(19)

(11)

EP 0 438 392 A1

(12)

(43) Date of publication: 31.07.1991 Bulletin 1991/31 (21) Application number: 88909347.0 (22) Date of filing: 31.10.1988

(51) International Patent Classification (IPC):  E21B 47/ 022( . ) (86) International application number: PCT/GB1988/000930 (87) International publication number: WO 1989/004909 (01.06.1989 Gazette 1989/12)

(84) Designated Contracting States: AT BE CH DE FR GB IT LI LU NL SE (30) Priority: 13.11.1987 GB 19870026597 (71) Applicant: MEASUREMENT DEVICES LIMITED Aberdeen AB2 8EW (GB)	(72) Inventor: BALL, Steven Banchory Kincardineshire (GB) (74) Representative: Fitzpatrick, Alan James, et al  Fitzpatricks, 4 West Regent Street Glasgow G2 1RS, Scotland Glasgow G2 1RS, Scotland (GB)