November 27, 2019
The EAN-70M tilt measurement system consists of two components – a portable tiltmeter and several Aluminium tilt plates. Individual tilt plates are usually anchored to the structure, but may also be bonded to a smooth clean surface i.e. granite, stone, tiles.
The portable tiltmeter is a high-resolution uniaxial tilt sensor that is designed to monitor inclination and vertical rotation in structures.
It is ruggedly-built and has superior temperature stability. The tilt variations in a structure can be caused by construction activities such as excavation, tunnelling or dewatering that may affect the ground supporting the structure.
The changes in tilt can also occur from the loading of a structure, such as a dam during impoundment, a diaphragm wall during excavation or a bridge deck due to wind and traffic.
The user places the portable tiltmeter on the mounted tilt plate to get the tilt reading at that location. The portable tiltmeter has to be carried from one tilt plate to another to get readings at different locations. Although it is a uniaxial tiltmeter, it can also be used to take biaxial measurements at any particular location by rotating the tiltmeter by 90° on the tilt plate and taking a second reading.
Here is everything about a portable tiltmeter – its installation procedure and how to take readings from it.
Encardio Rite deals with Model EAN-70M which is a portable tiltmeter, built around a precision accelerometer and suitable signal conditioning circuit mounted inside anodized aluminium housing.
The accelerometer senses force of acceleration due to gravity which is maximum when the accelerometer is rotated to full 90° tilt position and is zero (minimum) when the tilt angle of the accelerometer is 0°.
For in-between angles, the force experienced by the accelerometer is equal to the product of the sine of tilt angle and acceleration due to gravity.
The tilt sensor provides a bipolar DC voltage output which is proportional to the sine of tilt angle measured by the tiltmeter. The output is zero volts for a truly vertical sensor.
The portable tiltmeter provides a comparatively low-cost tilt measurement solution with excellent resolution, long term stability, and low thermal sensitivity.
The tiltmeter can be used to measure the change in the tilt of any vertical surface or horizontal floor by placing it on a suitable reference tilt plate, available separately.
The EAN-70M is not intended for absolute determination of tilt of structures. It measures the change in the tilt of structures to which the reference tilt plate is attached.
Initial tilt reading for each tilt plate is recorded after it is mounted on the structure to be monitored. Change in the tilt of structure over some time can be measured by subtracting the initial tilt reading from subsequent tilt readings.
A tilt plate is an aluminium disc about 142 mm in diameter. It is fixed to the structure with four Hilti anchors HPS 1-6/15×40 # 260350 or equivalent. In case the structure has a smooth surface like marble or granite, the tilt plate may be bonded to it by epoxy.
The four pegs on the tilt plate are used to orient the tiltmeter. The horizontally mounted tilt plate allows tilt readings in two planes that are at 90° to each other. Whereas, the vertically mounted tilt plate allows tilt readings along one vertical plane only.
| Read More:What Are Tiltmeters And Where Are They Used?|
The tilt plates are fixed on structural members that are representative of a large structure. When a single location does not represent the structure, additional tilt plates are placed at other locations. The number of tilt plates needed is determined by the stiffness of the structure and accuracy desired.
Stiffer structures require fewer plates. To achieve large accuracy, more plates are required. Tilt plates are generally placed with one set of pegs oriented to the expected direction of rotation. Location of tilt plate shall be chosen such that they are easily accessible.
Figure 2: Tilt mounting on horizontal surface
Horizontal plates provide two planes of measurement. Plane A is defined by pegs 1 & 3. Peg 1 is usually oriented towards the direction of tilt. Plane B is defined by pegs 2 & 4. Peg 4 is usually oriented toward the direction of tilt.
Vertical tilt plates should be aligned so that a vertical line can be drawn through pegs 1 and 3.
Tilt plates can be fixed to the structure with anchors and screws or with grout. When tilt plate experiences temperature changes or weather, a combination of both anchors and grout works best.
Anchors and screws
Tools & accessories required for installation
Allow time for tiltmeter to adjust to ambient temperature. If possible, store tiltmeter at the same temperature as at the time of taking readings. Connect tiltmeter to readout and power up. Take readings as follows:
Figure 3 below shows positioning of portable tiltmeter on tilt plate for taking readings in plane A
Take readings in plane B next. Plane B is defined by direction of pegs 2 and 4. Place “+” end of tiltmeter on peg 4, wait for reading to stabilize and record it.
Rotate tiltmeter 180° and place “-“ end of tiltmeter on peg 4, wait for reading to stabilize and record it. Figure 1 & 2 explain the positioning of portable tiltmeter on the tilt plate to take readings in plane A and B respectively.
Repeat steps 2-3 times to ensure readings are repeatable. In theory B+ and B – readings should be identical except for different sign (+/-). In practice, there is a difference up to 0.003 on Encardio Rite readout unit EDI-53UTM between two readings due to bias of sensor and small irregularities in the tilt plate.
Figure 4 below shows positioning of portable tilt meter on tilt plate for taking readings in plane B.
Vertical tilt plate allows reading in tilt plane A defined by the direction of pegs 1 and 3. The tiltmeter is aligned using alignment bars at ends of tiltmeter.
Place “+” end of tiltmeter against peg 1, wait for reading to stabilize and record it. Refer to figure 6.
Place “-” end tilt meter against peg 1, wait for readings to stabilize and record it.
Repeat these steps three times to ensure that you have good repeatable readings.
Repeat steps two to three times to ensure readings are repeatable. In theory, readings should be identical except for different sign (+/-).
In practice, there is a difference up to 0.003 on Encardio Rite readout unit EDI-53UTM between two readings due to bias of sensor and small irregularities in tilt plate.
Figure 5 below shows the position of portable tiltmeter on the tilt plate for taking readings in vertical plane B.
One is generally interested in finding change in tilt of a structure. To find change in tilt, subtract initial tilt from current tilt and convert result in degrees or displacement.
Encardio Rite readout unit Model EDI-53UTM display the readings in terms of sin (tilt angle).
Displayed readings = sin (A)
Where A = angle of tilt
Obtain two readings for each tilt plane, a “+” reading a “-“reading. In the data reduction process, add the two readings to eliminate sensor bias. Denote this value by “DIFF” (algebraic difference). A positive DIFF value indicate tilt toward + end of tilt meter.
DIFF = (+ reading) – (- reading)
To convert the DIFF value to tilt in degrees, divide it by 2 because the DIFF value comprises of two readings.
Take inverse sine of this value to get angle of tilt in degrees.
Angle of tilt = sin-1 (DIFF/2)
To calculate change in tilt use following formula:
Change in tilt = sin-1 (DIFF2/2) – sin-1(DIFF1/2)
Where DIFF1 is the initial reading (tilt reading taken just after installation) and DIFF2 is the current tilt reading (reading taken on a subsequent date)
Where DIFF1 is the initial reading (tilt reading taken just after installation) and DIFF2 is the current tilt reading (reading taken on a subsequent date).
That was all about portable tiltmeters – installation procedure, how to take readings and reduce data from the captured readings.
If you have any questions, feel free to drop them in the comments section below.
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