An optical fluid tester device for testing a fluid sample in an ampoule includes an ampoule cradle, a radiation source, a radiation detector and an analyzer. The ampoule cradle supports the ampoule. The ampoule with the fluid sample serves as a lens whose focal properties are dependent on the index of refraction of the fluid sample. The radiation source irradiates the ampoule. The radiation detector is located opposite the radiation source so that the ampoule, when supported by the ampoule cradle, lies between the radiation source and the radiation detector. The detector serves to detect the intensity of the incident radiation. The analyzer verifies the composition of the fluid sample based on the detected intensity, which is dependent on the focal properties of the ampoule and is, therefore, indicative of the composition of the fluid sample.