RFL 904 User manual

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INSTRUCTIONS
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Model 904
GAUSSMETER
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Copyright
1980
RFL Industries, Inc.
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Book MA-48540

CAUTION
FOR YOUR SAFETY
THE INSTALLATION, OPERATION AND
MAINTENANCE OF THIS EQUIPMENT
SHOULD BE PERFORMED BY
QUALIFIED PERSONS ONLY.
WARRANTY
The Models 904 and 904T are warranted
for 12 months from date of delivery for
replacement of any part, except for probes,
which fails during normal operation or ser-
vice. A deficient part should be returned to
the factory, shipping charges prepaid, for
replacement f.o.b. Boonton, N. J.
RFL Industries, Inc.
Boonton, New Jersey 07005
WARNING:
The Equipment Herein Described Contains High Voltage
Exercise due care during operation and servicing.
Read safety summary on reverse of this page.
II

CONTENTS
Description
5
II
Specifications
7
III
Theory of Measurement
9
IV
Operating Instructions
11
V
Probes
13
Flat, or Transverse-Field Probes
13
Axial Probes
13
Tangential Flat Probes
13
VI
Maintenance and Calibration
15
Maintenance
15
Calibration
16
VII
Circuit Details
19
Power Supply
19
Signal Circuits
19
Indication
20
Adjustments
20
VIII
Parts List
21
SAFETY SUMMARY
The following safety precautions must be observed at all times
during operation, service, and repair of this product. Failure to
comply wills these precautions, or with specific warnings
elsewhere in this manual violates safety standards of design,
manufacture, and intended use of the product. RFL assumes no
liability for failure to comply with these requirements.
GROUND THE EQUIPMENT
To minimize shock hazard, chassis, cabinets, and equipment
racks must be connected to an electrical ground. AC powered pro-
ducts should be equipped with a three-conductor power cable, or
equivalent connection on a terminal block. Power cables must
either be plugged into an approved three-contact electrical outlet
or used with a three-contact or two-contact adapter with the
grounding wire (green) firmly connected to an electrical ground
(safety ground) at the power outlet. The power jack and mating
plug of power cables provided meet International Electrotechnical
Commission (IEC) safety standards. Equipment provided with ter-
minal blocks, operating from either ac or dc, is provided with ap-
propriate means for connecting an electrical safety ground.
DO NOT OPERATE IN AN EXPLOSIVE ATMOSPHERE
Do not operate the product in the presence of flammable gases
or fumes. Operation of any electrical equipment in such an en-
vironment constitutes a definite safety hazard.
DO NOT OPERATE IN WET OR DAMP AREAS
Do not operate the product in wet or damp areas. Operation of
any electrical equipment in such an environment constitutes a
definite safety hazard.
KEEP AWAY FROM LIVE CIRCUITS
Operating personnel must not remove covers. Replacement of
components and internal adjustments must be made by qualified
maintenance persons. Disconnect power cable when replacing
components. Under certain conditions, dangerous voltages may
exist even with the bower cable removed. To avoid injuries
always disconnect power and discharge circuits by grounding
before touching them.
DO NOT SERVICE OR ADJUST ALONE
Do not attempt internal service or adjustment unless another
person capable of rendering first aid and resuscitation is present.
DO NOT SUBSTITUTE PARTS OR
MODIFY EQUIPMENT
Because of the danger of introducing additional hazards, do not
install substitute parts or perform an unauthorized modification to
the equipment. The product may be returned to RFL for service
and repair to ensure that safety features are maintained.
DANGEROUS -
PROCEDURE WARNINGS
Throughout this manual, warnings identify potentially
dangerous procedures. Instructions contained therein must be
followed.

