Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
  • G01N25/00—Investigating or analyzing materials by the use of thermal means
  • G01N25/56—Investigating or analyzing materials by the use of thermal means by investigating moisture content
  • G01N25/66—Investigating or analyzing materials by the use of thermal means by investigating moisture content by investigating dew-point
  • G01N25/68—Investigating or analyzing materials by the use of thermal means by investigating moisture content by investigating dew-point by varying the temperature of a condensing surface

Definitions

• the invention relates to a device and a method for determining the dew point of components of a gas mixture.
• dew point hygrometers in which the air is cooled until the water vapor condenses.
• the dew point temperature determined in this way corresponds to the saturation temperature of the air, from which the relative humidity can also be derived if the sensitive temperature of the air flow is known.
• a) dew point mirrors that fog when cooled below the dew point b) cooled electrode arrangements, which change the electrical resistance or the capacitance in case of condensate failure
• c) fog chambers in which fog can be observed when the temperature of the The measuring gas in the chamber drops below the dew point.
• the measuring gas is compressed without falling below the dew point and then relaxed.
• the moist air cools down considerably and almost homogeneously.
• Fog formation can be observed in an illuminated "fog chamber” if the dew point of the sample gas fell below as a result of the temperature drop due to the relaxation.
• the dew point temperature can be calculated from the wet gas temperature and the compression ratio.
• the measuring gas is cooled down to the limit of the thawing out, in which an attempt is made to adjust the cooling by repeatedly leveling the cooling temperature, at which just the formation of dew is recognizable.
• Devices are known in which a photocell arrangement always controls the cooling and / or heating of the dew point via a control amplifier in such a way that the dew limit is automatically - ie regulating - recorded.
• the task of automatic dew point adjustment poses some difficulties because the speed of dew formation depends very much on the respective dew point temperature.
• mirror dew point measuring devices are, in addition to the above-mentioned publication, also from DE-OS 19 57 322, DE-AS 12 99 437, DE-OS 26 4 * 0 663, DE-OS 29 45 445 and U.S. Patent 3,385,098 is known.
• Another disadvantage of cooled electrode assemblies is that, due to the poor thermal conductivity properties of these materials, it is very difficult to determine their exact surface temperature, i. H. to determine the exact surface temperature of the condensation surface. As a result, the dot temperature cannot be determined exactly.
• DE-OS 37 13 864 discloses an apparatus and a method for determining the dew point of components of a gas mixture, from which the invention is based in the preamble of claim 1.
• the known device has a moisture sensor, which can be both a dew point mirror and a cooled electrode arrangement.
• a condensate collector is provided which can be cooled or heated by means of a Peltier element.
• a temperature sensor is arranged in thermal contact with the condensate collector, by means of which the temperature of the condensate collector or the condensation surface is measured.
• Moisture sensor, condensate collector and temperature sensor are integrated in a single component. Therefore and since in this known system the precipitation of liquid is determined either by a dew point mirror or by cooled electrode arrangements, this arrangement also has the mentioned disadvantages of the respective measuring principles.
• the object of the present invention is to provide a device and a method for determining the dew point of components of a gas mixture which, at a low price, approximately achieve the measuring accuracy of mirror dew point measuring devices.
• the essential solution feature of the present invention is that the temperature-controlled condensate collector is arranged spatially separated from the moisture sensor. Liquid is separated in the cooled condensate collector when the dew point temperature is reached. This liquid is evaporated again while the condensate collector is on and leads into the vicinity of the condensate collector which the moisture sensor is arranged to an increased vapor content, which is clearly indicated by the moisture sensor.
• An important advantage of the invention is that it is no longer necessary to measure the temperature and the occurrence of condensate at the same time. Rather, according to the present invention, it is indirectly concluded, due to the occurrence of a significant change in moisture during the heating of the condensate collector, that the dew point temperature sought was passed through during the preceding cooling phase.
• the desired dew point temperature can thus be determined iteratively by changing the temperature differences traveled and thus by changing the final cooling temperature.
• the starting temperatures at the beginning of the cooling phase can also be changed for this purpose.
• At least the moisture sensor and the condensate collector in a measuring chamber.
