ROTRONIC TECHNICAL NOTE Dew Point Temperature F requently Asked Questions
Q. What is dew point temperature? A. Dew point temperature (com - monly referred to as ‘dew point’ or ‘dewpoint’) is the temperature at which water vapor in any gas at constant pressure begins to con - dense into liquid water at the same rate as it evaporates. If the air tem - perature is equal to or below the dewpoint, water condensation will occur meaning water vapor will transform from the vapor state into the liquid state. Condensed water will appear as dew on a surface and Industrial air compressors require reliable dew point measurement. may also appear as a cloud or fog suspended in the air. fairly common to measure the dif - When the air temperature is ference between ambient tempera - equal to the dew point temperature, ture and the dew point temperature. the air is at the saturation point and The alarm point for condensation, the relative humidity is 100%. The for example, can be set at a differ - greater the difference between air ence of 10 degrees between ambi - temperature and dewpoint, the ent temperature and dewpoint. lower is the relative humidity. Dewpoint is also an ideal meas - urement for very dry conditions (<10% relative humidity). Dewpoint Q What is frost point? Desiccant dryers for compressed air can be equated to the absolute typically operate at -40° frost point. A. Frost point is the temperature amount of water vapor present as (usually below 0°C or 32°F) at which long as the air pressure remains water vapor condenses as frost Q. When should I choose constant. Most direct dewpoint instead of liquid water. A more sci - dew point as the measurement technologies are entific definition; frost point is the more suited to measuring very low temperature at which air is saturated parameter I measure? amounts of moisture compared to with respect to water vapor over a A. Dewpoint is an ideal measure - relative humidity measurement surface of ice. Frost point will be a ment when your goal is to avoid technology. continued different measurement than dew - condensation or to know when point below 0C. condensation is about to occur. It is
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Dewpoint is also ideal for com - pressed air systems because con - OPTICAL BALANCE densation is a concern as the air is compressed in the system. The dewpoint changes with pressure which can create condensation in the compressed air system.
Q. What are the pros and cons of measuring SAMPLE FLOW C dew point versus relative I F I T N humidity? E I C S 115 VAC R
A. Dewpoint measurement tech - E D nology is usually more expensive N U H
THERMOELECTRIC T and more accurate than relative - LED HEAT PUMP E humidity measurement technology REGULATION 20.01 POWER C R U
at low dewpoints. Dewpoint sensor O S 115 VAC reaction time is usually slower than DEW POINT TEMPERATURE E G A
relative humidity sensor reaction M I time. In some cases, dewpoint sen - Schematic of conventional chilled mirror sensor. sors can take several hours to settle where RH sensors may take only a few minutes. Some dewpoint sen - ambient temperature (exception will become relatively dryer as the sors are highly sensitive to contam - noted above). Only changes in difference between dew point and ination in dirty process air causing pressure or the actual amount of ambient temperature increases. frequent calibration and sensor water vapor present will affect the This is why systems that are main - maintenance. dewpoint. tained in a vacuum state are typi - cally very dry. Q. Does the dew point Q. How does pressure change when the system affect dew point Q. What are the most temperature changes? measurement? common types of technology for measuring A. No. Dewpoint will not change A. As the pressure of a system as the system temperature changes increases, the dew point tempera - dew point? below the saturation point. If the ture will rise and approach satura - A. The condensation hygrometer system temperature is at or below tion. Dewpoint is a common or chilled mirror is the most well the dewpoint temperature in a measure in compressed air sys - known technology for dew point. closed system, the dewpoint will tems because it is possible to reach The chilled mirror is a direct meas - change because water vapor is the saturation point and introduce urement of the dew point tempera - removed from the air. It is a com - liquid water into the compressed ture as compared to a calculated mon misconception that changes in air by simply increasing the pres - measurement achieved with a rela - temperature will affect the dew - sure. As the pressure of a system is tive humidity sensor. Chilled mirror point. It is important to remember decreased, the dew point tempera - that dewpoint is independent of the ture becomes lower and the gas continued
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technology is very precise, very Q. How do I know Q. Where can I buy a expensive and requires technical expertise as well as high mainte - which technology is best dewpoint instrument? nance of the instrument. for my application? A. Rotronic Instruments offers a Another common technology is A. Your choice of technology will line of dewpoint instruments to measure relative humidity and depend on several factors including including portable meters and fixed temperature to calculate the dew your objective for the measurement , transmitters. The Rotronic dew point temperature with accepted the amount of water vapor present, point probe uses a relative humidity scientific formulae. For this type of the ambient temperature and pres - sensor and a temperature sensor to technology to be accurate in dry sure, the level of contaminants calculate the dew point tempera - conditions, the sensor must be present in the gas and your budget. ture at a fixed pressure. An enhanced with additional technology It’s best to consult with a measure - advanced, proprietary algorithm or processing software. ment expert or engineer to deter - allows the Rotronic technology to Aluminum oxide is another mine the best technology for your measure in very dry conditions (as common technology which is very particular application. It is fairly easy low as -70°C dew point. <5ppm effective in environments with trace to choose the wrong technology in water vapor, <.02% RH). Visit amounts of water vapor and dew - critical applications causing signifi - www.rotronic-usa.com/dewpoint points as low as -100°C. However, cant problems in sensitive processes. to learn more about the Rotronic this technology is not reliable as dewpoint instruments. moisture increases or in dirty process gas streams.
Q. Isn’t dew point temperature the same as wet bulb temperature? A. This is a common misconcep - tion and is both true and false. Wet bulb temperature is equal to dew point temperature when the air is saturated. At any other point below saturation, the two measurements are very different. The measure - ments are also made using very different technologies.
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