Different ways of reading vibrating wire and differences seen

The two readout units do read the vibrating wire in different ways. The AVW211 takes a reading using FFT or Fast Fourier transform whilst the handheld readout unit sweeps through its set range to find the frequency of the vibrating wire. There can be a slight offset between the two however this would not account for the declining pressure readings. 

Both the handheld readout unit and the AVW211 will give the peak frequency of the wire. If you are getting a difference of 15F²/1000 digits ie 6000F²/1000 and 6015F²/1000, although in the higher end of of offset that we have previously seen this would be an expected offset between the two. 

Yellow and orange boxes plucked wire excitation

Orange box:
The principle of operation is for the Data Recorder to supplya series of frequency signals across a user defined frequencyrange (sweep range) to the coil/magnet assembly within the instrument, which causes the wire within the instrument tovibrate at its resonant frequency.
The coil/magnet assembly then acts as a pickup as theoscillations of the wire through the magnetic field induce analternating current in the coil which is detected by the Data Recorder.
The Data Recorder converts the sinusoidal alternating voltageto a square waveform which may easily be timed using afrequency oscillator.
In this way the frequency of oscillation may accurately be measured, displayed and recorded.
The AVW200 series uses a spectral interpolation method which samples the vibrating wire and performs a fast Fourier transform (FFT). Digital signal processing combined with interpolation techniques provide a frequency spectrum that can distinguish between the natural frequency of the vibrating wire and noise frequencies with greater accuracy than ever before. (change wording)
Resolves the vibrating wire measurement to less than 0.001 hertz (industry standard is 0.1 hertz)

Yellow Box:
The Vibrating Wire Logger is suitable for reading most types of commercially available vibrating wire instruments requiring a "plucked” excitation signal.