Inclinometers

The answer involves both the inclinometer probe and the orientation of the inclinometer casing. 1. The inclinometer probe contains two accelerometers. One accelerometer senses tilt in the plane of the wheels. The other accelerometer senses tilt in a plane rotated 90 degrees to the wheels. The drawing at right is a top view of the probe. If the bottom of the probe is held in a fixed position, moving the top of the probe in the direction of the arrows results in positive and negative readings. 2. Inclinometer casing has grooves that control the orientation of the inclinometer probe. Usually one set of grooves is aligned with the expected direction of movement (For slopes, this is usually downhill. For excavations, this is usually toward the area being excavated). The probe travels in this set of grooves. If the probe is inserted so that the upper wheel points to the expected direction of movement, then A0 pass through the casing will show positive tilt if there is actually tilt in that direction. 3. On a graph, positive movment is shown on the right side of the graph, assuming that casing grooves are aligned with the direction of movement and that the probe was inserted with the upper wheels pointing in the downhill direction for the first pass.

It is not a good idea to use an English probe with a metric cable (or vice versa). You will introduce errors and lots of complications. Here’s a suggestion of how you can switch over those installations from English to metric:

1. Rent an English Digitilt system and take a final survey of the inclinometer installations with the English system.
2. On the same day that you take the final English readings, take two or three surveys of the same installations with your metric system.
3. Reduce the English data, evaluate the displacements and file all the readings from the past. This closes the book on the English unit data.
4. From now on, use only your metric system. Choose one of your metric datasets and use it as the initial. If you must report information in English units, use DigiPro for Windows to generate English unit graphs and data from your metric readings.

At one time, we recommended regular recalibrations. Now we recommend recalibrations only for the following three conditions below.

Condition l: Your probe requires repair. After we repair your probe, we must recalibrate it.

Condition 2: You need the probe certified for your company’s quality program. In this case, we may not actually open the probe to make adjustments. First, we put the probe on our tilt table to generate a calibration form. Then, if the probe is found to be within tolerances, we print out the calibration form. However, if the probe is found to be out of tolerance, we open it, make the necessary adjustments, and then run the calibration test again.

Condition 3: If you follow ASTM Standard D7299 (http://www.astm.org/Standards/D7299.htm) and readings in the check stand or in an inclinometer installed on stable ground show drifting or unusual readings.

Summary: If your readings and graphs are consistent, and there is no obvious need for repair, we recommend that you continue using the probe as is. Recalibration is unlikely to significantly improve the performance of a probe that is already working well.

Note that the probe is a special case. Other instruments, particularly readouts, can benefit from annual recalibrations.

These values are used for manufacturing quality checks. They assure us that the accelerometers are aligned properly and working properly. The values have no practical application for end users.

Slope Indicator’s manuals recommend that users lower the probe to the bottom of the borehole and wait 5 to 10 minutes before starting the survey. This “warm-up” period allows (1) the electronics of the probe to warm up and stabilize and (2) the mechanicals of the probe (wheels and housing) to match the temperature of the water in the casing. What happens if the user omits the warm-up period?

Readings taken during the first 5 or 10 minutes of the survey may contain an offset that later readings do not. When readings are processed, the offset error may look like movement at the bottom of the borehole. How severe are the errors? The offset error will be smallest when a warm probe (25 to 30°) is lowered into 12°C water, which is the typical groundwater temperature in moderate climates. In such conditions, a shorter, 3 to 5 minute warm-up period will provide good results. The offset error will be greatest when the probe is cold. For this reason, be sure to allow at least 10 minutes warmup time for cold probes. Note that the readout’s Ready signal does not monitor the warm-up period and should not be used for that purpose.

This is a problem that is exhibited by all inclinometer probes. The wheels of the probe, which travel in the ‘A’ grooves, are slightly narrower than the grooves in the casing. That means the wheels have some freedom to move from side to side within the grooves. The A axis of the probe is in-line with the wheels, so it is not affected by side to side movement of the wheels. Also, the spring loading of the wheels prevents any ‘wobbling’ in the A axis. The B axis of the probe is directly affected by side to side movements, which appear as small changes in inclination. Why don’t we make the wheels fit the grooves perfectly? If the wheels fit perfectly, the probe would be forced out of the grooves when the casing deformed, and then repeatability would be very bad. Slope Indicator is working on another solution to this problem, which we will announce in a few months.

