Length is fixed because that is how the sensor is built. Tension, however, changes as the
environment around the sensor changes. For example, the wire in a pressure sensor is attached at
one end to a diaphragm. As the diaphragm flexes with changes in external pressure, the wire is
stretched or relaxed. The stretch or relaxation is proportional to external pressure.
Depending on the purpose of the sensor, the manufactured lengths of vibrating wires vary
widely. Shorter wires vibrate at higher frequencies, longer wires at lower frequencies. For
example, one to two inchgages vibrate near 3000 Hz, while six inch gages vibrate near 900 Hz.
9.8.4 Sensor output frequency range and maximum
sample rate
Higher VWIRE 305 sampling rates require higher and tighter sensor frequency-output ranges.
The vibrating wire in a sensor must vibrate five to 10 times per measurement for the VWIRE 305 to
pick it up. Longer gages, such as those that vibrate near 900 Hz, cannot be measured at higher
VWIRE 305 scan rates because the wire does not vibrate fast enough to get the required number
of vibrations for a measurement.
As a general rule, the fastest that a vibrating wire gage can be measured is approximately 20
percent of the resonant frequency. For example, a gage that resonates at about 1100 Hz can be
measured at a maximum of 20 % of 1100 Hz, or at 220 Hz, so the 200 Hz measurement range is
used.
The third column of
(p. 23) lists the sensor frequency-output ranges that can be
measured at the VWIRE 305 sample rates.
Some applications may require that a sensor be used that can be sampled over a wide range of
sample rates. Sensors with higher resonant frequencies are more versatile. For example, as shown
in the previous table, a sensor with a resonant frequency of 2000 Hz can be measured at scan
rates 20, 50, 100, 200, and 333.3 Hz. On the other hand, a sensor with a resonant frequency of 290
Hz can be measured only at 20 or 50 Hz.
9.8.5 Temperature sensor
(p. 42) shows the location of the temperature sensor. Sensor manufacturers have
many thermistors and RTDs to choose from, so this manual has only general guidelines about
measuring an embedded temperature sensor. Additional information is in
Be aware that some manufacturers, Slope Indicator, for example, use different thermistors or
RTDs in the same model of sensor according to what the client orders. Take this into
consideration when specifying and ordering sensors.
VWIRE 305
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