Magnetic sensors
Many flow measurements are sensed as differential pressures with the indicator scale graduated in cubic feet per minute, gallons per minute, liters per second, and so forth. Rotating devices, such as the turbine, are used for accurate flow
measurements.
The devices are simple, do not require conversion to pressure or other medium, have low drag, can be constructed of inert materials that are resistant to corrosion, do not require regular recalibration, and are low maintenance.
The pick off is a magnetic sensor such as a Hall effect or an MRE device.
The Hall device gives an electrical impulse every time a blade passes under the sensor, whereas the resistance of the MRE device changes in a changing magnetic field.
Figure 12.14a shows the circuit used to shape the signal from an MRE into a digital signal. The MRE sensor contains four elements to form a bridge circuit as shown.
The Hall or MRE device does not normally require temperature compensation as they are being used as switches in digital configurations.
To measure flow rates a window is opened for a known time period.
The number of impulses from these devices are counted from which the rate of flow can be calculated.
These devices can also be used to measure the total volume in gallons or liters; in this case the number of pulses from the sensors can be counted and divided by the number of pulses per unit volume to give the total volume that has flowed.