The present invention relates to a new and improved apparatus for handling bulk material, such as powder or the like, having a weighing hopper or measuring chamber with a removable measuring receiver or scale for the material, wherein the weighing hopper or measuring chamber can be releasably connected with a pneumatic receiving or conveying system, the weighing hopper or measuring chamber being arranged vertically above the pneumatic receiving or conveying system. In its more particular aspects the invention relates to a weighing hopper or measuring chamber for bulk material and a device for handling such material, particularly powder, which comprises an inlet pipe for bulk material being fed to a supply area or a supply chamber, such as a tank or the like, for the bulk material and an outlet duct for feeding the bulk material to a weighing hopper or measuring chamber or scale, which weighing hopper or scale can be releasably connected with a pneumatic receiving or conveying system. At present there are utilized several methods and apparatuses for transferring bulk materials, such as powder, granular or pellet form, and other finely divided or comminuted material. Powder conveyors can be classified into five groups: bucket conveyors, vibration conveyors, pneumatic conveyors, chain conveyors, and linear motors. The weighing hopper or measuring chamber which is in a powder conveying system receives the bulk material, usually powder, is charged with the powder to be transported from the inlet pipe, and is subsequently closed. After the filling of the weighing hopper or measuring chamber it is connected with a pneumatic receiving or conveying system. The entire powder is conveyed through this pneumatic receiving or conveying system in order to transport the powder or bulk material, such as powder or the like, to the filling station of a container for filling, such as into the container of a powder or granulation press or a filter press. This type of apparatus for handling bulk material is already known from German published application DE-AS No. 12 77 523. However, this known apparatus has several drawbacks. There is no consideration of a possible leakage of powder. Also, the amount of powder which leaks or may be displaced during filling of the weighing hopper or measuring chamber into the container, is not detected, detected and compensated. With this known apparatus the weighing hopper or measuring chamber must be closed after filling with the powder in order to receive a container for a filling operation. Thus, the bulk material can only be handled for a short time after closing the weighing hopper or measuring chamber, which makes the handling apparatus unnecessarily expensive. Also, the filling is associated with a considerable amount of dust, so that there is the danger of injuring personnel. Finally, the charging of the weighing hopper or measuring chamber with the bulk material must be avoided during the conveying or delivery period so that leakage will not occur. There is further known from German patent DE-PS No. 23 23 232, a similar apparatus with a weighing hopper or measuring chamber for bulk material and a device for handling such material, which is constructed as a horizontal conveyor and is releasably connectable with a pneumatic receiving or conveying system. This known apparatus has the same drawbacks which have been described above. The powder has been found to undesirably discharge from the weighing hopper or measuring chamber through an outlet opening of the weighing hopper or measuring chamber, at least during the filling operation, which also results in considerable dust formation. It is further known from European published application No. EP-A 0 076 808 to provide the weighing hopper or measuring chamber with a closure, for instance a piston, to be operated by hand in order to open or close the weighing hopper or measuring chamber. However, this apparatus is also associated with the disadvantage that leakage cannot be detected or prevented. Another method for detecting the leakage in pneumatic bulk material conveyors is described in German published application DE-AS No. 11 37 882, wherein the output of a pressure measuring device at the outlet of the receiving or conveying system is compared with the inlet pressure, and the filling time of the receiving system is determined by a deviation from a predetermined reference value. However, with this known method, it is necessary to check the receiving system regularly for leakage after an appropriate time period, for instance after a certain number of cycles. Otherwise the pressure at the outlet of the receiving system will decrease, with the result that no leakage can be detected in a practical way. This type of control requires an adequate measuring response at the output pressure. However, because of possible disturbances, such as vibration, a significant decrease in pressure may occur with the above described known control apparatus without an effective decrease in output pressure. A method and an apparatus for compensating for leakage in pneumatic conveyors are known from U.S. Pat. No. 4,161,282. In this method a predetermined pressure is applied to the receiving system. This pressure is not constant but decreases with time. A