Inclinometer system – portable
A portable device used to monitor movements and deformations of soil, rock including retaining structures. It can be used vertically, inclined and horizontally, depending upon its configuration. It would be moved from installation to installation and commonly comprises of the following components:
- Probe – 500 x 30mm diameter wheeled instrument housing MEMS measurement sensor(s) which runs down and up inclinometer casing to obtain a measurement profile. In some cases, the calibration is built into the sensor(s) so that probes are interchangeable with other components.
- Cable – a Kevlar reinforced cable which connects the probe to a communication module. It has swaged cable markers every 500mm which are used to position the probe at precisely 500mm intervals along a casing.
- Cable reel – a lightweight reel for the cable which is connected to the probe and used to lower and raise the probe within the casing. It contains the wireless Bluetooth communication module which connects the probe to the PDA readout (Rugged Field PC).
- Readout - the PDA (Rugged Field PC) acts as a data collector, allowing data to be viewed at the borehole with graphical comparison to previous data sets.
- Accessories – include a carrying case, battery charger, spare battery and cable support gates.
Inclinometer system – In-place (IPI)
A series of MEMS tilt sensors in sealed bodies, used to monitor lateral movements and deformations of soil, rock including retaining structures. The string can be installed vertically, inclined or horizontally, depending upon its configuration and would be left in-place during its monitoring life. A system generally comprises the following components:
- Bottom sensor – A 500mm long x 30mm diameter wheeled sensor body which sits at the base of the borehole and has provision for a support rope to be attached for security during installation and retrieval.
- Extension rod – Rods which connect the series of sensor bodies together to create a complete profile within the installation. These can vary in length depending upon the spacing required.
- Standard sensor – A 500mm long x 30mm diameter wheeled sensor body which is connected, via an extension rod, to a series of other sensor bodies.
- Support – A component to support the complete string of sensors within the borehole, which includes a top hanger and top wheel assembly.
A specially manufactured tubing with four precise keyways formed at 90 degrees. The casing allows accurate installation of portable and In-place inclinometers.
At any position within the inclinometer casing, three readings are recorded. The depth and the inclination ( tilt ) of the sensors in two perpendicular directions, sometime described as A & B. In some cases the B direction is ignored or not used.
To minimise possible system errors, readings using portable inclinometers are each recorded twice. The probe (in which the sensors are housed ) is first read in the primary direction, then rotated through 180 degrees and the whole survey re-read. Readings are sometimes referred to as the A+ & A- directions. During data reduction, because there are two readings, The A- reading is subtracted from the A+ and the resultant is divided by 2. This maintains the direction (sign) of the movement , indicated by a positive or negative sign.
Prior to any monitoring of a manually read inclinometer installation, a series of sets of inclinometer surveys are recorded (usually 3sets ) in a particular casing. These are called the Base Readings and usually one set is selected as the basis for all future calculations for this installation.
- Deviation - An inclinometer sensor actually measures tilt, rather than lateral movement and therefore the basic principle requires calculations using the sine function, an angle, and the hypotenuse of a right angled triangle. This produces a value for the ‘Deviation from Vertical’. The software in the readout often carries out the conversion of the’ tilt’ to an ‘offset’ over the length of the probe, a value in mm will indicate that the top of the probe is offset from the bottom by this value; the Deviation from Vertical. The wheel centres are the ‘gauge length’ of the probe, over which the measurements are made.
- Displacements - Displacements are calculated by comparing the current readings to the ‘Base’ or previous readings. Changes in deviation are called displacements as any change indicates that the casing has moved from its original position. When displacements are summed and plotted, the result is a high resolution representation of movement.
- Checksum or Face Difference - A checksum or face difference is a function of the sum of an A+ reading and an A- reading at the same depth. In some cases it is divided by 2 and in other not. It serves as a ‘check’ on the quality of the data and the integrity of the inclinometer tubes.
Common graphical display of inclinometer measurements include:
- Cumulative displacement - a plot of cumulative displacement at depth against time. Starting, normally, at the base of the installation, the displacements at each elevation are added together to create an accumulative plot of displacements at each elevation from the base upwards. Sequential data sets generate a new plot line.
- Incremental displacement - a plot of incremental displacement at depth at each depth. Sequential data sets generate a new plot line.
- Cumulative deviation – a plot showing the profile of the casing relative to vertical (cumulative deviation against depth). Since inclination of the casing can contribute to error, the cumulative deviation plot is useful for diagnosing and correcting “rotational” errors.
- Incremental deviation - a plot of the readings converted to lateral offset at each reading interval (incremental deviation against depth).
- Checksum - shows checksums for each data set and can be used to evaluate the quality of the datasets. Spikes in the plot may indicate bad readings or a characteristic of the casing.
- Polar cumulative displacement – shows the cumulative displacement of the casing movement assuming the top or bottom of the casing as reference and as origin of the movements This is the change in position of the tube looking along its length.
- Polar cumulative deviation - shows the cumulative deviation of the casing assuming the top or bottom of the casing as reference and as origin of the measurements. This is the actual ‘shape’ of the installed tube looking along its length.
The difference between a probe’s non-zero value at true verticality is known as the probe’s bias. Every inclinometer probe has a very small bias, which can change through the life of the probe.
Bias shifts are not normally a matter for concern because the value of the bias is effectively eliminated by the standard two-pass survey and data reduction procedure.
Inclinometer Bias - Shift Error
A bias-shift error occurs when the data reduction procedure is unable to eliminate the entire value of the bias. This happens when:
- A survey does not contain data for the second pass through the casing.
- The bias changes during a survey, possibly due to thermal effects or some other subtle mechanical / electrical changes. A bias error would cause a plot of the cumulative displacement to gradually diverge from ‘normal’ plot lines, consistently over its whole length, starting from the ‘fixed’ end.
IP (or Ingress Protection) ratings
IP (or Ingress Protection) ratings are defined in international standard EN 60529 (British BS EN 60529:1992, European IEC 60509:1989). They are used to define levels of sealing effectiveness of electrical enclosures against intrusion from foreign bodies (tools, dirt etc) and moisture.
The numbers that follow IP each have a specific meaning. The first indicates the degree of protection (of people) from moving parts, as well as the protection of enclosed equipment from foreign bodies. The second defines the protection level that the enclosure enjoys from various forms of moisture (drips, sprays, submersion etc).
First Digit (intrusion protection)
- 0 No special protection.
- 1 Protection from a large part of the body such as a hand (but no protection from deliberate access); from solid objects greater than 50mm in diameter.
- 2 Protection against fingers or other object not greater than 80mm in length and 12mm in diameter.
- 3 Protection from entry by tools, wires etc, with a diameter of 2.5 mm or more.
- 4 Protection against solid bodies larger than 1mm (eg fine tools/small etc).
- 5 Protected against dust that may harm equipment.
- 6 Totally dust tight.
Second Digit (moisture protection)
- 0 No protection.
- 1 Protection against condensation.
- 2 Protection against water droplets deflected up to 15° from vertical
- 3 Protected against spray up to 60° from vertical.
- 4 Protected against water spray from all directions.
- 5 Protection against low pressure water jets (all directions)
- 6 Protection against string water jets and waves.
- 7 Protected against temporary immersion.
- 8 Protected against prolonged effects of immersion under pressure with product manufacturer specification based on testing e.g 16 bar.