Interesting question . I looked into the technical brichure of Gouda and Van den Berg, they don't say anything about frequency of calibration. CPT test is covered by an ASTM standard ( I don't work with it ). I have an old 1979 version of ASTM D3441 where they don't even talk about calibration ( they call for +/- 5% accuracy ).
I would say it's important to carry out one calibration before using the machine ( some parts can be exchanged in 100 and 200 KN heads from Gouda for example and can lead to results 4 times greater than reality ! ). If your tests are carried out in soft soils, i don't believe you need another calibration except if you change parts in the head ( mechanical ) or change the electric cone. If you are working in ranges close to the maximum capacity of the rig, or in case the head or the cones have been hit, it would be necessary to check again.
If you're referring to calibration of the instruments, and not "zero readings" (or baseline readings) which should be performed before and after every sounding, actual calibration frequency should probably depend primarily on frequency of use, and care/maintenance by the operator. I believe there is some guidance in the ASTM Standard (D 5778) as well as the International Reference Test Procedures (I believe established by ISSMGE TC16). The SGI also authored a report specifically on CPTu calibration - I think it came out around 1990, but cannot recall the report number.
FYI, all, a great free reference on the CPT (SCPT, SCPTu, etc etc) is the NCHRP Synthesis 368, which can be found at:
mdhshanwil - thanks for your comments; I went to uni with Paul. I'm talkin the gauges - like we get our compression machine gauges calibrated once per year. I was thinking that the calibrations should be at a minimum every 6 months. The drilling contractor that will be used by one of our contractors submitted calibrations that were nearly 3 years old - too old for me!
From Guide to Cone Penetration Testing for Geotechnical Engineering by Robertson, it mentions that "Calibrations should be carried out at regular intervals (approximately every 3 months)".
However for the contractor,it is commonly 1.5 years.
Thanks pagenel. Already told the contractor to tell his drilling sub "to take a hike" or get a new calibration. I'd be happy if it was within 6 months; being in Indonesia.