Frontispiece: Model 904 Gaussmeter.
The Model 904T Teslameter is similar in appearance.
Chapter I
DESCRIPTION
The Model 904 is a compact instrument designed for measur-
ing intensity of ac, dc, and permanent-magnet magnetic fields
with flux densities from 1.0 to 30,000 gauss (0.0001 to 3
Tesla). The Model 904 Gaussmeter reads flux density in gauss.
The Model 904T Teslameter reads flux density in Tesla. The two
models are identical in all other respects. DC and permanent-
magnet fields are read directly from a zero-left, analog meter. AC
field measurements require the use of an external ac meter or
oscilloscope connected to a rear-panel jack. The analog signal
available from this jack may also be used as a controlling signal for
automatic or semi-automatic magnetic processing systems, or as
input to a recorder.
Polarity of the dc or permanent-magnet field is indicated by two
LEDS located adjacent to the panel meter. This provides the con-
venience of a zero-center meter with the greater scale length of a
zero-left meter.
The instrument uses Hall—effect sensors. A wide choice of
axial,transverse-field, and tangential probes is available. A simple,
internal means for calibration without using a reference magnet is
included.
Both models operate from either 115- or 230-Vac, 48-63-Hz
power. They are enclosed in an impact-resistant plastic case, the
carrying handle of which serves also as a tilt bail to position the
front panel for easy viewing and control.
A simplified schematic of the circuit, Figure 1, outlines the
operation of the instrument.
A constant-current generator supplies excitation current to the
Hall probe, the signal from which is amplified in a two-stage,
direct-coupled amplifier. Magnitude of the signal is indicated on a
d'Arsonval meter; and sensitivity, or range, of the measurement is
set by selecting fixed values of negative feedback for the
amplifiers with a range switch.
Polarity of the field is sensed with voltage comparators and is
indicated with light-emitting diodes whose illumination cor-
responds to the polarity indicated.
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5

Chapter II
SPECIFICATIONS
Full-scale Ranges
Model 904 Gaussmeter: 0 to 30, 100, 300, 1K, 3K, 10K, and
30K gauss.
Model 9041 Teslameter: 0 to 0.003, 0.001, 0.03, 0.1, 0.3,
1.0, and 3.0 Tesla.
Accuracy
For permanent-magnet, dc, and ac fields from 10 to 400 Hz:
±(3% of range plus error of probe).
For ac fields from 401 to 1000 Hz:
±(5% of range plus error of probe).
Downscale Linearity
± 1%
of range.
Operating Temperature Range
12 to 32°C.
Calibration Method
Built-in electronic circuit calibrated against a 0.2% accurate
magnetic reference standard.
Analog Output
+ 1
volt full-scale for any range.
Range Selection
Pushbuttons
Operating Power
1
1 5/230 Vac, 48-63 Hz, approx. 3.5 watts
Size
8.25" wide, 9" deep, 2.75" high (209 x 228 x 70 mm).
Weight
3.8 lbs. (1.7 kg).
Figure 1. Simplified schematic of Models 904 and 904T.
J1
CONSTANT-
CURRENT
GENERATOR
ICSB
DS34
RANGE
,,-
[? RANGE
OFFSET I
OFFSET 2
NULL
J2
0
ANALOG
OUTPUT
METER •
CAL
FITTER
120
-REF /
-12
1055
+REF /
L
1
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tiAll
KW
CAL
3 RIT
1
CAL
READ
CAL
READ

