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Turndown ratio
Turndown ratio refers to the width of the operational range of a device, and is defined as the ratio of the maximum capacity to minimum capacity. For example, a device with a maximum output of 10 units and a minimum output of 2 units has a turndown ratio of 5. The term is commonly used with measurement devices and combustion plant like boilers and gasifiers.
==Flow meters==
In flow measurement, the turndown ratio indicates the range of flow that a flow meter is able to measure with acceptable accuracy. It is also known as rangeability. It is important when choosing a flow meter technology for a specific application. If a gas flow to be measured is expected to vary between 100,000 m³ per day and 1,000,000 m³ per day, the specific application has a turndown ratio of at 10:1. Therefore the meter requires a turndown ratio of at least 10:1. For example: if the meter had an advertised maximum flow of 2,000,000 m³ per day then the required turndown ratio would be 20:1.
The turndown ratio of each type of meter is limited by theoretical considerations and by practical considerations. For example, orifice meters create a pressure drop in the measured fluid proportional to the square of the velocity. Therefore the range of differential pressure can become too large and compromise accuracy. It can also create process problems such as hydrate formation, and in the case of measuring the discharge of a compressor, there is a limit to how much pressure loss is acceptable.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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