Design method for sampling unit for concentrates preparation device Field of the Invention The invention relates to a design method for the concentrates preparation device, especially to a design method for the sampling 5 unit for the concentrates preparation device using low pressure and low boiling point. Description of prior art The boiling point of water decreases with the pressure, so water can 10 be boiling with low temperature under low pressure, the boiling water changes liquid water into water vapor, mist water vapor condenses on the interface between cool and hot, a lot of mist collcets together to form waterdrop, and a lot of water drop collects together to form water flow. 15 So, there is provided with a device which can remove water from mixture using above principle, the device is a concentrates preparation device, which condenses hot water steam into liquid water, which is transformed to the container latter, thereby wanter content is reduced, so the concentrating function is achieved. 20 When the sampling unit takes samples of concentrates from the boiler, concentrates in the boiler can be taken out through the sampling bore of the sampling unit, the movable rod can take samples freely with the help of gas permeable passage for keeping 25 the pressure balance of ends of the movable rod. Inorde to incarese detecting accuracy, the handle is rotated fastly before sampling, referring to Fig.2, the remnant sample left in the sampling bore is thrown out due to the centrifugal force, then the 1/8 sampling bore is upwards with the help of the direction mark on the handle; the movable rod is drawn out to the withdraw position after the sampling bore is full of concentrates, detector detects the concentration of the sample in the sampling bore, or detector 5 detects the concentration of the sample in the glassware dropped from the sampling bore due to the gravity force after rotating the movable rod to make the sampling bore be downwards. The movable rod is pushed back to the insertion position after sampling process. Inorde to incarese the next detecting accuracy, the handle 10 is rotated fastly before sampling, the remnant sample left in the sampling bore is thrown out due to the centrifugal force. Fast rotating the handle before detecting is inevitably inconvenient, and it adds a process in detecting process, it is a time-consuming. 15 Summary of the present invention The object of the invention is to provide a design method for the sampling unit for the concentrates preparation device so as to solve the above problem. 20 Therefore, the invention provide a design method for the sampling unit for the concentrates preparation device, the sampling unit comprises providing a base body, a movable rod, a limiting pin, a sampling 25 chamber, a sampling bore, an axle sleeve and an isobaric chamber; arranging the sampling chamber to pass through the upper and lower surfaces of the base body; arranging the isobaric chamber to be a cylindrical blind bore; arranging for the isobaric chamber to pass through the front 30 surface of the base body and to pass through the front and back surfaces of the sampling chamber; arranging the axle sleeve to have an interference fitting with the isobaric chamber; 2/8 arranging for the movable rod to hermetically slidingly mate with the axle sleeve; arranging for the limiting pin fastened on the movable rod to mate with the front surface or the back surface of the sampling 5 chamber; arranging a gas permeable passage inside the base body; arranging for the head and tail ends of the gas permeable passage to respectively communicate with the isobaric chamber and the outside. 10 The method further includes providing the movable rod witha refreshing passage; arranging for the head end of the refreshing passage to extend to the bottom portion of the sampling bore of the movable rod; arranging for the tail end of the refreshing passage to extend 15 away from the handle and to extend to exterior cylindrical surface of the movable rod. Arranging the tail end of the refreshing passage inside the sampling chamber when the sampling bore is inside the sampling chamber, boiling concentrates in the sampling chamber flows into 20 the refreshing passage throuth the tail end of the refreshing passage and then flows into the sampling bore, or boiling concentrates in the sampling chamber flows into the refreshing passage throuth the sampling bore and then