHILLED MIRROR HYGROMETERS
Chilled Mirror Hygrometers are used in Standards and Metrology labs as
well as in industrial applications where precise and repeatable humidity
measurement and control is required. The inherent accuracy and long term
stability provides many advantages over other types of humidity
measurement technologies. Since chilled mirrors fundamentally measure the
dew or frost point temperature directly by controlling a reflective
surface to equilibrium between dew/frost formation and evaporation and
precisely measuring the temperature of the mirror at this point, the
methodology has been validated by standards labs worldwide.
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INTERCHANGEABILITY: KEY TO CHOOSING RH
SENSORS
A few simple performance tests should be conducted before building any
humidity sensor into the final product.
Humidity measurement is always a difficult task for the design engineer.
Few production facilities are capable of the in-process calibration
required for some humidity sensors before they are built into the final
product. Unwary engineers have been known to damage their companies'
reputations by relying on unproven RH sensors and performance data
extrapolated from manufacturers' specifications.
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ABOUT SAMPLING SYSTEMS
For many applications, a sampling system is required to extract and/or
condition a gas sample prior to its contact with the chilled mirror
sensor. For example, if the gas temperature exceeds the maximum sensor
temperature rating, a sampling system with a 5 ft (1.5m) coil of copper or
stainless steel tubing upstream of the chilled mirror sensor will reduce
the gas sample temperature to ambient and protect the sensor. The use of a
sampling system often provides better, more reliable, long-term dew/frost
point measurements.
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DEW POINT vs. FROST POINT
A conventional chilled mirror hygrometer grows water droplets on its
mirror for dew points above 0°C, and ice crystals on its mirror for dew
points below 0°C. Therefore, we speak of frost points below 0°. At
exactly 0°, dew point and frost point are equal - they correspond to the
same absolute concentration of water vapor.
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CONTAMINATION COMPENSATION SCHEMES
Contaminants on the mirror of a chilled mirror hygrometer will affect its
performance, so most automatic chilled mirror hygrometers will have a
scheme to compensate for contaminants. When discussing contaminants, it is
essential to distinguish between soluble and nonsoluble ones.
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OLDER TYPES OF CHILLED MIRROR HYGROMETERS
The original chilled mirror hygrometer was known as the dew cup. A cup
with a polished exterior surface is filled with alcohol and ice until it
becomes cool enough to condense water vapor from the surrounding gas. A
mercury thermometer in the cup reads the temperature. This device is quite
crude since the cooling is not controlled, and the temperature measurement
is not very accurate. But most importantly, the interpretation of when the
dew point is crossed is subject to operator opinion. In addition, this
device is useful for one measurement only - it is not continuously
controlling.
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WHAT IS THE DEW POINT OF AIR?
This is not a simple question. To start to answer this, let us first go
over the basic definitions of dew point:
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DEW POINT MEASUREMENT IN METAL HEAT TREATING
APPLICATIONS
A number of metal heat treating applications require controlled
atmospheres with the proper water vapor concentration to optimize the heat
treating process. A review of the principle of operation and theory of dew
point measurement of chilled mirror hygrometers, aluminum oxide and
thin-film polymer sensors is presented as well as a discussion of the
contaminants encountered in measuring dew points in heat treating
applications as well as the sampling systems employed. The use of dew
point measurement instruments are discussed in relation to three process
used in metal treating, Bright Anealing, Brazing Carburizing and
Carbonitriding are reviewed, however many of the principles apply to heat
treating operations for metals and semiconductor materials.
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MOISTURE FIELD CALIBRATION CHECK
Practical Check for Trace and RH Moisture Instruments
Moisture is one of the most difficult quantities to measure since there is
nothing to see or put in a container. Tough measurements often inspire the
development of many different measurement techniques -- with varying
success. With so many methods for measuring moisture in use, converting a
customer to our products can be difficult simply because people resist
change. But sometimes there is a more fundamental problem: the old
instrument or method used for years reads a different value than our
instrument in the same environment. These differences are sometimes hard
to reconcile. A simple test for any moisture measuring device is often
needed to provide an unbiased reference comparing our reading with a field
standard.
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COOLING GAS FOR POWER GENERATORS
Trace Moisture Measurement in Hydrogen Cooled Electrical Generators
In the generation of electricity at power plants, the
turbine generators produce a large amount of heat as a byproduct. Cooling
of the generators allows more efficient generation of power and prolongs
the lifetime of the generators and associated equipment.
