Presentation on theme: "AS-1 Calibration IRIS Kansas City Workshop September 19-21, 2007 John Lahr (with Kay Wyatt)"— Presentation transcript:
AS-1 Calibration IRIS Kansas City Workshop September 19-21, 2007 John Lahr (with Kay Wyatt)
Many of these calibrations and test need only be done once. Results of the tests should be saved in SAC format files. If you have trouble in the future, you can repeat the tests and compare them to the tests performed when you first received your seismograph.
Perhaps the easiest way to monitor the general health of your AS1 system is by comparing your records with those from other schools. See: http://www.iris.edu/hq/ssn/events for current records.
SIDE NOTE Keep in mind that the helicorder settings do not effect how the data are recorded on disk. They just control how the data are displayed on the screen.
1. Setting the zero level 2. Check the systems free period 3. Damping adjustment 4. Washer calibration 5. Adjusting the gain based on the background noise 6. Lift test calibration 7. Electronic noise calibration 8. Inductance calibration CALIBRATIONS TO BE PERORMED
Setting the zero level Setting the zero level is probably the most difficult part of setting up an AS1 system. The knob on the AS1 black box is VERY sensitive. Use the Show data values feature under Settings to monitor the level while adjusting the knob on the Black Box. Turn it slowly by hand to get within about 100 counts of zero and then make slight adjustments by tapping lightly with a pencil until the numbers bounce equally above and below zero. It is helpful to put a piece of tape on the Black Box and place a mark by the pointer on the black knob.
Check the Systems Free Period First, pull the magnet carrier away from the copper damping vane.
Check the Systems Free Period (contd) First, make sure the helicorder setting displays one hour per line and set the gain to 5. Gently tap the seismometer base. Undamped oscillations look like this.
Check the Systems Free Period (contd) Select one of the pulses and click the icon to display a single trace. The oscillations should continue for at least three minutes
Check the Systems Free Period (contd) First, change the helicorder setting so that it displays one minute per line. Undamped oscillations look like this.
Check the Systems Free Period (contd) Measure the time between two peaks or two troughs. This is the free or natural period of the system. Here we measured 1.4 seconds. 1.4 sec
Save a file in sac format showing the free oscillation decay so that it can be compared with a similar test in the future to determine if the spring or hinge have changed. Check the Systems Free Period (contd)
We just saw that without damping the system will continue oscillating for a few minutes following a single pulse. For this reason it would be nearly impossible to see any phases after the first P arrival of an earthquake. On the other hand, if the system is over damped, then the response will be attenuated. DAMPING ADJUSTMENT
This plot shows a damped oscillation. The ratios of the amplitude A1 to A2 equals the ratio of A2 to A3. A1/A2 = A2/A3 = A3/A4 etc. The proper damping ratio for the AS-1 system is 20. A1 A2 A3 DAMPING ADJUSTMENT (contd)
Prepare to Mark the Location of Magnet Block Tape a piece of paper to the area under the magnet block so that when the block has been positioned for proper damping, its location can be saved. If the instrument needs to be moved later, the block can be replaced in exactly the right spot. DAMPING ADJUSTMENT (contd)
Place a small ruler vertically next to the red magnet, perhaps holding it in place with some clay. Note magnets equilibrium position. Lift the magnet 10 mm, release it, and then note how much below the equilibrium position the magnet goes. Adjust the position of the damping magnet block so that the overshoot is 0.5 mm, which gives the desired ratio of 20. DAMPING ADJUSTMENT (contd)
The more overlap between the magnet damping block and the copper vane, the greater the damping. DAMPING ADJUSTMENT (contd)
If the damping is too little when the copper vane is completely within the damping block, then adjust nuts on the block so that the magnets are closer together DAMPING ADJUSTMENT (contd)
Mark the final position of the magnet block. DAMPING ADJUSTMENT (contd)
Tie a thread to a washer and place it near the boom magnet and coil. WASHER CALIBRATION
Pulling the washer away suddenly creates this calibration pulse: down, up, and then return to zero. WASHER CALIBRATION (contd)
The system is under damped on the left. The system on the right is correctly damped. Too little damping Proper damping WASHER CALIBRATION (contd)
Note that the overshoot during a washer calibration is primarily due to the AS-1 filters. This overshoot will be much more than 1/20 th of the first pulse, as it should be. WASHER CALIBRATION (contd)
When the washer was pulled away, which way did the magnet move? Which way did the seismogram trace move? Why? Save your calibration as a SAC file. WASHER CALIBRATION (contd)
When the ground moves down, the magnet will tend to remain fixed, which means it moves up with respect to the coil. WASHER CALIBRATION (contd)
As soon as your AS-1 is setup, perform a washer calibration. Save this initial calibration for future reference in a SAC file in case there is some question about the systems operation. WASHER CALIBRATION (contd)
In the Settings/Helicorder/Gain set the helicorder display gain so that the background noise is quite clear. Adjusting the gain based on the background noise
The gain on this record is too low. The gain may need to be reduced during storms and increased during quiet times. Adjusting the gain based on the background noise (contd)
To do a lift-test calibration of your system, make a 5/16-diameter hole in the top of the cover, near the hinge end. 14.5 cm from the end and 7.7 cm from the side. Lift Test Calibration
Mark the location for the lift-test weight. Make a white patch on the boom with whiteout and then draw a line across it 10 cm from the mast. Lift Test Calibration (contd)
A 0.034-gram weight is made from a 2 cm x 1 cm piece of a manila folder. The weight is lowered onto the boom, ready for a lift-test calibration. Lift Test Calibration (contd)
Quickly lift the weight. Here is an example lift-test calibration pulse. Be sure to save your test in a SAC file. Lift Test Calibration (contd)
To check for electronic noise, place a weight on the boom. Electronic Noise Test
The electronic noise when the boom is held fixed is just 3 counts peak-to-peak. Electronic Noise Test (contd)
For an inductance calibration, lift boom and place one loop of 32 gauge wire around the coil. Inductance Calibration
Here is a closer view of the loop of wire. Inductance Calibration (contd)
A pulse is generated when a AAA battery (1.4 V measured) is attached and another pulse when the battery is removed from the single loop of wire. Inductance Calibration (contd)
This calibration is independent of the AS1 spring and damping system. Again, save this calibration for comparison with future calibrations. Inductance Calibration (contd)
For more information or help regarding these calibration tests, contact: Kay Wyatt firstname.lastname@example.org CALIBRATION TESTS