MODEL 4420 VIBRATING WIRE CRACKMETER
| MODEL 4420-3 LOW PROFILE CRACKMETER |
25
APPENDIX E.
MODEL 4420-3 LOW PROFILE
CRACKMETER
GEOKON
Model 4420-3 low profile vibrating wire crackmeters use a Model 4150
strain gauge mounted to the underside of a stainless steel plate, an adjustable
oval point set screw, and an instrument cable, which is secured and
weatherproofed by a Swagelok connector.
The gauge is mounted as a cantilever, with the cable end of the crackmeter fixed
to the mounting surface by a cap screw installed into an expansion anchor, with
the opposite end left free. This free end allows the cone point set screw to
maintain contact with the mounting surface (or reference disc) as it moves.
Strains are measured using the vibrating wire principle. As the mounting
surface rises or falls, it increases or decreases the strain in the vibrating wire
inside the 4150 gauge. This change in tension is measured as a change in the
resonant frequency of vibration of the wire.
The threaded rod and accompanying jam nuts allow the zero reading to be
adjusted as needed.
17:
FIGURE 17:
Model 4420-3 VW Low Profile Crackmeter Layout
E.1
INSTALLATION
E.1.1 PRELIMINARY TESTS
Perform a preliminary check by completing the following:
1.
Connect the gauge to a readout box using the instructions in Section 4.1.
2.
Observe the displayed readout, which usually will be between 1800 and
2500 digits. The temperature reading should match the ambient
temperature.
Checks of electrical continuity can be made using an ohmmeter. Resistance
between the gauge leads should be approximately 50 ohms. Remember to add
cable resistance, which is approximately 14.7
Ω
per 1000 feet (48.5
Ω
per km) of
22 AWG stranded copper leads at 20 °C. Multiply this factor by two to account
for both directions. Resistance between the green and white conductors will
vary based on temperature; see Table in Appendix B. Resistance between any
conductor and the shield should exceed two megohms.
Should any of these preliminary tests fail, see Section 6 for troubleshooting.
STRAIN GAUGE MOUNTING BLOCK
TYPICAL
SEPARATION
STRAIN GAUGE ELEMENT
1 in. OR 2 in. LONG
OVAL POINT
SETSCREW
LOCKNUT
REFERENCE SURFACE
(IF REQUIRED)
DROP IN
ANCHORS
MOUNTING BLOCK
SIGNAL CABLE
SECT. A-A
(1.500 in.) [38.10 mm]
(2.000 in.) [50.80 mm]
(1.125 in.)
[28.56 mm]
(1.250 in.)
[31.75 mm]
(1.000 in.)
[25.40 mm]
(11.500 in.) [292.10 mm]
A
A
(10.500 in.) [266.70 mm]