Digitilt AT System FAQ
This FAQ compares Classic and AT systems. See the AT Reader FAQ for questions about operation.
Q: Can the AT System be used side by side with the classic Digitilt system?
A: Yes, if you upgrade your DigiPro software to DigiPro2. DigiPro2 can import all of your classic surveys. After that, you can keep AT surveys and Classic surveys in the same project database; however, it is not recommended that surveys taken with one probe (classic or AT) be compared to initial readings taken by another probe (classic or AT).
Q: Can I freely interchange Classic and AT probes?
A: No. The Classic probe is analog and the AT probe is digital. They have different power requirements and different readouts. Thus they are not freely interchangeable.
We do not recommend interchanging probes, in any case. Differences between probes are amplified by tilt and curvature of the casing, even with probes of the same type. The AT system features new cable marks and a cable gate, which introduces even more differences.
Best Practice: Some users obtain separate baseline surveys with two systems: an "active" system and a "backup" system. Subsequent surveys are taken with active system only. If that system fails, the backup system and its baseline survey can be used to continued the monitoring program. In this scenario, AT and classic systems would work well as either active or backup systems.
Q: Are AT cables different from classic cables?
A: Yes, there are three main differences:
Q: What is the advantage of measuring cable marks from the top wheels?
A: The AT system displays readings as mm or inches of tilt, and this reading applies at the depth of the top wheels. Scroll downwards to see a technical explanation with a graphic. Classic cable marks are measured from the middle of the probe, but this has some unexpected effects when readings are plotted:
Q: What is the advantage of the cable gate?
A: The cable gate provides an easy way to align cable marks to the top of the casing. With the pulley assembly, depth marks are typically aligned with the cleats at the top of the pulley, which is 0.3m or 1 foot above the top of the casing.
Summary: If classic surveys are not corrected for the pulley offset and the middle of the probe measurements, plots will show displacements a full interval deeper than they actually are.
Q: What is the technical reason for measuring cable marks from the top wheels of the probe?
A: The drawing shows a probe in tilted casing. We can overlay the probe with a right triangle. The vertical side of the triangle is the gravity vector. The hypotenuse of the triangle is the gauge length - the distance between the top and bottom wheels of the probe. You can see the tilt angle at the bottom of the triangle.
The tilt angle applies anywhere along the length of L. The Digitilt DataMate displays readings in sine units (the sine of the angle of tilt), which also apply anywhere along the length of L.
The AT Reader records sine units, but when it displays a reading, it multiplies the sine value by L, the gauge length of the sensor (500mm or 24 inches) to display a reading in mm or inches. This value applies only at the top of the triangle, as in the illustration, which is the elevation of the top wheels of the probe. Thus we measure cable marks from the top wheels of the probe, so that the reading is shown at the proper depth.
Q: My Classic system finally stopped working. Can I continue monitoring with an AT system?
A: Yes. We strongly recommend that you start a new baseline, whether the replacement system is classic or AT. By starting a new baseline, you avoid the complications (rotation and sensitivity differences) that can appear when one probe is substituted for another.
If starting a new baseline is not acceptable, you can achieve a better comparison by ensuring that the AT probe is positioned at the same elevation that the classic probe was. This would seem to be an easy task, but there are some details to consider.
Different Indexes: To control the elevation of the probe, we align depth marks on the cable to an index. The AT system uses a cable gate. The cable gate aligns depth marks with the top of the casing, so we can say that the AT index is the top of the casing.
With Classic systems, the index is often the top of the pulley assembly, which is 1 foot (0.3m) above the top of the casing. Thus for a given depth mark, the Classic probe is actually 1 foot (0.3m) higher than the AT probe.
Different Depth Marks: Depth marks on AT cable are measured from the top wheels of the probe. Depth marks on Classic cable are measured from the middle of the probe. The difference is 1 foot with English systems and 0.25m with metric systems. Thus for a given depth mark, the top wheels of the Classic probe are 1 foot (0.25m) higher than the elevation of the top wheels of the AT probe.
Summary: Assuming that the pulley assembly is used as the index, the top wheels of the Classic probe are a 2 feet or 0.55m) higher than the the top wheels of the AT probe. Even if the Classic system uses the top of the casing as the index, the top wheels of the Classic probe are still 1 foot or 0.25m higher than the AT probe.
If your Classic survey used a pulley assembly as index: Start the AT survey a full interval (0.5m or 2 feet ) shallower than the Classic survey. Also, edit the Classic survey to remove any depths shallower than 1m or 4 feet.
If your Classic survey used the top of the casing as index: Make a short casing extension (0.25m for metric systems or 1 foot for English systems) to use with the AT cable gate. Now the AT probe will be held at the same depths as the Classic probe.