The purpose of using ready-made applications for flow meters is only one: to detect the actual flow rate of the measured medium. However, many users ignore the difference between the on-site state and the scale state of the flowmeter (i.e. standard state), and directly use the scale reading of the flowmeter as the actual flow value of the measured medium. It is very obvious that this actual flow rate is not true, which will bring errors to the measurement of the flowmeter and cause errors in the detection results of the supporting instruments.
On site, obtaining the actual flow rate value from the scale reading of the flowmeter is essentially converting the flow rate value under the standard state of the flowmeter into the flow rate value under the on-site operating conditions. We set the parameter codes related to the on-site working conditions as actual flow rate Q, medium pressure P, and temperature T; the parameter codes related to the scale of the rotor flowmeter are flow rate Q, pressure Po compared to the standard state, and temperature To. According to equation (4):
By using equation (5), it can be seen that it is very convenient to obtain the actual flow rate value of the measured medium from the flowmeter reading on site. It should be noted that when using equation (5) for calculation, P、P。 、 T、T。 All values should be inputted, and P is the pre gauge pressure, which should be measured on the upstream side of the flowmeter and adjacent to the pipeline of the flowmeter.
Example: Using a certain air sampling device, during operation, the reading of the rotor flow meter on the sampling device is 500 milliliters per minute, the pressure in front of the measuring instrument is -100mmH 20 (negative pressure due to the use of an air pump), and the on-site temperature is 30 ℃. Calculate the actual flow rate of air at this time.
According to the measured data, there are
Here, because P. is measured in millimeters of mercury, the millimeter water column must be converted into millimeters of mercury. When calculating, simply divide the millimeter water column by 13.6.
From the above example results, it can be seen that although the on-site state is not significantly different from the standard state, there is a difference of 22 milliliters per minute (4.4% of the standard value) in the measurement results. In other words, if the route is not corrected, there will be a 4.4% error in the flow value!