The canister at GFM was replaced on November 15, 2016. text of email from November 13, 2017. Holly has found probable polarity reversals at GFM and DSNC. Attached is an image of arrivals from the M6.5 in Costa Rica last night. ASTN and BCRT are S-13 with Reftek. CCNC and CCRT are ISIS4 with S-13. DSNC is ISIS4 with S-13. GFM is ISIS4 with L4. So DSNC should be the same as CCNC and CCRT but its not. I don't think we have many L4's anymore but they should still be the same as the S-13. So these two are potentially problem. text of email from January 11, 2018. I was able to determine that the EHN channel is reverse polarity for DSNC, but looks okay at GFM. To check the EHE channels, we need a large teleseismic source to the east or west (mid-alantic ridge would be ideal). But I was unable to find one in 2017 suitable for checking polarity. So it might be best to visit them both with a portable, a steel plate, and a sledge hammer. Put the plat sufficiently far away so its approximately the same azimuth between the plate and both sensors but close enough to get an impulsive arrival. First do this on the east, then on the north and make sure the portable and the permanent both have the same motion. This might take two people, one to be the seismic source, and one to look at the data. Jim's email on Jan 11, 2018 I'll bet the EHE channel at GFM is also ok. Since it's still and L43D and uses a different breakout to interface with the I4 canister, I bet the horizontals are correct and just the Z is reversed. DSNC uses a standard S-13 breakout similar to all the other ETSZ SP stations so I'll be the problem is with all 3 channels in the I4 canister.