Hydra measures analog inputs of dc and ac volts, thermocouple and RTD temperatures, resistance, and frequency. It features 21 measurement input channels. In addition, it contains eight digital input/output lines, one totalizing input, one external scan trigger input, and four alarm output lines. Hydra is fully portable and can be ac or dc powered. An RS-232 computer interface is standard. An optional IEEE-488 computer interface is available for the Hydra Data Acquisition Unit (2620A) only. The Hydra Data Logger (2625A) adds substantial measurement memory capabilities. The RS-232 computer interface is standard, but IEEE-488 capability is not available for the Hydra Data Logger. The Hydra Data Bucket (2635A) adds more flexible storage for instrument setups and measurement data by adding a PCMCIA memory card and interface. The amount of storage can be easily changed by selecting a memory card of the appropriate size for the job. The Hydra instruments share many features and functions. The term "instrument" is used to refer to all three instruments. The model number (2620A, 2625A, or 2635A) is used when discussing features unique to one instrument. The instrument is designed for bench-top, field service, and system applications. A dual vacuum-fluorescent display uses combinations of alphanumeric characters and descriptive annunciators to provide prompting and measurement information during setup and operation modes. Channel Scanning Can be continuous scanning, scanning at an interval time, single scans, or triggered (internal or external) scans. · Channel Monitoring Make measurements on a single channel and view these measurements on the display. · Channel Scanning and Monitoring View measurements made for the monitor channel while scanning of all active channels continues. · Multi-Function Display Left (numeric) display shows measurement readings; also used when setting numeric parameters. Right (alphanumeric) display used for numeric entries, channel number selection and display, status information, and operator prompts. · Front-Panel Operation Almost all operations can be readily controlled with the buttons on the front panel. · Measurement Input Function and Range Volts dc (VDC), volts ac (VAC), frequency (Hz), and resistance (Ω) inputs can be specified in a fixed measurement range. Autoranging, which allows the instrument to use the measurement range providing the optimum resolution, can also be selected. · Temperature Measurement Thermocouple types J, K, E, T, N, R, S, B, and Hoskins Engineering Co. type C are supported.Also, DIN/IEC 751 (Pt 385) Platinum RTDs are supported. · Totalize Events on the Totalizing Input · Alarms Limits and Digital Output Alarm Indication · 4-Terminal Resistance Measurements (Ch. 1 .. 10) · RS-232 Computer Interface Operation · Measurement Rate Selection · Nonvolatile Memory Storage of minimum, maximum, and most recent measurements for all scanned channels. Storage of Computer Interface setup, channel configurations, and calibration values. · Features unique to the 2625A Data Logger. Storage of measurement data: storage for 2047 scans of up to 21 channels, representing up to 42,987 readings. · Features unique to the 2635A Data Bucket. Internal storage of measurement data for 100 scans of up to 21 channels, representing up to 2,100 readings. Memory card storage of instrument setup configurations so that instrument may be quickly set up to do different tasks. Memory card storage of measurement data for up to 4,800 scans of 10 channels on a 256K-byte card or up to 19,800 scans of 10 channels on a 1M-byte memory card. Enhanced RS-232 interface with higher baud rates and hardware flow control using the Clear to Send modem control signal.