CT-DOG (Coiled Tubing Diameter Ovality Gauge)

The coiled tubing diameter and ovality gauge (CT-DOG) is an economical, real-time monitoring device for detecting tubing dimensional imperfections.

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  • Ruler Specifications
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Detect imperfections while coiled tubing is running in or out

A microprocessor in the digital display unit analyzes the measurements and displays ovality as well as minimum, maximum, and average diameters. These data are also available through an RS232 interface to our Orion™ data acquisition system or other computers. The CT-DOG is made up of a sensor head with 12 probes, a dual-function cable for power and data transmission, an operator interface and signal processing module, and the power supply.

Principle of operation

The cylindrical sensor head surrounding the coiled tubing contains three rings, each with four eddy current proximity probes in a radial array. The 90° spacing between the radials gives six OD measurements with 30° spacing around the coiled tubing. The sensing element of each probe faces the work string in the center of the ring, and each probe measures the distance from its face to the surface of the coiled tubing. Two diametrically opposite probes form a measurement pair. For a given measurement pair, the difference between the probe-to-probe spacing and the sum of these gap measurements is the OD. Thus, each ring provides two orthogonal diameters and a measure of the ovality of the coiled tubing cross-section. Ovality is twice the difference of the two diameters divided by their sum: 2 × (ODmax - ODmin)/( ODmax + ODmin). The cylindrical sensor head splits lengthwise in a manner that allows it to close around the work string. Thus, CT-DOG can be installed on the coiled tubing between the reel and injector head at any time during operations. The sensor head attaches rigidly to the reel-level wind. A single cable connects the sensor head to the remotely located operator interface and signal processing module.

Operator interface and signal processing module

  • Supplies power for the probes
  • Converts the analog signal from each probe into an 8-bit digital signal
  • Calculates diameters from the probe readings
  • Calculates the maximum and minimum OD and the maximum ovality for the scanning interval (usually about 1 second)
  • Digitally displays for calculated values
  • Transmits calculated values to external computer via an RS232 interface
  • Compares calculated values against user-defined limits and trigger alarms
  • User interface for calibrating the sensor head and setting alarm limits


  • Digitally displays maximum, minimum, and average OD; ovality percentage; individual and paired sensor diameters; and alarm flags
  • User-defined alarms for diameters and ovality
  • Audible alarms for out-of-limit conditions
  • Display updates once per second
  • Keypad user interface
  • Field calibration of probe spacing
  • Installation on coiled tubing at any time during operations
  • Bidirectional measurement
  • Easily maintained

CT-DOG Specifications

 Diameter measurements  6 (30° radials)
 Coiled tubing OD range  1¼ to 2⅞ in.
 Axial resolution (100 ft/min)  0.120 in.
 OD measurement accuracy  + .010 in.
 Temperature sensitivity  0.0004 in./°C
 Bidirectional  Yes
 Self-contained electronics/display  Yes
 Orion™ compatibility  Yes
 Digital data output capability  RS232
 Tool head dimensions  20-in. OD × 19-in. L
 Tool head weight  57 lb
 Power  110 VAC or 12 VDC
 Integrated length measurements Yes
  • CT-DOG Flyer

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