Pulse tube. Models: HL 95 SP 90 T FS 310 FS 100 R FS 90 R - 4180 SP 90 HL 95 K BT 130 HL 100 FS 130 HT 131 FS 130 R KM 90 R HL 100 K HT 101 FS 87 R FR 130 T KM 100 R FC 100 FC 110 KM 130 R