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PPT Calibration

PPT Calibration

Owned by Trevor Carey
Last updated: Mar 12, 2021
8 min read

Equipment                                                                                                                                          

  • Pore Pressure Transducers (PPT)

  • Reference PPT w/ Cables

  • Computer w/ LabView

  • Metal Calibration Chamber

  • Calibration Chamber Accessories (Plugs, caps, etc)

  • Air Pressure Line

  • Air Pressure Regulator

  • Vacuum Grease

  • Adjustable Wrench

  • DI Water

Procedures                                                                                                                                         

  1. Login to the computer at the sensor calibration station

  2. Open the most recent version of the Labview program ResDAQ located in C:\\reddaqv5.

  3. Open a PPT config file and re-save using a different name (i.e. “PPT Config_APS”)

  4. Fill out PPT config file using the “Analog1-0” row for the Reference PPT and the Strain1-(Channel #) rows for the PPTs that need to be calibrated.

    1. Column C: use drop-down to change cell to “YES” for all rows that will be used, including Analog1-0 for the Ref. PPT

    2. Column D: Input SN of each PPT

    3. Column E: The Ref. PPT should say “Ref” all other PPTs should have the name of the PPT (i.e. 5863)

    4. Column F: The sensitivity of the Ref. PPT should be 1000 mV/V. All other PPTs should begin with a sensitivity of 35 mV/V (or previous value).

    5. Column H: Specify the range of the PPT in units of kPa. The Ref. PPT should be 1378.9515 kPa. For PPTs rated to 100psi, the range should be 689.47573 kPa

    6. Column J: Set the offset to zero.

    7. Save and close the config file.

  5. Remove two of the gold caps covering the ports on the calibration chamber. One port is to fill the chamber with DI Water, the other is to vent the air as the chamber fills.

    1. Ensure to remove two caps such that the Reference PPT does not get wet.        

  6. Fill the calibration chamber halfway with DI water using one of the ports in the middle. Then stand the chamber onto its side and letting the excess drain out of the chamber.

    1. Ensure to remove two caps such that the Reference PPT does not get wet.

    2. If you overfill the saturation chamber, water can be pushed into the air lines. If this occurs, notify a CGM staff member and correct the deficiency.  

  7. Replace the two gold caps on the calibration chamber.

  8. Remove enough gold caps from the calibration chamber to calibrate the your PPTs (1 cap/PPT). You can calibrate up to 5 PPTs at once if all the channels on the DAQ are functioning.

  9. Unscrew and remove the tips and pore stones from the PPTs.

    1. If you are calibrating PPTs for cohesive soils, this calibration method is not advised. Please seek assistance from sCGM Staff.

  10. Remove one gold cap with a hole in it from the accessory box. Thread the PPT through the gold cap, such that the gold cap can screw down on the threads of the calibration chamber.

  11. Remove two halves of a rubber stopper.

  12. Measure the length of wire needed to go into the chamber to ensure the PPT falls below the water surface (base of the calibration chamber once vertical).

  13. Pinch the PPT wire between the two halves of the rubber stopper at the pre-determined length, adding a bit of vacuum grease to ensure a good seal.

  14. Insert the PPT into the calibration chamber all the way to the rubber stopper and press the stopper into the hole.

  15. Tighten the gold cap onto the threads to secure the PPT in place and create an air tight seal.

  16. Repeat Steps 10 through 15 until all PPTs are inside the calibration chamber.

  17. Connect the Ground (Green Wire) from the calibration chamber into the DAQ.

  18. Screw the MilSpec connector on the Ref. PPT Cables into the Ref. PPT.

  19. Connect the Ref. PPT to the 24V-DC power in the lower right-hand corner of the DAQ.

  20. Connect the Ref. PPT to Analog1-Channel 0 on the DAQ.

  21. Connect the end of each PPT cable to the DAQ in the appropriate Bridge Channel # that corresponds with Strain1-(Channel #) for that specific PPT in the Config File.

    1. If the Config File has PPT 5863 listed in Strain1-2, then the jumper cable should be plugged into Bridge Channel 2 on the DAQ.