• a further increase in the significance can be achieved in that the flow of the measurement gas is stopped at least during the heating phase. At the same time, this also reduces the amount of sample gas required to determine the dew point.
• a further possibility for increasing the significance of the display is to arrange the moisture sensor spatially directly above the condensate collector so that the condensate evaporating during the heating phase acts directly on the moisture sensor.
• Both a mirror or an electrode arrangement can be used as the moisture sensor.
• Other components that change greatly due to the increased vapor content in the electrical resistance can also be used.
• the control effort or the process control is considerably simplified in contrast to the known mirror dew point measuring devices.
• the technical control difficulty is to determine the temperature at the time of the first separation of condensate as precisely as possible. That is, the exact time of the formation of dew and the prevailing temperature at that time must be measured together.
• the measurement of the moisture or the measurement of the occurrence of condensate and the temperature measurement are, so to speak, uncoupled in terms of measurement technology. It only has to be determined whether or not there is a strong change in humidity in a comparatively large temperature and time interval. The associated dew point temperature is measured by iteration.
• the hygroscopicity of the material used for the condensate collector also advantageously plays only a subordinate role, since the amount of liquid present before hygroscopicity reaches the saturation temperature is so small that the resulting indication pulses differ significantly from those of a condensate Differentiate collection after falling below the dew point temperature or when the dew point temperature is reached.
• Another advantage of the present invention is that the moisture sensor itself does not have to be heated or cooled and only insignificant amounts of condensate are deposited on it. This eliminates a major source of pollution and corrosion.
• the corrosion can still be avoided by applying alternating current to the moisture sensor during the measurement, so that no galvanic changes are possible.
• Another advantage of the present invention is that the temperature measurement on the surface of the condensate collector is comparatively simple, since metals which are good heat conductors and corrosion-resistant can preferably be used here.
• a further advantage is that the contamination occurring due to the frequent condensation and subsequent re-evaporation of liquid in the condensate collector affects the function or the measuring accuracy of the present invention only insignificantly.
• a particularly simple and compact construction results if a pelletizing element is used to cool and heat the condensate collector.
• a compact construction also results if one side of a heat exchanger, in particular an evaporative heat exchanger, is designed as a condensate collector.
• the condensate collector can be heated, for example, by means of a resistance heater or by means of microwave radiation.
• the use of a sintered metal heat exchanger as a condensate collector is particularly useful in the event that the dew point of several components is to be determined in succession in a gas mixture. In this case, the component with the highest dew point must first be completely removed from the sample gas volume. Only when this has happened can the next lower dew point of a component be determined. That is, the condensate collector also works as a refrigeration dryer.
• the dew point measuring device has a condensate collector 1 in the form of an aluminum plate. It is of course also possible to use differently shaped condensate collectors with other materials which are good heat conductors.
• One side of the condensate collector 1 is connected in a highly heat-conducting manner to a device for regulating the temperature of the condensate collector 1 in the form of a Peltier element 2.
• Condensate can separate out on the other side or an active surface 3.
• On the active surface 3 of the condensate collector 1 is a Temperature sensor 4 is provided, with which the temperature of the condensate collector 1 or more precisely the temperature of the active surface 3 can be monitored and with which the sought dew point temperature is measured.
• the Peltier element 2 has electrical connections 6 and 7.
• a moisture sensor 8 in the form of an electrode arrangement is provided spatially above the condensate collector 1.
• the moisture sensor 8 consists of an insulator 10 on which the two poles 11 and 12 of the electrode arrangement are arranged in the form of
• Moisture sensor 8 and condensate collector 1 are arranged in a measuring chamber 14.
• the measuring chamber 14 has a feed line 16 and a discharge line 17.
• the discharge line 17 can be shut off by a valve 18.
• a conveying device 20 is shown schematically, with which the measuring gas is conveyed into the measuring chamber 14. Any devices such as filters, heat exchangers, volume flow monitors, etc., which are not shown in the drawing, can be used between the conveying device 20 and the measuring chamber 14 or between the measuring chamber 14 and the valve 18.
