Flow measurement is one of the most challenging and sometimes the most difficult of all process measurements. Process temperature, pressure, fluid density, viscosity, velocity, and piping must be considered when choosing the correct flow meter. Unfortunately, a flow meter that will measure all applications does not exist. However, the strain gage type target flow meter is as close to a universal flow meter as any flow meter available today.

OPERATION AND CALIBRATION
The strain gage type target force transducer provides flow measurement by sensing the fluid force acting on the target suspended in the flow stream. The following equation describes the operation of the strain gage target flow meter:

Cd = Overall drag coefficient obtained from empirical data.
A = Target area
P = Fluid density
V = Fluid velocity at the point of measurement
g = Gravitational force of the earth

In a given flow application, the drag coefficient, target area, and gravitational force would be constant. The flow meter is actually measuring the following:

Fluid density x fluid velocity2

Flow is equal to the square root of the force. The transmitter amplifies the output signal, extracts the square root, and produces a linear analog (4-20ma) and digital (0-1000 Hz) output signals.

A typical strain gage target flow meter (figure 1A & 1B) consists of the sensing element, mounting flange or housing, and a terminal strip or transmitter enclosed in a junction box.

The sensing element (Figure 2) is the heart of all strain gage target flow meters. The sensing element consists of a wiring connector, target rod, calibrated target, mounting base, protective case, and the sensing tube where the actual strain gages are attached. The sensing element is constructed of 316 SS with the sensing tube being MP35N alloy. MP35N alloy is a nickel-cobalt resistance, making it the ideal material for the sensing tube. Each component of the sensing element is press fit together then welded, completely isolating the strain gages from the process fluid.

Four strain gages (variable resistors) are attached to the sensing tube, two on the leading side of flow, and two on the trailing side of flow. The strain gages are interconnected, forming a four active arm strain gage bridge circuit. At zero flow (no force on the target), the bridge circuit is balanced, producing zero output. Flow produces a
strain on the sensing tube, compressing the leading side strain gages and tensing the trailing side strain gages, causing their resistance to decrease and increase respectively. The change in resistance of the strain gages offsets the bridge circuit, producing an output (figure 3).

The most difficult process in producing the strain gage type target flow meter is the precise application of the strain gages to the sensing tube. Each of the four strain gages must be applied identically to ensure that they respond equally to changes in temperature and pressure, therefore, not affecting the bridge circuit output. The bridge circuit output must also be proportional to force (flow squared) and return to zero when the flow force is removed.

The calibration and range of the flow meter is determined by the target size. Given the flow parameters for an application and knowing the desired amount of stress to be applied to the sensing tube at full-scale flow, the approximate target size is determined. The flow meter is then tested in a flow test stand and the final target is obtained.

Since the flow meter is a force transducer and cannot determine what is causing the force, all fluid flow application can be mathematically converted to a water flow equivalent. This water flow equivalent represents the same force as the actual fluid application allowing water to be used as the primary calibration medium. The following applications all exert the same force on the target, producing the same bridge output:

UNIQUE FEATURES
As stated earlier, the flow range of the strain gage target flow meter is determined by the target size. By changing targets the flow range can be altered. For example, using water in a 3-inch line, a flow range of 5-75 GPM, 15-225 GPM, and 24-360 GPM can be obtained with each flow range having a unique target. The 3"line example, any full scale water flow equivalent force from 70 to 360 GPM is available, with each maintaining a 15:1 turndown. In other words, every flow meter is calibrated to match each individual flow range. This meter has no moving parts to wear out, has the ability to change target (flow ranges), and the capability to withstand a fifty percent over range without damage, providing tremendous flexibility. This unique combination also makes the flow meter very forgiving in terms of correcting for erroneous process information.

The strain gage target flow meter calibration can be easily verified in the field without the use of a flow prover. Again, taking advantage of the force transducer design, hanging a test weight (0-1000 grams) in the direction of flow simulates flow force. Each flow meter data sheet includes the 1000-gram force output, in mV/V, along with the target type and size. A visual check of the target will verify the target type. A set of calipers will verify the target size (three decimal places). Securing the flow meter as if installed in a vertical pipe with flow down will allow the test weight to be suspended from the target rod simulating flow. By taking the square root of the test weight divided by the full-scale weight, and then multiplying by the full-scale flow, the flow simulated by the test weight is obtained. This procedure allows complete system verification from the primary flow sensor, through the transmitter, to the final readout device. Each transmitter has a calibration circuit, which can be used to check the strain gage bridge circuit, transmitter, and the final readout device. The calibration circuit forces the bridge to have a known output which represents a specific flow rate. This procedure can only be performed under a no-flow condition. Both calibration procedures require zero verification (zero output at zero flow) as the first step. At installation, a functional check can be made by simply applying force to the target (by hand) in the direction of flow. This will also verify all wiring connections.

The strain gage bridge circuit, which measures the force produced by flow, will measure both forward and reverse force. The polarity of the output signal indicates the direction of the flow, making the target meter a true bi-directional flow meter. A special target is used in ensure accuracy in both directions.

SPECIFICATIONS
The sensing element, the heart of the flow meter, can be installed in any line size and in almost any mounting configuration. Inline flow meters, supplied with mounting housing such as wafer, flanged, MNPT, and flare tube, are available for one half to six inch line sizes. Fixed insertion type flow meters are available for line sizes of four to sixty inches. Retractable insertion type flow meters are available for line sizes of four to thirty six inches.

The type of mounting configuration limits the pressure rating of the flow meter. In flow meters that have a flange, the flange determines the maximum operating pressure. The strain gage sensing element is available in three pressure ratings: 1000, 5000 and 10,000 PSIG.

The strain gage target flow meter is available in three temperature ranges, from -65° to +425° F, -65° to +500° F, and -320° to +250° F. Design work is currently underway to develop a force transducer that will operate up to +1200° F.

The accuracy of the flow meter, with flow calibration, is ± 0.5%. For line sizes, 8 inches and larger, a ± 2.0% calibration is also available. All strain gage target flow meters have a repeatability and hysteresis of 0.15%.

STEAM FLOW MEASUREMENT
The strain gage target flow meter has all the features desired in a saturated or super heated steam flow meter. It has an all welded design, which eliminates potential leak paths created by seals, gaskets, or o-rings. It has a low-pressure drop; no moving parts (bearings, springs), and is not damaged by slugs of condensate. The retractable flow version allows the flow meter to be inserted into service without shutting off the steam flow. Seasonal flow ranges, such as large flow rates in the winter and small flow rates in the summer, can be easily obtained by changing targets.