Geokon 4422, Instruction Manual

Page: 7 / 21

1
1. INTRODUCTION
Geokon Model 4422 Monument Crackmeters are designed to measure movement across joints
and cracks in monuments. The small size is designed to render the crackmeter as unobtrusive as
possible. The shaft of the Monument Crackmeter has three small holes drilled in it. A metal pin
is supplied for insertion inside one of these holes. These holes and the metal pin are designed to
assist the user in selecting the range of the crackmeter so that it can be set to measure mainly
tensions, mainly compressions, or both, depending on which hole the metal pin is inserted. The
maximum range is four mm.
The instrument consists of a vibrating wire sensing element in series with a heat treated, stress
relieved spring, which is connected to the vibrating wire at one end, and to a connecting rod at
the other. As the connecting rod is pulled out from the gage body, the spring is elongated,
causing an increase in tension in the vibrating wire. The increase in tension (strain) of the wire is
directly proportional to the extension of the connecting rod. This change in strain allows the
Monument Crackmeter to measure the opening of a joint very accurately.
Monument Crackmeters are designed to be read by one of the various readout boxes available
from Geokon.
2. INSTALLATION
2.1 Preliminary Tests
Upon receipt of the instrument, the gage should be checked for proper operation (including the
thermistor). The Crackmeter normally arrives with its shaft secured at approximately 50% of its
range, by the metal pin placed inside the middle of the three holes, (see Figure 1). This holds the
instrument in tension in its midrange position. (This also helps protect it during shipping).
CAUTION! Do not rotate the shaft of the Crackmeter more than 180 degrees. This may
cause irreparable damage to the instrument. Never extend the crackmeter beyond its
working range.
Connect the gage to the readout box and take a reading. (See Section 3 for readout instructions.)
The midrange position should give a reading of about 4500 on Channel B. Gently pull on the
ends of the gage and the readings should be stable and in the range of 2000 to 7000 on Channel
B.
Checks of electrical continuity can also be made using an ohmmeter. Resistance between the
gage leads should be approximately 50 ohms, ±5 ohms. Remember to add cable resistance when
checking (22 AWG stranded copper leads are approximately 14.7
Ω
/1000' or 48.5
Ω
/km, multiply
by two for both directions). Between the green and white should be approximately 3000 ohms at
25° (see Table 6), and between any conductor and the shield should exceed two megohms.