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Selection of positive displacement flowmeters: The appropriate flowmeter should be chosen based on the medium being measured and the required measurement range. For oils with high viscosity, a scraper-type positive displacement flowmeter can be considered. For oils with low viscosity as well as water, elliptical gear or gearwheel-type positive displacement flowmeters may be suitable; for applications where high accuracy is not required, rotary piston or scraper-type flowmeters can also be used. For measuring gas flow, rotary drum or rotary piston type positive displacement flowmeters are generally employed. In the measurement of coal gas, diaphragm-type positive displacement flowmeters are commonly used. For measurements that require high accuracy, gear-type gas positive displacement flowmeters are also utilized. The measurement range of positive displacement flowmeters is typically determined as follows: the lower limit flow rate qmin is set according to the error characteristics of the flowmeter, meaning that the error at this minimum flow rate must remain within the allowable error range； The upper flow rate limit qmax is determined by taking into account the wear of the moving parts of the flow meter. Excessively high flow rates can cause accelerated wear of these moving parts, leading to an increase in leakage and errors. A typical value for qmax is approximately 5–10 times qmin