Model 791 Calibration Plate for Tiltmeters and Inclinometers

Transcription

1 User s Manual Model 791 Calibration Plate for Tiltmeters and Inclinometers Serial No. Baseline, L Date measured By mm Introduction The Model 791 Calibration Plate is designed for calibrating any tiltmeter or inclinometer (clinometer) that has a gravity-referenced output. The top surface of the plate contains threaded holes for attaching most Applied Geomechanics tiltmeters and clinometers. These holes also can be used to hold attachment fixtures for tiltmeters from other suppliers. The bottom surface of the plate contains three spherical tooling balls made of hardened steel. These balls, which are separated by a fixed distance L, comprise the legs of the calibration plate. To perform a calibration, NIST-traceable gage blocks are placed under one end of the plate to rotate the tiltmeter through a series of known angles. The output of the tiltmeter at each angle establishes the calibration. Figure 1 shows the calibration plate in use with a Model 701 Biaxial Platform Tiltmeter. Specifications are listed in Table 1. Figure 1. Calibrating a tiltmeter with the Model 791 Calibration Plate

2 page 2 of 6 Table 1. Model 791 Calibration Plate Specifications and Ordering Information MATERIALS BASELINE, L GAGE BLOCK SIZES SIZE & WEIGHT ORDERING INFORMATION Calibration Plate: 6061T6 aluminum Contact Points: 3 hardened tool steel spherical balls, diameter = inch ± inch ( ± mm) Gage Blocks: Hardened tool steel ± 0.03 mm Part No : 10 blocks ranging from 1mm to 25mm for angles between 0.2 and 6 degrees. Other sizes available on request. Calibration Plate: 13 x 6 x 1 in (330 x 152 x 25 mm), 3.6 lb (1.6 kg) Part No Gage Block Set: 2 lb (0.86 kg) Part No : Model 791 Calibration Plate Part No : Rectangular Gage Block Set Part No : Vertical Bracket Kit for Wall Mount Tiltmeters Part No : Tilt Plate Kit for Model 800P Portable Tiltmeter Using Your Model 791 Calibration Plate The Model 791 Calibration Plate is easy to use. Simply follow the steps below: 1. Locate the hole pattern that fits your Applied Geomechanics tiltmeter. Screw the tiltmeter to the calibration plate using the fasteners that were supplied with it or with your own fasteners. The following American thread sizes are used in the holes in the calibration plate: 4-40, 6-32, 8-32, 10-32, 1/4-20 and 5/ Place the calibration plate on a hard, flat and level surface. A granite table or an optical table are excellent choices. 3. If you plan to calibrate the tiltmeter over its full range (on both the positive and negative sides of level), elevate the end of the calibration plate with the two legs. Both legs should be elevated by the same amount. Gage blocks can be used for this purpose. 4. Connect your tiltmeter to a power supply and a readout device. Wait for the output to stabilize and then take the first reading. Enter the reading into a calibration table beside the height of the gage block. Table 2 is an example of a table in which the gage block heights and corresponding angles already have been entered. Table 3 is a blank table that you can copy and use. 5. Place the first gage block under the end of the calibration plate with the single tooling ball (place the block under the tooling ball)(figure 2). Record the output in your calibration table beside the height of the gage block. 6. Repeat step 5 until you have recorded the tiltmeter output for several steps over the desired calibration range. The number of steps that you should record is a function of your unique calibration requirements. 7. Compute the tilt angle θ at each step using the formula: θ = arc sin (h/l) where h is the height of the gage block and L is the baseline length (Figure 2), written on the first page of this document. Enter these values into your calibration table (Table 2 or 3). After completing the calibration procedure described above, your results can be used in a variety of ways. For example: The angular rotation between two gage block steps can be compared to the value indicated by the tiltmeter.

3 page 3 of 6 A linear regression line that best fits all of the calibration data can be computed (Figure 3) and the slope of this line taken as the scale factor of the tiltmeter. This is the method that Applied Geomechanics uses to obtain the scale factors of analog output tiltmeters in its factory calibrations. A polynomial approximation of the calibration data can be obtained using a spreadsheet application such as Excel. Maintaining Your Model 791 Calibration Plate Your Model 791 Calibration Plate and gage blocks are uncomplicated devices that will provide many years of trouble-free service if the following maintenance procedures are followed: Keep this equipment clean and dry. Prolonged exposure to moisture or to corrosive liquids will mar the finish and can damage the threads in the tiltmeter attachment holes. Keep the tooling balls (legs) coated with a thin coating of low-viscosity oil at all times to prevent rust. Avoid impacting the tooling balls against hard objects, as this may dent or flatten their surfaces, reducing the accuracy of the calibration plate.

4 page 4 of 6 Figure 2

5 page 5 of 6 Table 2 Calibration Table Model 791 Tiltmeter Calibration Plate. Baseline Length, L = 300mm Tilt Angle Gage block height, mm degrees arc minutes arc seconds microradians tiltmeter reading , , , , , , , , , , , , , , ,448 6 Output (volts) Degrees=1.021 x (Volts) Angle (degrees) Figure 3. Calibration data for a tiltmeter with analog voltage output. Scale factor = 1.021degrees/volt.

6 page 6 of 6 Table 3 Calibration Table Model 791 Tiltmeter Calibration Plate. Baseline Length, L = mm Tilt Angle Gage block height, mm degrees arc minutes arc seconds microradians tiltmeter reading Should you have questions about tiltmeter calibration, please contact us at the address below.