flows out from the tail end of the refreshing passage, therefore the boiling concentrates in the boiler can freely 25 flow into or out of sampling bore, thus the concentrates in the sampling bore is refreshing in real time, the density of the concentrates in the sampling bore is identical to the density of the concentrates in the boiler. Arranging for the tail end of the refreshing passage inside the 30 axle sleeve and near to the back surface of the axle sleeve when the sampling bore is inside the axle sleeve and near to the front surface of the axle sleeve, the sampling bore and the tail end of the refreshing passage are inside the axle sleeve, thus concentrates in the sampling bore or in the refreshing passage is sealed by the axle 3/8 sleeve and is isolated from the concentrates in the boiler. Arrang for the tail end of the refreshing passage inside the axle sleeve and near to the front surface of the axle sleeve when the sampling bore is outside the base body, because the tail end of the 5 refreshing passage is sealed by the axle sleeve, concentrates in the sampling bore do not flow out, it is limited in the sampling bore. Profitably, arranging for the tail end of the refreshing passage below the sampling bore, and arranging for the refreshing passage to be a 10 cylinder. Advantageous Effects Because concentrates in the sampling bore is refreshing in real time, it is no need to fast rotate the hand firstly when taking samples of 15 concentrates in the boiler. When the sampling bore is inside the axle sleeve and near to the front surface of the axle sleeve, the sampling bore and the tail end of the refreshing passage are inside the axle sleeve, so the sampling bore and the refreshing passage do not communicat the boiler with 20 the outside, the boiler is still in sealing condition. When the sampling bore is outside the base body, because the tail end of the refreshing passage is inside the axle sleeve, concentrates in the sampling bore is limited therein so as to be detected conveniently by detector. 25 Brief description of the drawing In the following, the invention will be described in greater detail by means of some embodiments with reference to the accompanying drawings, in which 30 Fig.1 is a 3d-drawing of the boiler with the sampling unit of the invention Fig.2 is a sectional 3d-drawing of the sampling unit; Fig.3 is a 3d-drawing of the sampling unit, wherein the movable rod 4/8 is drawn out and rotated; Fig.4 is a sectional front view of the sampling unit; Fig.5 is from Fig.4, wherein the movable rod is between the withdraw position and the insertion position; 5 Fig.6 is from Fig.4, wherein the movable rod is at withdraw position; Fig.7 is a detailed view of the part II in Fig.5; Fig.8 is a detailed view of the part I in Fig.4; Fig.9 is a detailed view of the part III in Fig.6; 1. the refreshing passage; 101. the head end; 102. the tail end; 10 14. the sampling unit; 1401. the base body; 1402. the limiting pin; 1403. the movable rod; 1404. the sampling bore; 1405. the sampling chamber; 1406. the handle; 1407. the shaft sleeve; 1408. the gas permeable passage; 1409. the isobaric chamber; 15 Detailed description of the preferred embodiment In order to distinguish between the front surface and the back surface, the surface near to the handle is called the front surface, the surface away from the handle is called the back surface. 20 Referrign to Fig.1, it is a boiler unit suitable for the concentrates preparation device of the invention. A sampling unit 14 is arranged near to the bottom portion of the sidewall of the boiler unit, referring to Fig.2, the sampling unit 14 is provided with a movable 25 rod 1403, a limiting pin 1402, a sampling chamber 1405, a sampling bore 1404, an axle sleeve 1407 and an isobaric chamber 1409. Inside the base body 1401 is provided with a gas permeable passage 1408, of which the head and tail ends respectively communicate 30 with the isobaric chamber 1409 and the outside atmosphere, the air permeable passage 1408 extends to the front surface of the base body 1401, air pressure of the isobaric chamber 1409 is equal to the outside air pressure with the help of the gas permeable passage 5/8 1408 so as to make the movable rod 1403 moves more flexibly. The movable rod 1403 near to the front surface of the base body 1401 is provided with a handle 1406, of which the diameter is greater than the interior diamenter of the shaft sleeve 