Guralp Systems Limited

Chapter 2. 2 First encounters

2.1 2.1 Handling notes

Although the 40T-1 has a rugged design, it is still a sensitive instrument, and can be damaged if mishandled. If you are at all unsure about the handling or installation of the device, you should contact Güralp Systems for assistance.

All parts of the 40T-1 are waterproof.

2.2 2.2 Connections

The instrument has an integrated cable ending in a 26-pin mil-spec socket which carries both power and output signals. This is suitable for connecting directly to a Güralp digitizer.

The breakout box, if ordered, provides individual signal and power connectors, or you can make up your own cable if you prefer.

2.2.1 The breakout box

If you are using a Güralp breakout box, it should be attached to the sensor through its SENSOR connector. Connectors are also provided at the CONTROL and RECORDER outputs, for attaching to a handheld control unit or a Güralp digitizer. If you have ordered a 40T-1 with optional high gain outputs, you will need to make up a suitable cable to expose these outputs.

The breakout box also provides a standard Güralp power connector on a 10-pin mil-spec plug. The 40T-1 draws a nominal current of 48 mA from a 12 V supply when in use; thus, using a 12 V, 25 Ah sealed heavy-duty lead-acid battery, you should expect the instrument to operate for around a week without recharging.

The CENTRE button switches the instrument into ACC/VEL mode whilst it is pressed. This mode allows you to monitor the mass positions whilst you adjust the offsets manually. If you prefer, you can use the equivalent switch on a Handheld Control Unit (see below.)

2.2.2 The handheld control unit

This portable control unit provides easy access to the seismometer's control commands, as well as displaying the output velocity and mass position (i.e. acceleration) on an analogue meter. Signal meter

The upper section of the HCU contains a simple voltmeter for monitoring various signals from the instrument. Calibration

You can calibrate a 40T-1 sensor through the HCU by connecting a signal generator across the yellow and green CALIBRATION SIGNAL inputs and setting the adjacent switch to ON. The sensor's response can now be monitored or recorded, and calibration calculations carried out. See Chapter 4, “Calibrating the 40T-1” for full details. Control commands

If you have ordered a 40T-1 with the remote null facility, you can zero its mass position offsets from the HCU. Banana plugs

The remainder of the HCU provides useful connections for each of the signal lines from the instrument, for attaching to your own equipment as necessary.

2.3 2.3 Zeroing the instrument

Before installing the 40T-1, you should check that the mass positions are not significantly offset from zero. The mass position offsets can be affected by any tilt to the instrument, as well as handling during transportation. The normal range of the mass positions is ±10 V; you should zero the instrument if any mass reads more than around ±3.5 V when the sensor is stationary.

The velocity outputs of the 40T-1 are set at the factory to a nominal value below ±3 mV. Once the instrument is installed and has reached thermal equilibrium with its environment, these outputs should be similar to the factory-set value.

2.3.1 Adjusting the mass position offsets manually

The 40T-1 has three potentiometers (“pots”) accessible within its casing, which should be used to remove any DC offsets electronically:

2.3.2 Adjusting the mass position offsets with a Handheld Control Unit

Some 40T-1 units are equipped with a remote mass centring option, which allows you to adjust the internal potentiometers by applying voltages across control lines to the sensor:

2.3.3 Zeroing a 40TD digital instrument

The offset potentiometers in a 40TD are in the same place as on the 40T. To access them, you will need to remove the digitizer module, which lies on top of the sensor itself. You can monitor the mass position outputs of the sensor using a Handheld Control Unit and an adapter cable, available from Güralp Systems.

To change the offsets of a 40TD without digital centring: