Chapter 2. Introduction
Mass recentring should not be necessary if the sensor is handled and operated in accordance with the user manual.
There may, however, be occasions when the instrument has received a strong mechanical shock which has resulted in the masses moving away from their central position.
This procedure describes the method for recentring the masses in Güralp 6-series instruments. It also applies to legacy EDU-T and EDU-V instruments.
Caution: Seismometers contain extremely sensitive mechanical components. Only ever open the casing of an instrument in a clean environment and take precautions to prevent any ingress of dust into the body of the instrument.
Caution - ESD : The circuit boards contained within the instrument include components which can be damaged by electrostatic discharge. Always work on a properly grounded dissipative surface and wear a suitable grounded wristband. Ground yourself by touching an earthed conductor before handling any of the circuit boards.
2.1 Applicability to 6TDs
The built-in CD24 digitiser in a 6TD only provides one-sample-per-second data-streams for monitoring the mass position outputs. The resulting packetisation delays make this process too slow to provide practical feedback to the operator. In order to recentre the masses in a 6TD, it is necessary to remove the digital lid and replace it with either an analogue lid, if available, or, if not, with an analogue test cable. This allows the analogue mass position output signals to be monitored in real-time using a hand-held control unit.
The analogue test cable can be ordered from Güralp as part number CAS-PEP-0040 or assembled using the schematic in section 4.1.
An alternative analogue test cable for use with a voltmeter rather than a hand-held control unit can also be used. This can be constructed using the schematic in section 4.2.