How Target Flowmeters Work
Target flowmeters measure flow by measuring the amount of force exerted by the flowing fluid on a target suspended in the flow stream. The force exerted on the target by the flow is proportional to the pressure drop across the target. Similar to differential pressure flowmeters, Bernoulli’s equation states that the pressure drop across the target (and hence the force exerted on the target) is proportional to the square of the flow rate. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale force. At 10 percent of full scale flow, the target flowmeter accuracy is dependent upon the transmitter being accurate over a 100:1 range of force. Transmitter accuracy is typically degraded measuring low forces in its range, so flowmeter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of target flowmeters. Remember that of interest is the accuracy of the flow measurement system --- not the accuracy of the transmitter.
How to Use Target Flowmeters
Target flowmeters inferentially measure the flow of liquids and gases, such as water, air, industrial gases, and chemicals. Be careful using target flowmeters on fluids that can coat the target, because the flowmeter accuracy can be degraded.
This flowmeter can be applied to sanitary, cryogenic liquids, relatively clean, and corrosive liquids. The materials of construction of all wetted parts, including the flowmeter body and seals, should be considered. Inline target flowmeters are available up to approximately 6 inches in size. Insertion target flowmeters are available for larger line sizes.
Applications requiring sanitary piping systems are often found in the pharmaceutical and chemical industries, such as in the manufacture of drugs and vitamins. Sanitary applications are especially prevalent in the food and beverages industry where cleanliness is important.
Applications for target flowmeters are found in the mining, mineral processing, pulp and paper, power, petroleum, chemical, and petrochemical industries. Common applications include the measurement of process and cooling water flows.
Application Cautions for Target Flowmeters
Be careful when operating target flowmeters low in their flow range because accuracy can be compromised at these flow rates. Avoid abrasive fluids because the target can be damaged and measure inaccurately. Fluids that coat the target can cause it to become heavy, such that the force measurement can become inaccurate and unresponsive to flow changes. These flowmeters should not be applied to slurries and high viscosity liquids.
Calibration is an important part of ensuring accurate operation of target flowmeters. Pressure effects, physical effects, and flowmeter orientation can affect the zero adjustment of target flowmeters. After calibrating the target flowmeter (typically with weights), be sure to zero target flowmeters after installation with a full pipe under no flow conditions to ensure compensation for these effects in the final installation. This procedure also compensates for the calibration shift that can occur when the orientation of the flowmeter during calibration is different than its installed orientation. In general, in-situ zero adjustment should be performed after re-installing target flowmeters.
Operating a target flowmeter above its maximum flow rate can cause the calibration to shift and potentially damage the suspension system of the flowmeter. Note also that operating target flowmeters continuously for extended periods of time can cause a mechanical shift to occur that can affect the zero because the target may not return to exactly the same position after being exposed to forces over time. Rapidly changing flow, such as slug flow conditions, two-phase flow, and start-up conditions (gas to liquid transitions), can similarly affect the mechanics of the target flowmeter and hence its zero. Therefore, it is advisable to periodically verify the zero adjustment of target flowmeters.