What should I know when choosing a flow meter? (3) Posted by : admin / Posted on : Mar 20， 2020

Flowmeter performance requirements
(6) Range
The range is the ratio of the upper limit flow rate and the lower limit flow rate of the flow meter. Linear instruments have a wide range, typically 1:10. The range of the non-linear flow meter is only 1: 3. Flowmeters generally used for process control or custody transfer accounting. If a wide flow range is required, do not choose a flowmeter with a small range.
At present, in order to promote the wide flow range of their flow meters, some manufacturers have increased the maximum flow rate in the instruction manual. For example, the liquid is increased to 7-10m / s (generally 6m / s); 75m / s (generally 40 ~ 50) m / s); in fact, such a high velocity is not needed. In fact, the key to wide range is to have a lower lower limit flow rate to meet the measurement needs. Therefore, a wide range flowmeter with a lower lower flow rate is more practical.
(7) Pressure loss
Pressure loss generally refers to the flow sensor's non-recoverable pressure loss due to the static or active detection element or the change of flow direction in the flow channel, which can sometimes reach tens of kilopascals. Therefore, the flowmeter should be selected according to the pumping capacity of the pipeline system and the inlet pressure of the flowmeter to determine the allowable pressure loss of the maximum flow. Improper selection will restrict fluid flow and cause excessive pressure loss, affecting flow efficiency. Some liquids (high vapor pressure hydrocarbon liquids) should also note that excessive pressure drops may cause cavitation and liquid phase vaporization, reduce measurement accuracy and even damage the flow meter. For example, a water flow meter with a pipe diameter greater than 500mm should consider the increased pumping cost due to excessive energy loss caused by pressure loss. According to related reports, the pumping expenses paid for the measurement of flowmeters with large pressure losses over the past few years often exceed the purchase costs of low-pressure loss and more expensive flowmeters.
(8) Output signal characteristics
The output and display of the flow meter can be divided into:
① flow (volume flow or mass flow); ② total amount; ③ average flow rate; ④ point flow rate. Some flowmeter outputs are analog (current or voltage), others output pulses. The analog output is generally considered to be suitable for process control, and is more suitable for matching with control circuit units such as regulating valves; the pulse output is more suitable for total volume and high accuracy flow measurement. Long-distance signal transmission pulse output has higher transmission accuracy than analog output. The method and amplitude of the output signal should also have the ability to adapt to other equipment, such as control interfaces, data processors, alarm devices, open circuit protection circuits and data transmission systems.
(9) Response time
When applied to pulsating flow applications, attention should be paid to the response of flowmeters to step changes in flow. Some applications require flowmeter output to follow fluid flow changes, while others require slower response outputs to obtain a composite average. Instantaneous response is often expressed in terms of time constant or response frequency. The former ranges from a few milliseconds to a few seconds and the latter is below several hundred Hz. Equipped with a display instrument may considerably increase the response time. It is generally believed that the asymmetry of the dynamic response when the flow rate of the flowmeter increases or decreases will accelerate the increase of the flow measurement error.