Sensor Installation Handbook Preface 3rd edition - Siemens ...

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Humidity 72 Measuring humidity Humidity is generally measured by use of hygroscopic materials. i.e. materials which respond to changes in humidity. Relative humidity (% r.h. = �) Principle: physical change in length Textile fibres (cotton, nylon etc.) expand as a function of relative humidity (producing a deflection). The deflection can be used as follows: To move a pointer (hygrometer), to operate a switch (hygrostat), to adjust a potentiometer, to adjust nozzles/baffles etc. (pneumatics), and to change inductivity (electrical variable). Humid air Insulating plate Pole % r.h. Cotton fibre Principle: change in electrical capacitance Capacitive elements respond to relative humi - dity by changing their electrical charge-storing capacity. Two electrical poles are connected to an insulating plate. In conjunction with the moist air, these poles store the electrical charge. The charge-storing capacity between the two poles depends on the ambient humidity.
Absolute humidity x g H 2O per kg of dry air Salts (such as lithium chloride, LiCl) have hygroscopic properties. Their electrical conductivity changes according to the amount of moisture absorbed by the salt. A temperature sensor is wrapped with in a woven fabric which holds the salt. The fabric is wound with two non-touching electrical wires connected to an alternating current. As the air humidity rises, the moisture content in the salt increases, so reducing the electrical resistance between the wires. The smaller the resistance, the higher the electrical current, so that the two wires act as an electric heating element. The heat produced causes the water to evaporate. As the moisture content is reduced, so the resistance increases again, and the heat output is reduced. This alternating process is repeated until the amount of water evaporated is equal to the amount of water absorbed. At this point, a state of equilibrium (constant temperature) is reached. This temperature (the transformation tempera - ture) is a measure of absolute humidity. Combined humidity sensors There are also combined humidity sensors on the market, which operate simultaneously with the temperature and humidity measurement principles described above. By measuring the temperature and relative humidity, these sensors can be used to calculate the dew point (°Tp) or the moisture content (g H 2O). Alternatively, the relative humidity (�) can be calculated on the basis of the temperature and the moisture content (g H 2O) or dew point (°Tp). Humidity 73
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Sensor Installation Handbook Prefac
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Sensor Installation Handbook Prefac
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General notes for all sensors Alway
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Always allow a sufficient length of
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Temperature Immersion sensors · Wa
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Water-resistant seal Chilled water
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If the active length (a) of the sen
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Sensors mounted without immersion p
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Temperature Probe sensors · Air En
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The ambient temperature at the devi
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Temperature Averaging sensors Allow
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Do not bend the capillary too tight
Page 29 and 30: When installing on "drawer"-type un
Page 31 and 32: Use spacing clips to maintain a 50
Page 33 and 34: Do not install on external walls. A
Page 35 and 36: Temperature Outdoor sensors The sys
Page 37 and 38: Seal gaps between cable/plastic tub
Page 39 and 40: Temperature Surface-mounted strap-o
Page 41 and 42: Temperature Wind sensors Install on
Page 43 and 44: Avoid shade (from trees or neighbou
Page 45 and 46: A test hole must be provided for ev
Page 47 and 48: Distance for adiabatic humidificati
Page 49 and 50: Humidity sensor Average humidity me
Page 51 and 52: Seal gaps between cable/plastic tub
Page 53 and 54: Where there is a risk of condensati
Page 55 and 56: Avoid using tips which protrude int
Page 57 and 58: Where more than one sensor is used,
Page 59 and 60: Pressure Liquids Pressure tapping p
Page 61 and 62: Pressure measurement in conjunction
Page 63 and 64: Flow monitoring Water Correct insta
Page 65 and 66: Avoid recesses (e.g. shelving) and
Page 67 and 68: Floats: These are used in open and
Page 69 and 70: Principles of operation General All
Page 71 and 72: Example of response characteristic
Page 73 and 74: Stepless devices Stepless devices (
Page 75 and 76: Bimetal rod and tube Material Const
Page 77 and 78: Electrical resistance sensors Metal
Page 79: Thermocouples These consist of two
Page 83 and 84: Electronic systems Piezoelectric el
Page 85 and 86: Measuring water velocity and flow L
Page 87 and 88: Room temperature measurement Room s
Page 89 and 90: Calibration of measuring sensors Th
Page 91 and 92: Practical tips Frost protection the
Page 93 and 94: Preventive frost protection thermos
Page 95 and 96: Pressure control in VAV systems The
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