This would apply to all probes as in HAM, BSC etc.

We found the MicroSense did not report the standoffs and when asked were surprised it was not 2mm.  They do record them but don't list the number on the calibration sheet.  I'm asking for them all but to-date only have a few that they told me.  One was listed as 2.259mm (89mil) and four of the seven were greater than 2.16mm!  They will 'mess' with them if they are too far below the nominal and I don't know all the numbers yet.  This was a source of much of our confusion when we were jigging these in to get a noise spectra.  Eric ended up using 6 10mil shims on the test jig to get the outputs near zero.

Bottom line here is I wanted to post an email explanation given by MicroSense showing why our gaps are not exactly 2mm.  Sounds like we could specify that if we think it is needed but apparently we did not do so.

From Roy Mallory 1 July 2010:

Hi Hugh,

We calibrate to achieve a particular scale factor and to minimize
linearity error, but don't attempt to tweak the standoff.  The nominal
standoff is determined by design parameters, but a number of things, like
component tolerances, can cause it to be slightly different from unit to
unit.  It's also tricky to measure and define.  For example, is the
standoff the shortest line from the center of the probe to the target?  Is
it the distance between the point on the target and the point on the probe
that will first contact if the two are translated until they touch?  If
the probe and target aren't perfectly parallel, the standoff will be
affected, but is that the standoff to report?

What we do--mostly because it's simple--is to run the probe and target
together until they touch.  The target, which is the back side of a
mirror, is held to its mover by a light magnetic force.  The front side of
the mirror is monitored by an interferometer.  We note the
interferometer's reading with the mirror resting against the probe face,
and then move the mirror until the cap gage is at the center of its range,
noting the amount the interferometer reading has changed.

This method will be affected by parallelism, by any nonflatness in the
probe, and by any mote of dust on the probe or mirror.  Our gages do have
a front-panel zero control that allows the standoff to be adjusted,
however most of our customers want incremental accuracy and aren't overly
concerned about absolute standoff.  Does the LIGO application require some
particular accuracy in standoff?  If so, we can adjust the zero control at
the time of calibration to achieve your desired standoff.

Roy