Chapter
III
THEORY OF MEASUREMENT
THE HALL EFFECT
When electrons move in a conductor perpendicular to a
magnetic field, they are deflected toward one side of the conduc-
tor (left or right depending on the direction of the magnetic field)
in a direction normal to the direction of both the initial electron
flow and the magnetic field. The force deflecting the electrons is
directly proportional to the electron velocity (current) and the
magnetic field intensity. This deflection process continues until
sufficient charge has accumulated at the sides of the conductor to
establish a transverse electric field which opposes further deflec-
tion of charge carriers. The transverse potential difference thus
created is called the Hall voltage and the phenomenon causing it is
known as the Hall effect.
The magnitude of this potential in volts is given by
RIH
where E is the Hall voltage, H is the magnetic flux density in
Oersteds, t is the thickness of the sample in centimeters, and R is
the Hall coefficient in volt-cm/ampere gauss.
Under the conditions shown in Figure 2, terminal B will be
negative with respect to terminal A for metals and semiconduc-
tors in which the current is carried by electrons. Reversing the
field reverses the output polarity. As shown by the equation
above, the width of the strip is not a factor of the output as long
as it is large compared to the thickness. The strip should be twice
as long as it is wide or the output drops off. For thin strips the out-
put is unaffected by the frequency of the field in the audio range.
The Hall voltage is maximum when the plane of the element is
perpendicular to the magnetic field.
Practical materials for Hall elements are films and slabs of
specially refined semiconductors. The sensitivity, temperature
stability and linearity to fields can be varied through wide ranges
by selection of materials. One characteristic can usually be im-
proved at the expense of another.
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The output voltage from a gaussmeter probe is proportional to
the excitation current, and if this current is held constant then the
Hall voltage will be solely proportional to the incident magnetic
flux, although the precise Hall voltage for constant flux may vary
slightly from one probe to the next. Probes supplied for use with
the Model 904 are standardized at the factory, for use with this
instrument, by marking them with a number which compensates
for this variation. Then, the instrument is calibrated by varing the
magnitude of the constant-current excitation until the meter reads
the number stamped on the probe.
Figure 2. The Hall effect.
Chapter IV
OPERATING INSTRUCTIONS
CAUTION
Be sure that the gaussmeter is unplugged from
the powerline before setting S2, the line-voltage
switch, or before opening the case and changing
Fuse Fl.
INSTALLATION
Determine whether the instrument is to be operated from
1 15-
or 230-volt primary power. The instrument is shipped from the
factory with the voltage-selector switch, S2, Figure 5, set at the
115-volt position, and with a fuse rated for 0.25 ampere at Fl.
For operation from 1 15Vac,
the instrument need only be con-
nected to the power source.
Figure 3. Front and rear panels of Models 904 and 904T.
11
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For 230-volt operation, the case should be opened, Fl should
be replaced with a fuse rated for 0.125 ampere, and S2 should be
moved to its 230-volt position, after which the instrument may be
powered.
OPERATION
(1)
Connect the Hall probe to its socket on the rear panel. The
location will be seen on Figure 3.
(2)
Set all switches on the front panel, Figure 3, to their out
position. Connect the instrument to its source of power, depress
the POWER switch, and allow the instrument to warm up for ap-
proximately ten minutes before continuing.
(3)
Depress READ/CAL button to CAL.
(4)
Depress 10K/CAL switch to CAL.
(5)
Using the 0-10 scale on the front-panel meter, adjust the
CAL potentiometer so that the meter reads the number which is
stamped on the body of the probe. The instrument is now
calibrated to the characteristics of the individual probe used.
(6)Move CAL/READ button to READ by depressing to release it.
(7)
Depress the 30-gauss (0.003 tesla) range button.
(8)
If a zero-field chamber is available, place the active end of
the probe in the chamber. If no chamber is available, orient the
probe in space so that the earth's magnetic field causes the
minimum deflection on the meter, Then, in either case, adjust the
NULL control for zero on the meter. The instrument is now ready
for use.
It is advisable to recheck the zero condition periodically.
Although under normal conditions it is stable, there is always the
possibility that either the NULL or the CAL control may have been
moved inadvertently.
The tip of the probe is color coded to indicate polarity of the
magnetic field. When the red-marked side of the flat probe, or the
front end of an axial probe is pointed toward a magnetic pole
which attracts the North-seeking end of a compass needle, the
polarity-indicating lamp marked N will be illuminated.
Although every effort has been expended to make the Hall
probe as rugged as possible, it should be remembered that the Hall
element is a fragile component. Use care when handling the
probe. Do not force it into narrow gaps, or apply twisting or bend-
ing forces while the probe is in the confines of a gap. A properly
handled probe will last many years, but carelessness can destroy
the element in a few seconds.
12
Chapter V
PROBES
The Models 904 and 904T use Hall-effect probes for measuring
flux density. RFL has nine standard Hall-effect probes available for
use with this instrument. Questions on choice and application of
probes and search coils, as well as questions on unusual applica-
tions, may be direct to RFL's Magnetics Engineering Department.
Flat or Transverse-Field Probes
The transverse Hall-probe measures magnetic fields perpen-
dicular to its flat surface. RFL flat probes are supplied in thickness
ranging from 0.015 inch to 0.039 inch. The 0.025 and
0.039-inch probes are enclosed in rugged fiberglass/epoxy
shields to provide maximum protection for the sensitive Hall ele-
ment. All flat probes incorporate a pencil-slim handle and semi-
flexible leads from the element to enable use in a wide variety of
applications. A lightweight, five-foot cable is standard with all flat
probes.
Axial Probe
This configuration is used to measure fields parallel to the probe
handle, such as solenoidal fields. The Hall element is encapsulated
in epoxy and the handle is rigid for several inches beyond the tip.
Various sizes are available. The axial probe, because of its tip con-
struction, is generally less subject to accidental damage than flat
probes. Care must be exercised, however, not to allow the flat
face of the tip to contact abrasive surfaces where any motion is
involved. The Hall element is located only a few thousandths of an
inch behind the face and could be damaged if undue pressure is
placed on it.
Tangential Flat Probe
The tangential probe is similar to the flat probe with the main
difference that the element is placed as close to the edge of the tip
as possible. The flat-probe element is centered between the out-
side edges of its enclosure. This construction puts the element in
close proximity to the outside edges of the enclosure. Care must
be taken, therefore, that the probe will not be subjected to rough
handling.
13