drop in pressure is detected by a drop in pressure below a predetermined reference pressure, which also serves as the output pressure of the receiving system, so that there is no or a drop in output pressure, and leakage in the receiving system is considered to exist. In contrast thereto, it is an object of the invention to provide an apparatus for handling bulk material, which includes a weighing hopper or measuring chamber and a device for handling such material, which is connected with a pneumatic receiving or conveying system, and which permits accurate measurement or detection of leakage in the device for handling the bulk material, such as of leakage in the pneumatic receiving or conveying system, the detection and compensation of leakage and other changes in operating conditions. The foregoing object of the invention is achieved in the provision of a second, pneumatic receiving or conveying system, for instance an air filter system, which is connected with the outlet opening or discharge duct of the weighing hopper or measuring chamber and has a pressure measuring device for measuring the pressure at the outlet of the second pneumatic receiving or conveying system. It has been found that there is a substantial difference in the pressure between the outlet of the weighing hopper or measuring chamber and the outlet of the second pneumatic receiving or conveying system, and especially in those areas where the material is present in excess, for instance in the case of overfilling of the weighing hopper or measuring chamber. It has been found that the pressure change in the second pneumatic receiving or conveying system, which is associated with a corresponding change in output or discharge pressure, is very quickly detectable by a pressure measuring device at the outlet of the second pneumatic receiving or conveying system, and that this pressure can be used to evaluate a change in the output pressure, for instance a decrease in output pressure in order to detect leakage in the weighing hopper or measuring chamber or measuring chamber, and to transmit this information to the measuring device of the second pneumatic receiving or conveying system. In this manner a proper determination of the output pressure can be made. The determined output pressure value is utilized to determine the degree of filling of the weighing hopper or measuring chamber. This degree of filling is taken into consideration when correcting the output pressure of the measuring device of the second pneumatic receiving or conveying system. The difference between the output pressure and the reference pressure of the weighing hopper or measuring chamber can be used to measure the leakage in the weighing hopper or measuring chamber, and any changes in output pressure which might occur. A comparison of the reference value, the output value and the reference value, is carried out by a computer which determines the extent and rate of change and transmits this information to a display device or a display instrument of the apparatus. Also it has been found that with such an apparatus in which the weighing hopper or measuring chamber is charged with a pressure medium, such as pressurized air or the like, the output pressure which corresponds to a desired filling or charging degree of the weighing hopper or measuring chamber, can be determined from the pressure or the pressure and the volume of the weighing hopper or measuring chamber, even at a given temperature of the bulk material. A pressure equalization process of the type described above can be used to achieve such charging or filling of the weighing hopper or measuring chamber. This permits a desired filling degree of the weighing hopper or measuring chamber to be reliably achieved. Such filling degree can be maintained for an extended period of time. An apparatus with a weighing hopper or measuring chamber can have a third pneumatic receiving or conveying system which is connected with the outlet duct of the weighing hopper or measuring chamber. In the apparatus according to the present invention a third pneumatic receiving or conveying system is connected with the outlet duct of the weighing hopper or measuring chamber, so that in this manner an output pressure of the weighing hopper or measuring chamber can be monitored in the second pneumatic receiving or conveying system, and can be adjusted, for instance when it deviates from a reference value for charging. This makes it possible to compensate for leakage, and/or a decreased output pressure by increasing the output pressure, or by discharging the weighing hopper or measuring chamber, without a reduction in the quality of the handling of the material. The determination and display of the output pressure of the second pneumatic receiving or conveying system and/or the first pneumatic receiving or conveying system can be used for other purposes, such as for regulating the operation of the device for handling bulk material. If the apparatus includes several weighing hoppers or measuring chambers, which are controlled separately, they can have separate second pneumatic receiving or conveying systems. The invention makes it possible for leakage of bulk material, particularly powder or granulated material, to be detected and compensated