I have developed a mini system is to automatically maintain the desired temperature and air humidity in the oven, covered boxes, as well as in greenhouses or other spaces. The design of it is basically block-aggregation (Fig. 1); mounting quite on forces to everyone who at least once in your life going even without soldering, and using connectors and twisting some simple electro – or radio circuit.
Sensitive sensors of the system is familiar from school physics lessons two mercury in glass electric contact thermometer type TLC, mounted on a plastic stand (Fig. 2). To the flask with mercury one of the thermometers, called the WET (abbreviated W) and used to visually identify, install, and maintain the desired humidity of the ambient air, stretched ointment stocking dipped in a small container of water. The other thermometer is called the DRY (for short ST) as a mercury bulb, which does not pull. It serves for direct detection, installation, and maintenance of the desired temperature. The lower part of both thermometers protected by a grid.
There is no “maintenance” DRY thermometer is not required. The target temperature set regular magnetic device, and does all the basic automation, which is the extension of bodies (in this case mercury) during heating and compression during cooling. Being inherently electrically conductive, the mercury inside a glass bulb closes or opens the circuit, turning the heater and thereby maintaining the ambient temperature at the specified level.
The WET thermometer is a little “capricious”: for proper operation it is necessary that the stocking on the bulb with the mercury never dries out. I have to watch that and, as necessary, pour into a small container of water. In obedience to the immutable laws of physics, it evaporates from the surface of the stocking in accordance with the relative humidity of the air, thereby cooling the mercury in the bulb WATTS.
So, the difference between the readings of DRY and WET thermometer, you can judge humidity. Furthermore, to control the state of the environment in the case of deviations of moisture content from the desired “norm” to enable or disable electroclinical. Automatic execution of such responsibilities provides electrical contact mounted magnetic device of the WET thermometer in accordance with these psychrometric tables.
Control subunit for each of your thermometer. However, they are collected on the same wiring diagram four valves Д247, D226 diode, thyristor КУ202, Д814Д Zener diode, two transistors: МП41 and КТ117Б. Built-in modern appliances, such devices work reliably with different sensors and is already well familiar to a wide readership, so that now divert attention to the peculiarities of their functioning is hardly appropriate.
The other algorithm of the system itself. To automatically maintain the required air humidity is necessary first of all to look at… the DRY thermometer and by defining a scale for the steady-state temperature (TW), remember its significance. Then must do the same, but on TUES.
After securing the source data, you go to a psychrometric table. Finding her the value of TW , and then tвт,
should be at the intersection of horizontal and vertical lines to determine the established value of the humidity.
In particular, when TW = 20 °C and tвт = 15 °C, then ψ = 59%. Well, if TW = 25 °C and tвт = 21 °C, then ψ = 70%.
If the humidity is less than necessary according to the specifications, focusing on actual temperature TW, which shows DRY bulb thermometer, we find the horizontal table, the desired humidity, and to determine the (vertical!), what values should be consistent with that on the scale of WATTS. Mounting the magnetic device of mobile contact of the WET thermometer to the scale division equal to the desired value tвт, ask the most necessary humidity, which our obedient system and will work by connecting (disconnecting) to (from) the household AC, humidifier, and if necessary a heater.
For example, if TW = 25 °C and need to obtain ψ = 50%, tвт = 18 °C. Well, since TW = 29 °C and you want to achieve, say, 66% humidity, then (in accordance with a psychrometric table) must put the mobile contact of the WET thermometer on the scale division corresponding to a tвт temperature = 24 °C.
By the way, the table is given in a somewhat abbreviated form. It is possible to detect the humidity with only a 10 percent correction, based on the use of the most common thermometers, the pitch of the scale marks which is equal to 1 °C. For more accurate temperature setting and humidity of the air is needed W I ST, with a smaller error, then use advanced psychrometric tables will be justified. For example, if a temperature scale with increments of graduations of 0.1 °C can increase the setting accuracy humidity 1%!
A. NORVATOV, engineer, Volsk, Saratov region.