4-14
4 Making Measurements
4.2.4 Optimal Stand-Off Distances for the Sensor Heads
OFV-303/-353
The stand-off distance is measured from the front panel of the sensor head
OFV-303/-353. The optimal stand-off distances are:
, n = 0; 1; 2; ...
i.e. at 232 mm; 435 mm; 638 mm; etc.
OFV-511
The stand-off distance is measured from the shoulder of the connector for the
mini sensor or the fiber head. The optimal stand-off distances are:
, n = 0; 1; 2; ...
i.e. at 135 mm; 338 mm; 541 mm; etc.
OFV-512
When making two point measurements the stand-off distance is the difference
between the stand-off distances of both arms. The optimal stand-off distances
are:
, n = 0; 1; 2; ...
i.e. at 0 mm; 203 mm; 406 mm; etc.
When making single point measurements with the reference head OFV-151
the stand-off distance is measured from the shoulder of the connector for the
mini sensor or the fiber head. The optimal stand-off distances are:
, n = 0; 1; 2; ...
i.e. at 63 mm; 266 mm; 469 mm; etc.
Maxima of
visibility
The light source of the vibrometer is a helium neon laser. This is a multimode
laser in which a maximum of two modes can exist. The interference of the two
modes leads to the intensity of the resulting optical signal varying periodically
with the stand-off distance. The intensity increases to a maximum i.e. a
maximum of visibility is present if the optical path difference is an even
numbered multiple of the length of the laser cavity (203 mm). As the optical
path difference is equal to twice the stand-off distance (the beam goes there
and back), a maximum of visibility is present once per laser cavity length.
In practice, it is not usually necessary to search for the maximum of visibility
as the vibrometer is sensitive enough to make a measurement even close to
the minimum. A minimum is indicated during the warm-up phase by periodic
fluctuation on the signal level display.
232mm
n 203mm
⋅
+
135mm
n 203mm
⋅
+
0mm
n 203mm
⋅
+
63mm
n 203mm
⋅
+