1407, the sampling 5 chamber 1405 of the sampling unit 14 is inside the boiler, the handle 1406 is outside the boiler. Referring to Fig.2 and Fig.3, the sampling chamber 1405 passes through the upper and lower surfaces of the base body 1401, the 10 isobaric chamber 1409 is configured to be a cylindrical blind bore, the isobaric chamber 1409 passes through the front surface of the base body 1401 and passes through the front and back surfaces of the sampling chamber 1405. The axle sleeve 1407 has an interference fitting with the isobaric chamber 1409, the movable 15 rod 1403 hermetically slidingly mates with the axle sleeve 1407, the limiting pin 1402 is fastened on the movable rod 1403. The handle 1406 of the movable rod 1403 is provided with a triangular arrow mark, and the front surface of the base body 1401 20 is provided with a triangular arrow mark, the sampling bore 1404 arranged on the movabe rod 1403 is uptoward when the two triangular arrow marks are corresponding to each other. Referring to Fig.4 and Fig.8, the head end 101 of the refreshing 25 passage 1 extends to the bottom portion of the sampling bore 1404 of the movable rod 1403, the tail end 102 of the refreshing passage 1 extends away from the handle 1406 and extends to exterior cylindrical surface of the movable rod 1403. The movable rod 1403 is at the insertion position when the limiting pin 1402 mates with 30 the back surface of the sampling chamber 1405, at this point, the sampling bore 1404 is inside the sampling chamber 1405, the tail end 102 of the refreshing passage 1 is inside the sampling chamber 1405, boiling concentrates in the sampling chamber 1405 flows into the refreshing passage 1 throuth the tail end 102 of the refreshing 6/8 passage 1 and then flows into the sampling bore 1404, or boiling concentrates in the sampling chamber 1405 flows into the refreshing passage 1 throuth the sampling bore 1404 and then flows out from the tail end 102 of the refreshing passage 1, therefore the 5 boiling concentrates in the boiler can freely flow into or out of sampling bore 1404, thus the concentrates in the sampling bore 1404 is refreshing in real time, the density of the concentrates in the sampling bore 1404 is identical to the density of the concentrates in the boiler. 10 Referring to Fig.5 and Fig.7, the movable rod 1403 is drawn toward the withdraw position, when the sampling bore 1404 is inside the axle sleeve 1407 and near to the front surface of the axle sleeve 1407, the tail end 102 of the refreshing passage 1 is inside the axle sleeve 1407 and near to the back surface of the axle sleeve 1407, 15 that is the sampling bore 1404 and the tail end 102 of the refreshing passage 1 are inside the axle sleeve 1407, thus concentrates in the sampling bore 1404 or in the refreshing passage 1 is sealed by the axle sleeve 1407 and is isolated from the concentrates in the boiler, that is the sampling bore 1404 and the refreshing passage 1 do not 20 communicat the boiler with the outside, the boiler is still in sealing condition. Referring to Fig.6 and Fig.9, the movable rod 1403 moves outwards to the withdraw position, at this point, the sampling bore 1404 is 25 outside the base body 1401, the tail end 102 of the refreshing passage 1 is inside the axle sleeve 1407 and near to the front surface of the axle sleeve 1407, because the tail end 102 of the refreshing passage 1 is sealed by the axle sleeve 1407, concentrates in the sampling bore 1404 do not flow out, it is limited in the 30 sampling bore 1404, at this moment, the density of concentrates in the sampling bore 1404 can be detected conveniently by detector, or detector detects the concentration of the sample in the glassware dropped from the sampling bore 1404 due to the gravity force after rotating the movable rod to make the sampling bore 7/8 1404 be downwards. The tail end 102 of the refreshing passage 1 is located below the sampling bore 1404, such arrangement is good for concentrates in 5 the boiler flow into or out of the refreshing passage 1, boiling water drives concentrates moving from the bottom layer to the top layer in the boiler, concentrates moved to the top layer returns to the bottom layer. 10 The refreshing passage 1 is configured to be a cylinder, such arrangement is good for manufacturing, the refreshing passage 1 can be easily achieved by drilling process, and it also can be easily achieved by electric spark drilling process. 8/8