Air-flow Meter for Testing Milking Machines

New Zealand Journal of Agriculture, Volume 85, Issue 3, 15 September 1952, Page 213

Air-flow Meter for Testing Milking Machines

THOUGH in the milking machine the "vacuum" is the fundamental • working force, the relation between vacuum and air flow in the various parts of the machine is only very poorly understood. The vacuum in the machine is in fact only a partial vacuum, so that the machine is filled with air at a pressure below normal atmospheric pressure (actually half of atmospheric pressure at Isin. mercury vacuum). This means that the vacuum levels in the machine, which determine its whole action, are completely decided by the very rapid flow of this air in the machine. Air is continuously flowing from air inlets on the machine through the pipes to the vacuum pump, which forces it out into the atmosphere again. It is not surprising then .that the major factors in machine operation can be determined from a thorough understanding of the air flow in the machine, and in this article D. S. M. Phillips, Physicist, Department of Agriculture Animal Research Station, Ruakura, describes a meter for measuring air flow.

THE direct measurement of air flow provides much more useful information than the measurement of vacuum level alone. Much field investigation work in the last few years has been devoted to the mapping of air flow in typical machines in various states of repair and under all conditions. From these figures and other detailed studies have come accurate tables of data for use in the testing of machines and improved and simplified methods of measurement and testing. This work has now reached the stage where it can be applied in the field. Importance of "Reserve Air" The control of vacuum in the machine is extremely important in obtaining efficient milking and depends almost entirely on the provision of a sufficient quantity of “reserve air” and an efficient relief valve to handle it. This reserve air is provided by the vacuum pump, which must run at a speed which .allows it to pump more air than the machine normally uses, thus making a reserve available to cope with additional air inlets. The relief valve is very important, as it must regulate the admission of this reserve air m such a way that the machine vacuum remains constant. The accurate measurement of reserve air is fundamental in obtaining efficient milking, and is possible. only by the use of some device for measuring air flow.

Causes of Low Reserve Air The measurement of reserve air alone is not sufficient; the cause of the deficiency must also be located so that appropriate adjustments can be made. In this respect the air-flow meter is of the utmost value, as it enables the serviceman to check rapidly the airflow levels in all parts of the machine under working conditions. Factors of importance include: — 1. Vacuum pump efficiency: The efficiency of a vacuum pump can be. determined accurately under working conditions only with an air-flow meter. 2. Milking machine tests: The leaks in the various parts of the machine are frequently very difficult to locate by normal means and here also an airflow meter' provides a rapid and accurate test. 3. Milking machine air consumption: The air-consumption figures for the various parts of the machine provide much essential • information on operation and indicate immediately the presence of leaks, which cause low reserve. 4. Machine design faults, which cause loss of vacuum owing to the flow of air through pipes and various

obstructions. This is a very important factor in determining vacuum pump efficiency and correct operation of releasers and pulsators. None of these measurements, .of fundamental importance to machine operation, can be made unless some form of air-flow measurement is possible. Correct Air-flow Figures Detailed air-flow tables are now available from research work carried out at Ruakura Animal Research Station in the last few years. Airconsumption figures for various types of machines under different conditions have been provided by measurements made on milking machines in the field. The determination of the level of reserve air in relation to machine air consumption and machine construction was a somewhat more difficult problem and remained unsolved until recently. Its solution lay in a detailed investigation, made with a recording vacuum gauge, of a . typical milking machine during milking. In this particular investigation the pump speed was adjusted so that any required reserveair level could quickly be obtained by admitting air to the machine through an air-flow meter; at the same time recordings of vacuum were made at a number of places on the machine,

including at the cups, while milking progressed. The results showed quite clearly that the importance of the reserve lay in the recovery time of the machine after an air admission, such as when cups were changed. Reserveair and vacuum tank volumes had relatively little effect on the initial fall in vacuum to about lOin. of mercury, but they did have a very marked effect on the recovery of vacuum once the air admission was stopped. It was also found that if the recovery time was about 3 seconds, the fall in vacuum became barely noticeable at the cups owing to the cushioning effect of the milk in the dropper. This figure was used as a basis for the mathematical calculation of “correct” reserve-air figures for various sizes of milking plant, the air-consumption figures obtained from field tests being used. To be correct these figures should be calculated for any particular machine in question, but the calculation is far too long for this method to be useful, and tables for general types of machines can provide sufficiently accurate results. Such tables, for a common .'type . of milking plant, are given on page 215. Some latitude in the figures is given, as it is not always possible to set pump speeds exactly, and precise figures are quite unnecessary, especially when a general table is used. (Accuracy in air-flow measurement is important, however, in

