17
2. THRESHOLD B
–
(THS-B) (BIAS)
Setting range 1-99
Factory preset at 50
History of THS-B and NULL
To explain what THS-B NULL is, I need to first explain what the Ground
Balance is actually doing.
The Mineralised ground in our goldfields produces a reaction in the
detector the same as a target response. However, this base response
can be neutralised (NULLED) by performing a Ground Balance, whereby
this mineralised response is effectively nulled out until you get no
response from the ground.
This sets the detector up to be able to detect responses from targets that
are either side of the ground balance point (i.e. rising or falling pitch
target responses).
When you ground balance the QED, you listen for the response when
you lower the coil to the ground, if it’s a rising (high) pitch (in other words
a short time constant response) you lower the MGB towards the GB
NULL point (and vice versa if the response is a falling (low) pitch) until
you get no response from the ground in either direction. The QED is then
said to be “ground balanced” (i.e. the ground is NULLED).
When the QED is ground balanced, targets with a short time constant
(small targets) will then produce a rising (high) pitch target response and
long-time constant targets (large targets - roughly 2.5 g or larger) will
produce a falling (low) pitch response, this equates to either side of the
ground balance point.
Now back to THS-B. In the very early QED development days, Howard
found that temperature changes caused the GB NULL point to drift, so
introduced an adjustment to compensate for this drift. This adjustment in
those early days was called “NULL”.
Further experimentation with this adjustment, Howard found that by
adjustin
g “UNDER NULL” resulted in enhanced target response to short
time constant (small) targets, and “ABOVE NULL” resulted in enhanced
target response to long time constant (large) targets.
The further either under or over NULL, the greater the enhancement,
until you could go no further either way as a result of the noise being
introduced.
Howard was on to something unique, and as a result the THS-B
adjustment was born as a user adjustable setting.
When the QED was first released this adjustment was called “BIAS”,
because you were able to bias the QED from NULL for better (enhanced)
target response for either small or large targets or adjust to NULL for
standard responses to small or larger targets.
In later releases this feature was renamed to THS-B, with the
“B”
meaning “BIAS”.
Summary of Contents for PL3
Page 56: ...56 NOTES...
