How to calibrate flow meter

1. Overview

There are many types of flow measurement instruments, and their measurement principles, instrument structures, use conditions and installation methods are different. In order to unify the flow values ​​of various instruments produced by various manufacturers and achieve a certain measurement accuracy, it is necessary to calibrate or verify the newly manufactured or used flow meters. The equipment that can realize the calibration or verification of flow meters is various flow standard devices.

2. Verification method of flow meter

In order to obtain the flow value of the flow meter and its measurement accuracy, each flow meter must be calibrated or calibrated. There are generally two ways to calibrate flow meters: direct measurement method and indirect measurement method. The

direct measurement method is also called the real flow verification method. The actual fluid flows through the instrument to be verified, and then another standard device (standard flow meter or flow standard device) is used to measure the actual flow flowing through the instrument to be calibrated, and compared with the flow value of the instrument to be calibrated, or the instrument to be calibrated is divided. This verification method is also called wet calibration. The flow value obtained by the real flow calibration method is both reliable and accurate. It is the method currently used for calibration of many flow meters (such as electromagnetic flowmeters, volumetric flowmeters, turbine flowmeters, vortex flowmeters, float flowmeters and Coriolis mass flowmeters, etc.), and is also the method currently used to establish standard flow.

The indirect measurement method is to measure the structural dimensions of the flow meter sensor or other quantities related to the calculated flow, and use them according to the prescribed method to indirectly calibrate its flow value and obtain the corresponding accuracy. This method is also called dry calibration relative to wet calibration. The flow value obtained by the indirect method is not as accurate as the direct method, but it avoids the difficulty of having to use a flow standard device, especially for large flow devices. Therefore, some flow meters have adopted the indirect calibration method. For example, the orifice plate, nozzle, venturi tube, etc. in the differential pressure flowmeter have been standardized, and a wealth of test data have been accumulated, and there are corresponding standards. Therefore, the flow value of the standard throttling device can be indirectly measured by checking the geometric dimensions of the throttling device and calibrating the matching differential pressure gauge (differential pressure transmitter). The indirect calibration method of the target flowmeter is to calculate the flow value by hanging weight method based on the measurement target and pipeline size. Some foreign manufacturers have carried out indirect calibration method on float flowmeter, that is, only check the geometric size of float and cone tube and float weight to determine the flow rate, and its accuracy is slightly lower than that of the instrument with real flow calibration. Although elbow flowmeter has not been standardized, due to the low requirement for its measurement accuracy, the indirect calibration method can also be generally adopted. The super-body transducer and the super-body flowmeter parameter setting clamped outside the pipeline are actually dry calibration. Although the indirect calibration method is always the most important flow calibration method for some flow meters, even for the standardized standard throttling device, sometimes the use conditions exceed the range specified by the standard, or in order to obtain higher measurement accuracy, the real flow calibration method is still required for calibration.

There are many methods for calibrating flow meters, but the real flow calibration method is always the most basic and important calibration method. The reason why some instruments can be calibrated by indirect calibration method is also based on the accumulation of rich test data by real flow calibration. Therefore, this chapter mainly briefly introduces the laboratory and field real flow verification methods, types and various types of flow standard devices and flow value transmission systems. (end)