  22. Rotate the valve on the pressure regulator to the “Vent/Off” position (vertical).

  23. Connect the main air supply line (plastic yellow line) from the wall to the pressure regulator.

  24. Turn on the air by rotating the valve on the wall for the air supply.

  25. Rotate the power knob on the pressure regulator to the right so the pressure reads in “psi.”

  26. Ensure the air supply is not currently plugged into the calibration chamber.

  27. Test the system to make sure the pressure regulator is completely off by rotating the valve counter clockwise to the “On” position. The pressure reading on the display should be about 0 psi. If the screen is not reading zero, rotate the pressure regulator knob counter clockwise until the reading is at 0.

  28. Rotate the valve on the pressure regulator back to the “Vent/Off” position.

  29. Use the air-line to connect the calibration chamber to the pressure regulator.

  30. Rotate the valve on the pressure regulator to the “On” position.

  31. Pressurize the calibration chamber to test that it has an adequate seal by rotating the knob on the pressure regulator clockwise until the pressure reads about 20psi.

  32. Identify the location any leaks by looking leaking water or listening for hissing air.

  33. De-pressurize the calibration chamber by rotating the knob counter clockwise until the display screen reads about zero.

  34. Turn the valve on the pressure regulator to the “Vent/Off” position.

  35. If leaks are found, correct them by switching out hardware/rubber stoppers or applying more vacuum grease. If not continue to step 36.

  36. Click the “Play” button in ResDAQ.

  37. Select the appropriate Config File.

  38. Click “Set” in the header window.

  39. Verify sampling rate is 2000 Hz, and then click “OK”.

  40. Click the toggle “Show Graph” so that it turns green and the live data is plotted.

  41. Rotate the valve on the pressure regulator to the “On” position.

  42. Click “Save Data” and wait for the green light to turn on.

  43. Rapidly rotate the knob on the pressure regulator clockwise until you reach about 25% of the capacity of the PPT (~25psi for PPTs with 100psi capacity). Allow the pressure to stabilize, then rapidly rotate the knob counter clockwise to de-pressurize the calibration chamber.

  44. Repeat step 43 but increase the pressure to 50% capacity (~50psi for PPTs with a capacity of 100psi).

  45. Click the “Stop” button and wait for the green light to turn off.

  46. Ensure the calibration chamber is depressurized and rotate the valve on the pressure regulator to “Vent/Off” and ensure.

  47. Open the most recent version of Plot Fast Data from Labview. Located on the desktop.

  48. Click the “Play” button and open the “data” folder.

  49. Select the most recent BIN file that has “@999.0rpm” at the end of the file name.

    1. This file produces a better fit, as the other file has a smoothing algorithm that can eliminate some valuable data.

  50. Select the “Zoom Page” tab.

  51. Ensure “Time/Data” toggle is set to “Time”.

  52. Select the appropriate Bridge Channel (Bridge #) to plot from the drop down that corresponds with the Channel the PPT is plugged into.

  53. Turn on “Zero Offset” for Ref. PPT and the PPT(s) being calibrated.

  54. Verify results match reasonably well. Make a note if the PPT and the Ref. PPT are extremely different at any point in the record.

  55. Switch “Time/Data” Toggle to DATA

  56. Ensure “X-axis” dropdown is set to “HOC-0”

  57. Verify the slope of the PPT being calibrated is linear. If the slope is non-linear then the PPT is faulty and needs to be replaced.

  58. Verify that the slope of HOC-0 is 1.

  59. If the slope of the PPT being calibrated is accurate to two decimal places (1.00 or .99), then the calibration is complete. Otherwise, the PPT sensitivity needs to be adjusted.

  60. Copy the slope of the PPT being calibrated.

  61. Open the config file.

  62. Multiply the old sensitivity by the PPT’s slope copied in the previous step. Paste the new sensitivity into the cell for the PPT being calibrated. Save and close the config file.

  63. Repeat Steps 36 through 62 until the slope of the PPTs is accurate to within 2 decimal places (1.00 or 0.99).

  64. Once the PPTs are successfully calibrated, disconnect the PPTs and the Ref. PPT from the DAQ.

  65. Disassemble the calibration chamber and return all accessories to the accessory box.

  66. Drain the calibration chamber, ensuring that the Ref. PPT does not get wet.

  67. If desired, the calibration file can be saved as a .txt file or as a BIN file using the “misc” tab and the Dump Button.

Photos                                                                                                                                               

 

Equipment Overview

Step 5

Step 6

Step 6

 

Step 13

Step 15

 

Steps 17-21

Steps 22-31

Steps 36-45

Step 67