• the active surface 3 of the condensate collector 1 is at the initial temperature level T Q.
• the arbitrary output temperature T Q is usually the ambient temperature or the entry temperature of the measuring gas into the measuring chamber 14.
• the condensate collector 1 or its active surface 3 is cooled to a temperature T by the temperature difference dT.
• the temperature T is an alarm threshold value defined temperature or around the desired dew point temperature of the respective monitored dryer.
• the heating phase is initiated by reversing the polarity of the electrical connections 6 and 7 of the Peltier element 2.
• the duration of the cooling phase or the duration of the heating phase and the holding time at the respective temperature level are based on the respective application. During the cooling phase it must be ensured that sufficient condensate can separate out and during the heating phase it must be ensured that the condensate formed is completely evaporated again. Does the moisture sensor 8 show an indication pulse, for example, until the freely selectable heating end temperature, which is oriented towards the area of use, is reached. B. in the form of a sharp drop in resistance, the cooling temperature reached during the preceding cooling phase is the sought-after dew point temperature or a temperature below the dew point temperature.
• the resistance remains constant, for example, the dew point temperature was not reached.
• a higher temperature for example +1 K
• a lower temperature for example +1 K
• the exact parameters of the control strategy for example the temperature steps, cooling and heating speed, will be based on the respective application. If a permanent display is desired, the present dew point temperature is defined, for example, in such a way that the last temperature at which there was an indication corresponds to the respective dew point temperature.
• the permissible or suitable temporal length of the measuring interval will likewise depend on the respective application and can be influenced by the performance of the installed cooling device. Furthermore, the installation of several cooling surfaces on one sensor or the placement of several arrangements in a measuring chamber is possible, so that a quasi-continuous measurement is possible.
• the display of the moisture sensor 8, the signal from the temperature sensor 4 and the activation of the Peltier element 2 are combined in a control device (not shown in more detail) .
• the described dew point measuring device according to FIG. 1 can be used to achieve measuring accuracies of the mirror dew point measuring device.
• a plate made of epoxy resin (15 ⁇ 15 mm), for example, as an insulator 10 with a width of 300 ⁇ m, a thickness of 35 ⁇ m and a distance of 150 ⁇ m is arranged as the moisture sensor 8 Copper conductor tracks are used as poles 11 and 12.
• An aluminum plate with the dimensions 3 x 15 x 15 mm is used as the condensate collector.
• the distance between the moisture sensor 8 and the condensate collector 1 is 2 mm.
• the Peltier element 2 has a current consumption of 1.8 A at a maximum of 3 volts.
• the measuring chamber 14 has a volume of 20 cm 3 .
• optical sensors In addition to the conductor track sensor shown, optical sensors, ultrasound sensors and other components suitable for capacitive or resistance measurements can also be used.
• the sample gas chamber can be designed for any high pressure.
• the arrangement described as being preferred is very robust and, in contrast to the usual optical and capacitive ones, insensitive to contamination, calibration-free and including the peripheral devices from cheap industrial standard components, so that the realizable prices are of the order of 1 / 10 of the mirror dew point measuring devices are located.
• the basic arrangement described permits many variants, for example the cascade connection of several Peltier elements to achieve particularly low cooling temperatures, or high recooling temperatures or the use of other cooling systems.
• the senor also undergoes a temperature treatment in order, for example, to selectively determine the presence of several condensable vapors, or to place several sensors around one cooling surface or several cooling surfaces around one or more sensors.

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• Physics & Mathematics (AREA)
• Health & Medical Sciences (AREA)
• Life Sciences & Earth Sciences (AREA)
• Chemical & Material Sciences (AREA)
• Analytical Chemistry (AREA)
• Biochemistry (AREA)
• General Health & Medical Sciences (AREA)
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• Immunology (AREA)
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• Investigating Or Analyzing Materials Using Thermal Means (AREA)

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• 1990
  • 1990-07-26 DE DE19904023796 patent/DE4023796C1/de not_active Expired - Fee Related
• 1991
  • 1991-07-26 WO PCT/EP1991/001415 patent/WO1992001926A1/de not_active Ceased
  • 1991-07-26 JP JP3512755A patent/JPH05501764A/ja active Pending
  • 1991-07-26 CA CA 2066765 patent/CA2066765A1/en not_active Abandoned
  • 1991-07-26 EP EP19910913867 patent/EP0494291A1/de not_active Withdrawn

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