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OZONE GENERATION IN WATER TREATMENT
Trace Moisture Measurement in Supply Gases for Ozonizers
Handling and treatment of water for drinking, industrial
processes, or other uses, as well as the treatment of wastewater, requires
much concern and often large expenditures. The purification of fresh water
for human consumption requires particular attention to removal of various
naturally occurring bacteria and organic substances.
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SEMICONDUCTOR MANUFACTURING
Using the HygroTwin 2850 to Reduce Costs, Improve Quality
The semiconductor manufacturing and packaging industry must
measure trace moisture in many types of pure gases, including Ar, CF4, C2,
CIF5, C2F6, C3F8 (gaseous freons), H2, He, Kr, N2, N2O, Ne, O2, SF6, Xe,
and clean dry air.
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PAPER MANUFACTURING
Moisture Monitoring Saves Money in Paper Mills
Much of the cost required to turn a tree into a sheet of
paper is in the form of energy, rather than raw materials. After
completion of the pulping process, which reduces the wood to a usable
fiber, paper forming takes place within the paper machine. In this
process, the materials begin as 99 percent water and 1 percent pulp. This
ratio is nearly reversed by the end of the process, with only 10 percent
moisture content remaining in the dried paper.
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PETROCHEMICAL APPLICATIONS
Moisture Measurement Basics in the Petrochemical Industry
Many hygrometer manufacturers are capable of measuring in
petrochemical applications. The GE General Eastern gold/aluminum oxide
sensor is ideal because of its ability to offer continuous measurement
from direct installation in the process stream, without a sampling system
in many cases.
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THE CRYOGENIC PROCESS
Trace Moisture Measurement at an Air Separation Plant
The fundamental principle for the production of pure
nitrogen (N2), argon (Ar), oxygen (O2), and other industrial gases is that
the components of air condense from gas to liquid at different cryogenic
temperatures. These temperatures are extremely low and usually in the
range of -300° to -450°F.
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DRIED COMPRESSED AIR
Moisture Measurement in Compressed Air Applications
Compressed air is used in almost every industrial plant. It
is used for cooling components, actuating valves and other mechanical
components, powering air-driven tools, purging enclosures, cleaning and
blow-off, and pressure testing. Each use determines the quality of
filtration and dryness required of the air supply. Generally, compressed
air can be classified in terms of dryness and cleanliness: as Shop Air,
Plant Air, and Instrument Air. (not standard industry terms.)
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ETO STERILIZERS FOR MEDICAL EQUIPMENT
Monitoring Moisture in Ethylene Oxide Sterilization Procedures
Many medical instruments, including one-time use items for urological and
blood work, are sterilized after packaging by exposure to ethylene oxide (ETO)
gas. These medical devices are usually packaged in a sealed plastic or
polymer envelope and boxed in cardboard cartons for shipment. Palletized
cartons are then placed inside a humidity-controlled warehouse. Humidity
(60-70% RH) helps precondition the packaging, enhancing penetration of the
ETO gas.
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HUMIDITY CONTROL IN CLEAN ROOMS
Monitoring Relative Humidity in Moisture-Sensitive Environments
Many products manufactured and processed in a clean room environment are
moisture-sensitive. For this reason, clean room specifications often
include relative humidity (RH) control. These control points range from
35-65%RH for year-round operation. These RH levels generally are
maintained in a narrow band ±2 percent RH at temperatures below 70°F.
The effects of higher humidity levels in close tolerance environments can
be detrimental to product quality and production schedules.
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MOISTURE IN BLAST GAS FOR STEEL MAKING
Replacing aging sensors in difficult measurement
Blast gas in the steelmaking industry is used to oxidize (burn) the coke
to melt the “heat” of ore to produce the metal. This gas has to be
moist in order to carry heat from the exhaust gasses into the furnace and
to keep the process of the burn efficient. Steam has been added to control
this mix and thus the gas must be analyzed for moisture. This measurement
is critical to the steelmaking process and in the past Foxboro has
supplied their DewCel to make this measurement. Their cells have to be
resensitized every 1-3 months and this procedure is expensive and a
maintenance headache.
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TRACE MOISTURE IN PAD AIR
Monitoring Compressed Air by Another Name
GE General Eastern measures moisture in more compressed air applications
than in any other application. Compressed air is referred to by many
names, depending on the use of the air. In this discussion, we'll explore
a compressed air system called “pad air.”
