When purchasing a new flowmeter to measure gas flows in heat-treating applications, it’s important to remember the distinction between the meter’s operating range and its 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 0-2,000 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.” That is, size it 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.
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. However, there is an easy way to calculate the effect of these changes.
Equation 1 allows us to calculate the actual flow when a change in temperature occurs.