CH663407A5 - GAS FLOW METER. - Google Patents

Description

663 407

2

Claims (6)

PATENT CLAIMS 1. Gas flow rate measuring device consisting of a piston (3) which can be displaced in a cylinder (2) with engine power (4) and has a filling valve (11) and a discharge line (14) with a metering valve (12), characterized in that that the cylinder (2) with the piston (3) is built into a container (1), whereby the cylinder (2) can be filled from the container (1) via the filling valve (11) and via a pressure difference measuring device (10 ) with an electrical output signal, cancels the differential pressure that occurs when gas is withdrawn from the metering valve (12) with the filling valve (11) closed with the aid of a switching amplifier (9) by motor-driven displacement of the piston (3). 2. Gas flow meter according to claim 1, characterized in that the piston rod actuates a counter which indicates the change in volume of the cylinder (2). 3. Gas flow meter according to claims 1 and 2, characterized in that the container (1) can be evacuated. 4. Gas flow meter according to claims 1-3, characterized in that a constant gas pressure can be set in the container (1) via a gas inlet (5) with the aid of a metering valve (6), a pressure regulator (7) and a vacuum pump (8). . 5. Gas flow meter according to claims 1-4, characterized in that the piston rod of the piston (3) exits through an intermediate chamber (13) into the atmospheric environment and the gas inlet into the container (1) through this intermediate chamber via the metering valve ( 6) with the piston rod slidably sealed in both walls of the intermediate chamber. 6. Gas flow meter according to claims 1-5, characterized in that the pressure in the intermediate chamber (13) is greater than or equal to the ambient pressure. The invention relates to a device for measuring gas flows in vacuum technology according to the preamble of claim 1. Gas flows of the order of magnitude > 1 mbar • 1/s are measured with variable area flow meters. For smaller gas flows, thermoelectric flow meters are often used today, which determine the gas flow from the amount of heat transported by the gas flow. These devices are limited in their measuring range by the smallest detectable heat flow. The smallest measurable gas flows are 1 • 10-2 mbar 1/s. Furthermore, measuring devices are known which have a labyrinth of very narrow channels in which molecular flow prevails below a certain pressure. By precisely measuring the difference in pressure between the inlet and outlet of these channels, the gas flow rate can be calculated if the flow resistance is known. The lower measuring limit of these devices is 2 • 10-4 mbar 1/s. All these devices have the disadvantage that their display depends on the type of gas and that they have to be calibrated. If the measuring devices are dirty, significant errors can occur that the user cannot easily identify. The present invention has for its object to provide a gas flow meter that Measures gas flow from the elementary values of pressure, volume change and time, so no calibration is required. In addition, even smaller gas flows than those specified above should be able to be measured. The object is achieved by an arrangement as described in claim 1. The gas flow rate Q is determined by measuring the pressure P in container 1, which is equal to the pressure in cylinder 2, and the volume change A V in cylinder 2, which is caused by displacement of the piston during the time À t with valve 11 closed using the formula The advantage of the invention over the currently known devices is the flow rate measurement with the aid of elementary measurable variables such as pressure, time, volume, with the volume display advantageously taking place according to claim 2 . Any leaks between the piston and cylinder are irrelevant, since there is no pressure difference > 0.3 mbar between the interior and exterior of the piston. By choosing a cylinder capacity of 0.0011, a lower measurement limit of 1 • 10~3 mbar 1/s can be achieved when the cylinder volume is emptied in 1000 s at atmospheric pressure. The lower measurement limit drops to 5 • 10 -5 mbar • 1/s when the pressure 15 is reduced to 50 mbar according to claim 4 . In this context, an embodiment of the invention according to claim 5 and an operation according to claim 6 are advantageous. In this way, contamination of the measuring gas by the ambient air is prevented if there are any leaks in the piston rod seal. Being independent of the type of gas, the device is very well suited for calibrating other flow meters. The function of the gas flow meter is to be explained in more detail with the help of the following example. First, the entire apparatus is evacuated with the metering valve 12 closed and the metering valve 6 and filling valve 11 open. Then the piston 3 is brought into the lower extreme position. The gas to be measured is then allowed to flow into the container 1 through the gas inlet 5 and the intermediate chamber 13 via the metering valve 6 . With the help of the pressure regulator 7 and the vacuum pump 8, a specific working pressure P is set in the container 1, which is measured on the manometer 15. The gas consumer is connected to connection 14. The metering valve 12 is set to the required gas throughput. The measurement begins when the filling valve 11 is closed. By sucking gas out of the cylinder 2, the pressure in the same decreases. The pressure gauge 10 uses the switching amplifier 9 to set the piston 3 in motion via the motor 4 when a pressure difference of >0.3 mbar occurs. When the pressure equalizes on the pressure gauge 10, the piston is stopped again. By determining the piston displacement during a time At, the flow rate Q is calculated as above. After the end of the measurement, the filling valve 11 is opened again and the piston is returned to its starting position. After closing the filling valve 11, a new measurement can begin. 5 10 15 20 25 30 35 40 45 50 55 60 c 1 sheet of drawings