Lake Shore Model 480 Fluxmeter User’s Manual
Magnetic Measurement Overview
2-3
Important Integrator Characteristics (Continued)
Other integrator characteristics that may affect measurements are drift, maximum input voltage, and
maximum and minimum rate of input change. These characteristics are a result of fluxmeter design;
the user often has little control. Check specifications carefully before choosing a fluxmeter for any
application or designing a coil for a fluxmeter.
2.1.4 Reducing Integrator Drift
Drift is the most noticeable and often the largest source of error in integrating fluxmeters. Drift is a
slow change in reading when no change in flux exists. It is caused by any small error voltage at the
integrator input.
Manufacturers spend significant time and effort reducing the drift in instrument integrators.
Component type and value, circuit board layout and manufacturing methods are all optimized to
reduce drift. Temperature change contributes so much to drift that critical components are often
thermally isolated from other parts of the circuit.
Low drift is a result of good fluxmeter design, but users can do things to maintain low drift:
1. Use the instrument on the range specified for lowest drift.
2. Attach sensing coil leads tightly and avoid unnecessary junctions or connections.
3. Keep drafts or other temperature changes away from the coil lead contacts.
4. Allow the instrument to warm up before drift is adjusted and adjust drift as often as practical
during use.
5. Reset the integrator often, before every critical measurement if possible.
Some instruments have built in software algorithms that help adjust drift to zero before measurement.
Other algorithms work in a different way to cancel drift during measurement. It is important to
understand the difference and the affects on measurements.
2.1.5 Dielectric Absorption
All capacitors exhibit a characteristic that can be described as a tendency to rebound from any fast
change. When capacitors are discharged to zero volts momentarily, a small voltage will rise a few
seconds later across the capacitor. Likewise, a rapid charge of a capacitor to some voltage will be
followed by a slight reduction of that potential occurring over several seconds. This characteristic is
usually referred to as Dielectric Absorption. The effect of dielectric absorption in the Model 480
fluxmeter is a slight reading change over several seconds after a larger reading change. This occurs
predictably during reading changes from 0 to some level and more notably occurs when the reading
is reset. A reset from a large, full scale reading will show a “creeping up” of the reading for several
seconds after the reset. The level of this effect is approximately 0.03% of the reading change. The
effect is most noticeable in the first few seconds and stabilizes after 20-30 seconds. For the most
accurate reset of larger measurements an initial reset should be followed by a second reset a few
seconds later.
As inconvenient as this is, capacitor limitations create this condition and cannot be easily remedied.
The capacitor selection for the Model 480 included testing of many vendors and capacitor dielectric
types. The selected capacitors offer the best overall characteristics including that of dielectric
absorption. It is felt that even though this is certainly a source of error for all analog integrating
fluxmeters, the Model 480 is capable of seeing this characteristic with it’s increased resolution while
others have simply ignored it. During instrument factory calibration readings are taken 1 to 2 seconds
after any signal transition. DC Peak, AC and AC Peak readings do not suffer from this anomaly.