Calculate the difference between the inside diameter of the casing and the outside diameter of the probe. For example, 85 mm casing has a 73 mm inside diameter and the probe body has an outside diameter of 24.5 mm, so the maximum allowed deformation would be 48.5 mm.

Complex Answer: This answer assumes a constant radius curve and describes tracking limits in terms of reading units. 

 Probes are interchangeable when they leave the factory – they can be interchanged and still meet the system accuracy spec that we list on our brochures ( .025 feet per 100 feet). However, soon after being put into service, each probe begins to acquire its own characteristics – a bump here, a knock there, a severe curve in the casing, etc. After a while, correcting the output of one probe to match that of another probe becomes quite difficult and would involve a lot of record keeping. Control cables can also acquire their own characteristics, depending on how they are used. For this reason, we recommend using the same probe and same cable for any particular installation. Also see the question below.

If you are using the same probe and cable as the previous consultant, you should have no problem using the original baseline data. However, if you are using a different probe (or a new probe), several variables come into play. The most important of these are the profile of the casing as installed and the condition of the two probes and cable.

If the casing was installed vertical and straight, differences between probes will be minimized. Vertical casing shows less than 3 degrees of cumulative deviation from the bottom to the top of the casing. You can check this by making a cumulative deviation plot. and calculating the overall degree of tilt. If you find more than three degrees of tilt in either axis, there is a good chance that you’ll see a difference in the readings of the two probes. Tilt in the B axis will affect A readings, and tilt in the A axis will affect B readings.

Casing is considered straight if it does not snake from side to side within the borehole. You can check this with an incremental deviation plot. In the ideal plot, lines would be vertical. Non-vertical lines indicate waviness in the casing. In “wavy” casing, depth control becomes an issue, and slight differences between your old control cable and your new control cable can make difference in readings.

The easiest way to check compatibility is to take about three surveys with your own probe and compare them to the latest survey taken with the previous probe. (If you have a completely new probe, run it up and down in the casing about five times to break it in). If the resulting cumulative displacement plots overlay the latest survey from the old system, you’re in luck. But if you see significant differences, then you have two choices:

1. Complete the old series of surveys by printing a final summary of cumulative displacement and time vs displacement. Also print a comparison of the final survey with the old system to the initial surveys take with the new system. Then archive that data. Use one of the three surveys you obtained with the new system to start a new series. This is generally the most satisfactory method in the long term. Here’s the logic behind it: you know the magnitude and rate of movements you were detecting with the old system. If movements are increasing, you will catch them with the new series. If the ground has stabilized, the new series will tell you that, too.

2. Your second choice is to correct every survey taken with the new system to make it compatible with the old system. This is a lot of work, for very little reward. For this reason, we recommend the choice above.

 A jump in checksums does not necessarily indicate a problem with data or the probe. However, if your probe becomes less consistent – i.e. you start seeing frequent jumps in checksums, it may be time to have your proble checked. The SD of checksums that you report indicates that there is no problem now.

Checksums of 10 or 20 are actually within the spec of the probe as it leaves the factory. So long as the checksums are consistent, there will be no problem with the data. The main component of a checksum is the bias of the probe. (A check sum is 2 x the bias of the probe). The 0 and 180 surveys effectively cancel the bias, so data are not affected. However, there are two conditions that you should watch for:

(1) Check for large changes in the mean checksum from survey to survey. Use DigiPro’s checksum graph: If plots are close to each other, there is no error. If plots are separated widely, there may be bias-shift errors.

(2) Check for drifting checksums – checksums growing consistently larger or smaller from bottom to top. Use DigiPro’s checksum plot again. Each plot should be vertical. If the plot tilts to the right or left, there is drift. This is probably an indication that an electronics board is bad.

This is a very common question, so we have prepared a tech note on inclinometer accuracy. Hopefully, it will answer your question fully.