determining leaks and vacuum pump efficiency.) The tables give preliminary figures and are for standard types of machines with 4-gallon vacuum tanks, l/32in. air admission, and soft inflation cups. TABLE I—NOT MILKING (Pulsators on, Relief Valve off) (Measurements in cubic feet of air per minute where 1 cub. ft. 61 gallons) Size of Reserve Pump Machine plant air capacity consumption Single: 2- .. 9-10 13-15 4-5 3- .. 10-11 16-18 5-7 4- .. 11-12 18-20 6-8 5- .. 14-15 21-23 7-9 6- .. 15-16 24-26 8-10 7- . . ' 17-18 27-29 9-11 8- .. 18-19 29-31 11-13 9- .. 19-20 32-34 12-14. 10-cow . . . 20-22 35-37 13-15 Doubled up: 2- .. 11-12 18-20 5-7 3- .. 13-14 ■ 23-25 8-10 _4-cow . . 15-16 27-29 U"l3 5- . . 18-19 32-34 13-15 6- . . 20-21 36-38 16-18 7- .. 22-23 41-43 18-20 8- . . 24-26 45-47 20-22 < TABLE 2— MILKING (Measurements in cubic feet of air per minute where 1 cub. ft. 61 gallons) Size of Machine Reserve plant consumption air Single: 2-cow . ■ • • 6-8 7-9 . 3-cow .. ■ • 8-10 7-9 4- .. • • 10-12 8-10 5- . . . • 12-14 9-11 6- ~ • • 14-16 9-11 7- .. ' 16-18 10-12 , 3-cow .. - ■ 19-21 10-12 9-cow . . . • 21-23 . 10-13 10- .. ■ ■ 23-25 11-13 Doubled up: 2- .. •• 10-12 7-9 3- ~ .. 15-17 8-10 4- . . • - 19-21 8-10 5- . . • • 23-25 8-11 6- ... ■ • 28-30 8-11 . 7-cow ' ... • • 32-34 9-12 8-cow . . . . 37-39 9-12 Vacuum Pump Capacities The pumping capacities of a number of types of vacuum pumps are given in Table 3. These have been taken from the highest figures in field tests and represent the figure to be expected if a pump is in good order. These tables will no doubt. be • superseded when air-flow measurement is more general and further figures come to hand. TABLE 3— PUMP TABLES Standard Masport Masport Masport (Ridd) Mac Ewan Speed Mac Ewan Senior “Super” “Super” 150 . . 11 16 22 200 ..14 19 .21 29 230 .. 16 19.5 24 33 250 .. 18 20 26 36 270 . . 19 22 28 39 280 . . 20 23 29 40 290 . . 21 24 30 42 300 .. 22 25 31 . 43 320 . . 25 26 33 46 Measurement of Air Flow Neither the 'tables given nor an air-flow meter are of any value alone; reliable figures will be useful only if applied with the use of an accurate measuring device. To this end a thorough study of air-flow measurement techniques has been carried on at Ruakura over the

last year or two. In the course of this work many types of instruments have been developed and tried out. As the air-flow work progressed the requirements of an air-flow meter suitable for servicing could be stated fairly definitely as follows: — 1. Range: The meter should be capable of measuring pump capacities of up to 50 cub. ft. per minute. It should be capable of distinguishing between air flows differing by as little as 4 cub. ft. or less per minute, making it possible to measure the air consumption of a single bail pulsator. ' 2. Accuracy: Over-all accuracy should be about 3 to 4 per cent, to give a reliable check on vacuum pump performance. 3. Simplicity: The meter must be as simple as possible so that its use is easily understood and so that incorrect use by unskilled persons is not readily possible. 4. Robustness: It must be mechanically strong and capable of withstanding the type of handling to be. expected from everyday use by machine fitters under all conditions. ■■ 5. Calibration: The meter must , retain its calibration over long periods and

under adverse conditions, as frequent checks on calibration will be impossible except by cqmparison with a similar instrument. 6. Portability: It must be readily portable and easily set up and attached to the milking machine, in spite of difficulties in finding a suitable point for connection. In addition, the meter should not be of such a type that the position of attachment will affect its accuracy. 7. Non-continuous flow: The meter must be capable of giving a reliable reading of mean, air flow when a pulsating air flow is passing through it, as is generally the case in machine testing. ' 8. Production: If the meter is to be used in commercial testing and servicing, it must be produced in quantities and be reasonably low in cost. . Therefore, it must be suitable for quantity production and must not involve too much individual work in calibration and adjustment. Multi-orifice Type Meter The multi-orifice type air-flow meter was originally developed at Ruakura to fill the requirements given. In principle it consists of a metal, plate through which . are drilled a number of small specially shaped orifices, arranged in a semi-circle so that they can be opened or closed one at a time by rotation of a close-fitting sliding cover. The hole diameters and the entrance shape of these orifices are adjusted by reaming so that each will admit exactly J • cub. ft. of air per minute when the vacuum across them is 15in. of mercury. Measurement of pump capacity or reserve air is done by connecting the meter to the pump or machine respectively and opening the orifices until the vacuum level comes to 15in. of mercury; the number of orifices open then gives the air flow. These orifices can be made so that each will admit exactly 1 cub. ft. of air per minute and thus the number open gives the air flow in cubic feet per minute. The illustration on this page shows air-flow measurement, including measurement of the reserve air, and indicates the

