MORROW INSTRUMENTS TECHNICAL PRODUCTS DIVISION

OPERATING & SERVICE MANUAL

MODEL 7

FIELD RESONANCE RECORDER

DOCUMENT MI-7-OSM-B
REVISION B
ISSUED JUNE 1978
NOTICE

This publication contains operating and service information applicable to Model 7 instruments fitted with Series B control assemblies. Earlier instruments may differ in minor panel details.

1

GENERAL DESCRIPTION

The Model 7 Field Resonance Recorder is a portable, self-contained indicating instrument intended for comparative field measurements where conventional reference equipment is impractical.

Measurements are displayed directly on a damped moving-coil meter. Three selectable operating ranges permit observation of both low-level background conditions and localized variations. A self-test reference circuit is provided for field calibration.

NOTE

The Model 7 provides comparative readings only. Instrument indications shall not be interpreted as absolute measurements unless correlated against a known field reference.

FIGURE 2-1. FRONT PANEL
FIELD LEVEL RANGE ZERO INPUT A INPUT B CAL REF 1 2 3 4 5
  1. Indicating meter
  2. Range selector
  3. Zero control
  4. Field inputs
  5. Calibration control
2

CONTROLS AND INDICATORS

CONTROL FUNCTION
RANGE Selects nominal sensitivity of 1, 10, 25 or 50 units.
ZERO Establishes meter reference prior to measurement.
INPUT A Primary field probe connection.
INPUT B Auxiliary or comparison probe connection.
CAL Applies internal reference signal to the indicating circuit.
REF Illuminates when internal reference conditions are within tolerance.
3

PREPARATION FOR USE

  1. Place the instrument on a stable surface with the front panel approximately level.
  2. Set RANGE selector to 1.
  3. Disconnect both field inputs.
  4. Allow the instrument to stabilize for not less than three minutes.
  5. Adjust ZERO until the meter rests at the left calibration mark.
  6. Connect the required field probe before selecting a higher range.
CAUTION

Do not move the RANGE selector beyond position 25 with an unterminated probe lead connected.

4

FIELD CALIBRATION

Field calibration should be performed at the beginning of each operating period and following any movement exceeding approximately five miles.

STANDARD CHECK

  1. Disconnect INPUT A and INPUT B.
  2. Set RANGE to 10.
  3. Establish zero.
  4. Depress and hold CAL.
  5. Observe meter indication after two seconds.
0
CAL
25

The pointer shall remain within the marked CAL region. If the indication falls outside this region, repeat the zero adjustment. Persistent error indicates battery or reference-circuit service is required.

NOTE

Readings obtained within 20 ft of permanent survey markers may be unreliable. Relocate the instrument before repeating the calibration procedure.

5

OPERATING PROCEDURE

Select the lowest range which permits the meter indication to remain on scale. Record range setting, approximate position, probe orientation and environmental conditions with each reading.

When traversing a line, maintain probe height and orientation as consistently as field conditions permit. Readings should normally be taken at intervals not exceeding 100 ft.

FIELD RECORD — EXAMPLE 
STA.      RANGE      READING      REMARKS
17+00       10         3.2        normal
18+00       10         3.5        —
19+00       10         7.8        repeat
19+25       25        16.1        localized
20+00       10         3.6        normal
6

INTERPRETATION OF READINGS

Isolated readings shall not be considered significant unless reproducible after relocation and recalibration. Where practical, approach the area from a second direction and compare results.

A slowly varying indication generally reflects broad background change. Rapid localized deflection should be checked against buried conductors, vehicles, fencing and survey hardware before being recorded as a field variation.

NOTE

If the pointer remains above 18 with INPUT A and INPUT B disconnected, move the instrument not less than 50 ft and repeat the complete zeroing procedure.

FIGURE 6-1. FUNCTIONAL ARRANGEMENT
FIELD
PROBE
→
INPUT
BRIDGE
→
SELECTIVE
AMPLIFIER
→
INDICATING
METER
REFERENCE OSCILLATOR CAL
7

MAINTENANCE

No routine internal adjustment is required. Keep connectors free of moisture and abrasive material. Clean the front panel with a damp cloth only. Solvents shall not be used on meter glazing or control markings.

Battery condition should be checked following extended storage and before each field assignment exceeding five days.

WARNING

Do not attempt calibration if the reference tone is audible without headphones. Remove power and return the instrument for service.

8

TROUBLESHOOTING

CONDITION PROBABLE CAUSE CORRECTIVE ACTION
No meter indication Battery discharged or probe open Check battery and cable continuity
Cannot establish zero Insufficient stabilization time Allow five minutes and repeat
CAL indication low Battery below operating voltage Replace battery
Reading changes when case is moved Nearby metallic object or reference disturbance Relocate instrument
Repeated deflection with inputs disconnected Undetermined Record location. Discontinue measurement.
9

SPECIFICATIONS

INDICATION
Damped moving-coil meter, 0–25 scale
RANGES
1 / 10 / 25 / 50
INPUTS
Two shielded field-probe connectors
STABILIZATION
3 min nominal at 68°F
OPERATING TEMPERATURE
20°F to 120°F
POWER
Internal dry battery assembly
WEIGHT
11.8 lb including battery
CASE
Sealed portable enclosure with carrying handle

REVISION RECORD

REV. DATE DESCRIPTION
A FEB 1977 Initial issue
B JUN 1978 Field calibration procedure revised