Flowmeter Selection Basics
The first step in selecting a flow sensor is to determine if the required flow-rate information should be continuous or totalized and whether these data are needed locally or remotely. If remotely, should the transmission be analog, digital or shared? If shared, what is the required minimum data-update frequency? Once these questions have been answered, the properties and flow characteristics of the process fluid (gas or liquid) and the properties and configuration of the piping that will accommodate the flowmeter should be evaluated.
Next, determine the required flowmeter range by identifying the minimum and maximum flows (mass or volumetric) that will be measured. After that, the required flow-measurement accuracy needs to be determined. Accuracy is typically specified in percentage of actual reading, percentage of calibrated span or percentage of full-scale units. Accuracy requirements should be separately stated at minimum, normal and maximum flow. Unless you know these requirements, meter performance may not be acceptable over its full range.
When purchasing a new flowmeter to measure gas flows in heat-treating applications, it’s important to remember the distinction between the instrument’s operating range and design range.
Some variable-area flowmeters offer full-scale operation, while others offer a very limited range – “not below 25% and not above 90% of scale capacity,” for example. In other words, if the flowmeter is rated for 0 to 2,000 cubic feet per hour (cfh), you can only obtain accurate readings when the flow is between 500 and 1,800 cfh.
If flow measurement has to cover a wide flow range, then select a flowmeter that has a high turndown. An alternative but costly approach is to install several different-size flowmeters with automatic or manual switching based on flow range.
A good rule of thumb for sizing a flowmeter is to purchase one “in the middle third.” It should be sized so that the actual flow will be no less than 33% and no higher than 67% of the scale you select. This gives you the ability during actual operation to compensate for unexpected changes in flow requirements that may occur. Over the life of a heat-treating furnace, process requirements and operating conditions often change, sometimes dramatically, and you want your gas measurement to remain accurate.
If knowing the proper flow rate is required for a variable-area rotameter, be aware that a change in temperature, pressure or specific gravity of the gas from that for which the meter was calibrated will cause a serious error in the indicated scale reading. It is quite common in a heat-treat shop to find flowmeters operating at pressures and temperatures different from those for which they were calibrated.
References are available at the end of part 2 in this series.
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