An embodiment provides a method, including: operating a motor to position sample fluid within a fluid channel of a cuvette; transmitting light through an optical chamber of the cuvette; measuring a value of received light that has been transmitted through the optical chamber; comparing the measured value of light to one or more thresholds; determining a position of the sample fluid within the fluid channel based on a comparison from the comparing step; and generating a response based upon the position of the sample fluid with the fluid channel. Other aspects are described and claimed.