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MEDICAL GAS
Moisture Measurement in Medical Compressed Air Systems
Few process control applications are as critical as those that monitor the
quality of systems that deliver vital, life-sustaining air to hospital
patients. Medical compressed air pipeline systems are essential in health
care institutions, so engineers must design these systems to deliver the
best quality air with the highest degree of safety for the patients.
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PRESSURE DEW POINT CONVERSION
Determining Comparable Readings Among Multiple Platforms
When installing a moisture analyzer, sometimes the reading is different
than what you expected. This difference is often due to a change in the
pressure at the measurement point where the new probe has been installed.
For example, a probe is installed at line pressure in a process where the
previous method of measurement was at atmospheric pressure. The problem
then becomes one of predicting the pressure effect on the probe and
achieving the desired result.
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NATURAL GAS
Measuring Moisture at Transfer Points in Natural Gas Streams
People expect to get what they pay for. Imagine that you bought 10 gallons
of gasoline for your car, and half of it turned out to be water.
Obviously, this would not be acceptable. When consumers and companies
purchase natural gas from suppliers, water in place of the natural gas
would be equally unacceptable.
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INCREASING SENSOR RESPONSE SPEED
Practical Tips for Moisture Analyzer Installations
There are many process parameters that impact the speed of response of any
moisture measurement. Improvements can be made to minimize this impact in
many cases but some issues will be with us forever and to understand them
can enhance compensation techniques for control systems. Listed below are
some of these parameters. They affect all moisture analyzers because they
are oriented to the behavior of the moisture in the process/system, not
that of a given sensor.
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FREEZE DRYING END POINT MONITORING
Adding Precision to a Difficult Measurement
Foods, pharmaceuticals, and even whole animals for museums are examples of
freeze-dried products. The largest freeze dryer market for process
moisture analyzers is the pharmaceutical industry. Its products are the
most costly and have the highest potential for loss if a process is not
totally under control. A specific formulation can be tested and approved
by the FDA, but marketing the product can be a problem if it has a short
shelf life. Freeze drying solves this problem in many products, such as
injectable drugs, allowing these medications to be more readily available.
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EMI/RFI/ESD PROTECTION
Making Sense of the EMI/RFI/ESD Alphabet Soup
You may have heard about these letters. No, they're not a military
specification or project code designation for some secret weapon system.
They stand for electromagnetic interference/radio frequency
interference/electrostatic discharge. But why do you need to know this
acronym? So you can check instrument specifications and avoid installation
problems.
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ELECTROSTATIC POWER COATING
Monitoring Moisture in a Coating System’s Compressed Air Supply
For decades, the electrostatic powder application process has been used to
finish metal parts. Major appliances such as washers, dryers, and
refrigerators have been powder coated for more than 25 years. The cost
savings from reduced waste (high transfer efficiency) and the durability
of the finish make powder coating the finish of choice. This finishing
system is also popular for automotive components and furniture.
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PETROLEUM REFINING AND PETROCHEMICAL
Trace Moisture In Liquid Hydrocarbons
Many products we use every day begin with crude oil as the raw material.
Refining and separating the various fractions of crude oil produces the
building blocks for synthetic fibers, plastics, agricultural chemicals,
and pharmaceutical products. Most of these chemical reactions take place
under tightly controlled conditions where moisture is a contaminant or
needs to be controlled in some way.
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SYNTHETIC FIBER PRODUCTION
Quench Air Moisture Measurement for Polymer Fiber Production
Modern chemistry has produced many new synthetic fibers for so many
products we use every day from the clothes we wear and the tires on the
family car to new military and police armor. The unique properties of each
fiber are not only the result of the chemical formulation but the process
of spinning them as well. For example, some fibers are used in long
strands as they are spun and are stored on long reels with diameters to
1.5 meters while others are crimped, chopped and baled. Another example
has to do with the environment surrounding the spinnerets and the air in
the column where the fiber cools as it falls. The temperature and the
humidity in this environment are often critical to the final properties
imparted to it.
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SEMICONDUCTOR MANUFACTURING
Trace Moisture and Oxygen in CVD Furnace Atmospheres
Chemical Vapor Deposition (CVD) epitaxial reactors are used in the making
of semiconductors. The wafers are processed through these reactors where a
stable solid film of single crystal doped silicon is deposited on them.
