Specifications for the Gx Data-Logger

Given the nature of the environments in which pipeline surveys are typically conducted and the nature of the actual voltages that are logged by data-loggers on these types of surveys, the voltmeter component of the data-logger has to satisfy a number of important performance criteria.

Voltmeter Specifications

The performance criteria that are particularly significant for a voltmeter employed in pipeline survey work are as follows:

High Voltage Isolation.

Since surveys are typically conducted in the vicinity of electrical devices and power line systems, and, since transient over-voltages can occur frequently and at high levels in today’s electrical systems, it is imperative that a high degree of voltage isolation be provided to ensure operator safety.

The IEC (International Electrotechnical Commission) has specified that voltmeters employed in Category IV environments, which are environments typically encountered during survey work (outside areas in the vicinity of electrical devices and power lines), must provide at least 1kV isolation (steady-state voltage) and at least 12kV transient (momentary) overload protection. It is critical that purchasers of data-loggers that are intended for use in such environments be aware of these (minimum) requirements.

The M. C. Miller Co. (MCM) voltmeter , which is used in all MCM data-loggers including the G1, is designed for 3kV (steady-state) isolation (tested to 1kV) and 50kV transient voltage isolation (tested to 15kV), and thus is designed to exceed the IEC Standard for Category IV environments. On the other hand, the National Instruments voltmeter models (PCMCIA NI4050 and NI4060) provide no high voltage isolation, while model NI4070 only provides up to 300V (steady-state) isolation. [NI voltmeter cards are used, for example, in Allegro data-loggers]. Consequently, it is imperative, from an operator’s safety standpoint, to be aware of the specific voltmeter PCMCIA card that is employed in any data-logger you might be considering for site survey work and to be aware of any associated liability issues.

Voltmeter Response Time and Resolution.

In cases where fast rectifier current interruption cycles are to be in-effect during surveys [on the order of 1Hz, for example (perhaps 0.7 sec. On/0.3 sec. Off cycles)], it is imperative that the voltmeter response time be fast in order that true On/Off voltages can be measured.

It is particularly important that the response time be fast even when the input impedance of the voltmeter is on a high setting.  A high input impedance setting is required for a voltmeter when the source resistance associated with a measurement is high, for example, in a high soil resistivity situation, in order that a voltage measurement is not affected by the source resistance and a true reading is presented.

Consequently, in survey work where poor contact (of reference electrode data-probe with the soil) can occur and high soil resistances can be encountered (both of which impact the “source resistance”), it is important to employ a voltmeter that has a high input impedance setting option. In addition, it is critical, in the case of fast interruption cycling, that the high input impedance setting also be a fast response setting. Furthermore, the voltmeter setting should also provide a high resolution (high degree of accuracy).

The MCM voltmeter has a setting which satisfies all three of these criteria simultaneously, which is the 5.7Volt, 400MΩ setting. This setting provides a response time of 80ms and a resolution of 0.17mV, while presenting an input impedance of 400MΩ.

Noise and AC Interference Filtering on Fast Cycle Setting.

Since AC interference and noise can be encountered on buried pipelines, it is imperative that the voltmeter does not saturate at the voltage levels that might typically be induced, which would lead to erroneous On/Off pipe-to-soil voltage readings. It is also imperative that the fast cycling On/Off potentials can be “extracted” from the background noise.

The MCM voltmeter will not saturate even with an AC signal as large as 120V appearing at the input and will maintain sufficient speed to process the fast cycle waveform data. The MCM voltmeter accomplishes this feat by using a sophisticated combination of analog pre-processing and digital signal processing (DSP). The sampling rate for the DSP is 3600 times per second.

Other Key Specifications (Hardware & Software):

CATEGORY	Gx  SPECIFICATION
Computer Related Specifications
Rugged Classification	Ultra-Rugged Handheld Computer (Die Cast magnesium alloy and high impact plastic)
Environmental Specifications	Exceeds MIL STD 810F Rating Survives 6 foot drops to concrete IP 67 Rating (dust/water intrusion) Operating Temp. -4°F to 122 °F (-20°C to 50°C) Storage Temp. -4°F to 140°F (-20°C to 60°C)
Processor	Xscale processor (624MHz)
Memory	256MB SDRAM;
Non-Volatile File Storage (including survey files)	2GB SD card
Operating System	Windows CE 5.0
Main Power Supply	Two re-chargeable 2500mAh Li-Ion batteries (5000mAh total capacity)
Battery Life (Main Power Supply)	40+ hours continuous operating
RAM Retention (survey programs are stored on RAM)	If main battery pack is allowed to fully-discharge or is being replaced, a back-up battery provides 40 hours of RAM retention time.
Display Screen	320x240 pixel (3.5 inch) color LCD touch-screen (indoor/outdoor viewing)
Communication Port (for survey data)	Serial (RS232) & USB ports, Bluetooth-enabled
Dimensions	12.5”(L) x 5.31” (D) x 2.31” (W)
System Weight	2.64 lbs
CP Survey Related Specifications
Annual, bi-monthly etc. Site Surveys (both continuous and current-interrupted)	Yes (Gx-SS app)
Trigger-mode CIS	Yes (Gx-PLS app)
Timer-mode CIS	Yes (Gx-PLS app)
DCVG surveys	Yes (Gx-PLS app)
Combined DCVG/CIS surveys	Yes (Gx-PLS app)
Stationary data-logger surveys	Yes (Gx-Time Chart Recorder app)
Internal GPS receiver	Yes
External GPS receiver ready	Yes
Built-in MCM Voltmeter Specifications
Options for DC range and input impedance settings	5.7V, 400MΩ  57V, 75MΩ 57V, 400MΩ    570V, 75MΩ 40V, 75MΩ      40mV, 10MΩ 400V, 75MΩ     400mV, 10MΩ
Options for AC range and input impedance settings	40V, 75MVΩ 400V, 75MΩ
Voltmeter reading style options	Single reading On/Off pairs On/Off pairs (min/max) On/Off pairs (GPS Sync)