It is well-known to use springs to hold evaporation covers down against a test element in an analyzer. However, some springs are not readily removable and others are difficult to manufacture and assemble. Described herein is a spring (8) which overcomes the disadvantages associated with known springs. The spring (8) comprises two portions (10, 20) bent to form the letter "J". The longer portion (10) includes an aperture (40) which engages with a post (104) for retaining the spring (8) in the incubator station (100), and the shorter portion (20) includes an aperture (30) and a lateral extension (50) for retaining the cover (110, 112, 116) under the spring (8).