Accessories for ocean bottom instruments
Deep-sea release mechanism
The Güralp Systems Deep-Sea Release Unit is a robust, compact, electrically actuated cable release mechanism for instrumentation deployment and recovery systems.
The release mechanism in use.
The release unit employs simple and efficient internal linkages for complete reliability over extreme time periods with very low power requirements. An oil-filled housing permits use at deep ocean sites, with each unit being functionally tested at its working pressure. The mechanism has been tested in the field to a depth of 7 km over 700 cycles without failure.
Limit switches are fitted to prevent mechanical over-travel. Control is straightforward, performed using a standard Subconn two-pin connector.
The standard unit is rated at 500kg with a maximum release link thickness of 10mm. Other sizes may be supplied to customer requirements.
Precision real-time clock module
This autonomous module is designed to provide a precision timebase for data acquisition systems. The unit contains a microprocessor-controlled, temperature compensated quartz oscillator and real time clock counters. Once synchronized with a GPS time source, it can maintain time-keeping to an accuracy of 1.2 × 10-8 using a built-in lithium primary cell. The graph below shows a Real Time Clock unit performing with a drift below 1 ms in 48 hours.
Low drift of the RTC unit against GPS signals (click to enlarge).
The unit provides several different output frequencies and a real-time calendar clock (in BCD format) via an I2C bus. It provides all the frequencies used by the DM24 or DM16. The primary frequency is used by the system microprocessor and the 1.024M and 2kHz signals are used by the analogue-to-digital converters, providing accurately sampled data.
The clock runs from the system's 5 V logic supply in normal use, and only needs to use the internal battery for standby operation (between system setup and test and deployment.) A software-controlled switch on the battery allows you to save power by switching the clock unit off in storage and transit.
The unit is useful wherever GPS is not available, or power consumption must be kept to a minimum—particularly for ocean bottom applications.
Features and specifications
| Crystal frequency | 8.192 MHz |
| Stability | 1.2 × 10-8 (0 − 50°C) |
| Output frequencies | 8.192, 1.024 MHz; 2.000 kHz; 50 Hz; 1Hz (aligned to UTC) |
| Data output (I2C) | 4-year calendar clock, y,mm,dd,hh,mm,ss,hh BCD format |
| Control inputs (TTL/CMOS levels) | Internal Battery Switch (I2C) 1Hz reference (from GPS 1pps) Frequency trim (serial 2400 Baud) |
| Power Supply | 5 V @ 10 mA from system 3.5 V @ 7 mA on internal backup battery (provides typically 100 days standby operation) |
Using the real-time clock with Güralp digitizers
The unit provides all the frequencies required for the CMG-DM24 and CMG-DM16R8 digitizers. The primary frequency is used by the system microprocessor and the 1.024 MHz and 2 kHz signals are used by the analogue-to-digital converters, providing accurately sampled data. The digitizer software automatically synchronises the unit to a GPS receiver and trims its frequency for minimum error.