This process is the result of the decomposition of gaseous silicon
compounds in the presence of the heated wafer. During the process any
water vapor or oxygen in the reactor will contaminate the film and reduce
the yield.
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METAL TREATING
Trace Moisture and Oxygen in Heat Treating Atmospheres
Even with all the plastics and polymer advances chemists have given us,
metal products still dominate our lives with their usefulness, strength
and durability. Today’s metal products used in all kinds of objects
affecting our everyday life are better than those of just a few years ago.
This is due, in large part, to the improved methods of heat treating that
add strength and durability to those metal products.
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GLOVE BOXES
Trace Moisture and Oxygen Measurement in Glove Box Applications
Many manufacturing processes and laboratory procedures must be carried out
in a modified atmosphere. Producing a modified atmosphere simply means
maintaining a controlled environment with conditions that differ from
normal, or ambient conditions.
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MOISTURE IN SOLIDS
Moisture Measurement for Control when Drying Solid Materials in Solids
Many industrial drying processes call for moisture analysis as an
indication of product quality or process control. An example can be found
in the food industry where product specifications often include a
tolerance for moisture content. The moisture content of most breads,
chips, cookies, and other baked goods greatly affects the quality of the
product. In applications where materials are being loaded into railroad
cars or aboard cargo ships the moisture content can make a significant
difference in the density of the material. In these cases the moisture
measurement would give an indication of the weight of a given load or the
storage space required to hold it. In processes where solid materials are
mixed and blended the moisture content of the ingredients can effect the
consistency, flow characteristics, and quality of the end product.
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CONFECTIONERY MANUFACTURING
Moisture Measurement in Confectionery Manufacturing Processes
One of confectionery manufacturers' worst fears is high humidity in their
production facilities. Candy quality deteriorates when humid conditions
exist in three primary areas; coating, cooling, packaging and storage
areas. These areas require a means of dehumidification, especially if the
plant is located in a region of the country known for having high
humidity. Dehumidifiers come in three types; refrigerated, desiccant, or a
combination of the two. A refrigeration unit removes moisture by cooling
the air until moisture condenses. The air is then re-heated to lower the
relative humidity. A desiccant dehumidifier removes moisture by exposing
the air to desiccant material, which removes moisture from the air through
absorption. The desiccant dehumidification unit is more efficient at
removing moisture, is capable of producing drier air, and is more
expensive than a refrigeration unit.
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PROTECTIVE PROBE CAPS
Regular or Sintered Trace Probe Caps and How They Work
The trace moisture sensors are thin films of metal that form a capacitor
in direct contact with the stream to be measured. This capacitor is 1000
times thinner than a piece of copy paper and therefore can be damaged by
touching it. In order to insert this sensor into the process it must be
protected from any such damage with a protective cap.
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MOISTURE INSTRUMENT STANDARDS
We Stack up to NIST - the US Benchmark for Measurements
The US national standards for all common measurements are maintained at
the National Institute of Science And Technology, NIST, formerly National
Bureau of Standards, NBS. We have worked very closely with NIST and with
the vendors we use for equipment associated with all of our calibration
systems. These systems are maintained in a “traceable” condition per
ISO 9001 procedures for periodic checking of the calibration systems’
accuracy. The industry we serve demands this standardization.
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HIGH TEMPERATURE SAMPLING
Monitoring moisture when temperatures are too high
Today’s processes continue to demand more from moisture measuring
sensors. Higher pressures, temperatures, flow rates, and contamination are
just a few of the process conditions where moisture monitoring is desired.
In the past, moisture sensors could not approach these conditions so other
methods were used to control the process, since moisture measurement was
simply not possible. Sensors today have become more robust and the market
has revisited control methods to take advantage of these sensor advances.
Monitoring moisture in these demanding processes can enhance control
element performance and control energy usage, therefore optimizing profit.
Thus the market pressure is to provide instrument systems for these
conditions.
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LEAK DETECTION IN PURE SAMPLES
Looking for Leaks from Inside the Pipe
There are many processes that have been designed to operate under very
pure conditions. Heat- treating, semi-conductor manufacturing, and optical
coating are just a few process requiring high purity gases. Another
application is purging of a system to insure there are no leaks. All of
these applications are primarily concerned with identifying the
contamination of the system with moisture as the identifiable contaminant.
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