manner in which the factors pump capacity, machine air consumption, and reserve air are related. The meter is seen m use in the illustration at the head of this article. . Some of these meters have been used in the early field trials conducted by two officers of the Dairy Division of the Department of Agriculture and in field work by . Ruakura research workers, so that many will be familiar with their appearance and operation. The meter was developed by the author more than 2 years ago and its possibilities were recognised. However, preliminary work suggested that it would be difficult to prevent such small orifices from wearing or becoming contaminated and that the fine work involved, with the need for special air-flow metering apparatus in

making and testing the orifices, would make the manufacture of 50 orifices for each meter too expensive. Therefore, another type of meter was developed as a possible commercial type, but it was found unsuitable for field work, though its manufacture would have been relatively simple. In the meantime, several firms had become interested in air-flow meters and several prototype models of various types were submitted for test. None of them measured up to requirements. However, further work at Ruakura showed that commercial manufacture of the Ruakura-type multi-orifice meter was possible at reasonable cost, provided the necessary precision measuring equipment was available and a method of ensuring protection of the small orifices was

perfected. Such a meter is now in commercial production and is being used by Dairy Division officers.

The essentials and construction of the Ruakura-type multi-orifice meter are shown in the illustration on this page. The meter consists of a hollow body (A) with a tube (B) for connection to a machine. The front orifice plate . (C) carries a series of small orifices (D) which can be opened or closed by rotation of the cover (E), turned by the knob (F) engaging pin (M). The sliding cover plate (E) is held firmly on the surface by the vacuum and also by the spring (G). The air-flow level (that is, the number of holes open) is indicated 2,9 the scale (J) by the pointer (K). The scale also acts as a protecting cover for the meter and has a number of holes round it to let in the am. h , e cover (E) can be made from a flexible plastic sheet or other suitable material which requires no lubrication and seats well on the front surface. The accuracy of the meter and the manner in which the orifices retain their calibration are determined entirely by the precision and care taken m their manufacture and by the accuracy of the apparatus used for checking. Normal drilled holes are useless, as small irregularities could cause errors of up to 20 per cent, or more; therefore the design of an orifice which would permit accurate reproduction and calibration was the first to successful manufacture. With the use of these methods accuracies of better than 1 per cent, are quite possible at reasonable cost. Some appreciation of the importance of this factor can be gained from'the fact that an error . of l/10,000in. in orifice diameter can alter the air-flow level by 1 per cent. Inaccuracy in the vacuum gauge affects the air-flow reading to a small extent when “internal air flow” is measured but it has no effect on tree-air flow into the orifices as tongas the true vacuum level is above 13in of mercury. The important property of, the small orifice is that free-ai. entry is limited by the velocity of air flow and remains constant as long as the vacuum is above 13in. of mercury The error in internal flow is approximately 24 per cent, for a gauge error -2-m. of mercury. For this reason, where conventional types of vacuum gauges are used some method of checking the gauge accuracy periodically should be employed. . . As the value of any air-flow meter be in its accuracy, it is most important that farmers should have some guarantee that the instrument used in testing milking plants is accurate. This is even more important in the multiorifice type meter, in which the accuracy depends on design and precision workmanship. Accordingly, arrangements 1 have been made for Ruakura Animal Research Station to test and seal all meters submitted by manufacturers. Meters once sealed could not be altered without the seal being broken, The seal and stamp used by Ruakura. f 1 1?;. in diameter and bearing the initials of the station, are shown in the illustration on page 215. This z seal and a certificate which accompanies any tested meter are a = guarantee of accuracy, and readings taken with such a meter can be relied on. Farmers should always see that the instrument used in testing is sealed m this way.

Care Necessary in Carrying Firearms

U SE S °L f • f irear ™ S should always make sure that the weapons are held or carried . with the muzzles inclined upward. In the illustration on the left above-the man in front is holding his gun with the triggers .downward. If the gun accidentally discharged the man behind could easily be injured, especially as neither man . is aware of the danger. . If the gun was held with the triggers' upward as shown in the .illustration on the right above, the danger would be eliminated. . , , Always make a habit of holding firearms with the muzzles pointed upward and-be especially careful when shooting with a party. , —G. G. KELLY