ACTIVE AREA
.071 X MIT
ACTIVE AREA
078 X .167
904.200
4 1/4
I —I 14-- 5/16 DIA.
I 1 ,
..„
0
ACTIVE AREA
904.312
6
1561
ts_
.060,1.150
0
ACTIVE AREA
904.156
.082 0.125
014
11.200—
1
4—
.
1
8-3/4
4,--
ACTIVE AREA " '
.060 .060 X .150
NOTE:
RFL reserves the right to change probe dimensions as required.
The tangential probe is used for measuring tangent to the sur-
face of a material where it is necessary to place the probe element
as close to the surface as possible. This enables measurements of
flux densities close to and parallel to a surface where either flux
leakage or anomalies in the material occur.
General
Test-data sheets are supplied with every probe. These contain
electrical and temperature-stability values plus calibration data for
each range of the instrument on which they are used. It is impor-
tant when ordering replacement probes that the model and serial
number of the Gaussmeter be provided. This will assure proper
calibration and assembly of the correct connector.
FLAT PROBES
—011.4-- .020
0
904.020
—4125/14 R- - - 1
1/I9
0
ACTIVE AREA
.040 0.090
904.015
904.039
1 -or
-5/32
I-114-- .025
0
ACTIVE AREA
.076 X .197
904.025
5/32
.039—•1141--
TANGENTIAL PROBE
.040 —4114-
-a l
AXIAL PROBES
.030 X .060
3
t
.
i= 1
1
f
—DIA.
16 t
DIA.
VI"
8 3/4
C= I
1
9
0
904-100
ACTIVE AREA • -
Figure 4. Standard gaussmeter (Hall-effect) probes.
14
Chapter VI
MAINTENANCE and CALIBRATION
MAINTENANCE
The content of this chapter is primarily a description of pro-
cedures for adjustment of the circuits of the instrument. Failure of
the equipment to perform as required is usually evidence of some
defect in procedure or equipment, and the nature of the failure is
generally a most valuable symptom, when used in conjunction
with the schematic of the circuit, leading to discovery of the
defect. Signal tracing with an oscilloscope is also an effective ap-
proach.
Figure 5 locates the most significant components and ad-
justments.
Figure 5. Location of major controls. Models 904 and 904T.
15
ACTIVE AREA
.078 X .187
9/32
3/16-
3/8
904-040

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CALIBRATION
When performing any of the following calibration procedures, it
is recommended that the instrument first be turned on and allow-
ed to warm up for at least ten minutes. With the case removed,
the chassis may be protected from air currents by using a piece of
transparent lucite equipped with a hole for passing the
screwdriver used for adjustments. Then, internal calibration con-
trols are set as follows:
(14) Plug a standard probe into J1.
(15)
Set the READ/CAL switch to the CAL position by depress-
ing it.
(16)
Depress the 10-K (1-T) range switch.
(17)
Using the 0-10 scale on the meter, adjust CAL control, R19,
on front panel so that the front-panel meter, M1, reads the
number stamped on the body of the probe. The instrument
and probe are now calibrated and ready for use.
(1) At the socket for the probe, Terminals 1 and 2 must be
shorted together, and Terminals 3 and 4 also must be in-
dependently shorted together. These connections can be
made either by preparing a suitably shorted plug for insertion
in J1, or by clipping leads across the appropriate socket ter-
minals inside the instrument.
(2)
Set all range switches to their OUT position.
(3) Connect a jumper from TP3 to TP2. This grounds the input
to IC5A.
(4)
Adjust R25, OFFSET 2, for minimum deflection on the front-
panel meter.
(5)
Move the grounding jumper from TP3 to TP1. This grounds
the input of IC4.
(6)
Set NULL on front panel, R8, to center of its range and
depress the 30-gauss (0.003-T) range button.
(7)
Adjust R16, OFFSET 1, for minimum deflection of the meter.
(8)
Remove the jumper at TP3 and the shorting plug or jumpers
used at J2. Then connect a stable, adjustable dc source,
capable of a few tenths of a volt, across J2-1 and J2-2, with
its positive side at J2-1. This serves as an input signal.
(9) Depress the 30-K (3-T) range button.
(10)
Adjust the dc input voltage until a digital voltmeter, con-
nected to J1, reads precisely 1.00 volt. Then adjust METER
CAL, R30, until M1 reads exactly full scale.
(11) Observe that the N polarity indicator, DS2, is illuminated
and that DS3 is not.
(12)
Reverse the polarity of the input signal so that J2-1 is
negative. The digital voltmeter should read — 1.00 volt.
Observe that the polarity indicator, DS3, is illuminated and
that DS2 is not.
(13)
The instrument is now calibrated and the combination of in-
strument and probe is now ready for calibration.