An apparatus comprising: a first carousel comprising: a first annular array of slots (108a-108n) to receive a first reaction vessel; a first track of rotation about the first carousel having a first diameter; and a second track of rotation about the first carousel having a second diameter smaller than the first diameter; a first diverter to move the first reaction vessel from the first track to the second track; a second carousel coaxial with the first carousel, the second carousel comprising a second annular array of slots (110a-110n) to receive a second reaction vessel; a first pipetting mechanism disposed outside of a first outer circumference of the first carousel and outside of a second outer circumference of the second carousel; a memory (1013, 1014, 1016) including coded instructions; and a processor circuity configured to execute the coded instructions to cause the first pipetting mechanism to: aspirate a sample from a first container disposed outside of the first and second outer circumferences and dispense the sample into the second reaction vessel on the second carousel; and after the sample mixes with a reagent in the second reaction vessel, aspirate a mixture of the sample and the reagent from the second reaction vessel and dispense the mixture into the first reaction vessel on the first carousel. a first carousel comprising: a first annular array of slots (108a-108n) to receive a first reaction vessel; a first track of rotation about the first carousel having a first diameter; and a second track of rotation about the first carousel having a second diameter smaller than the first diameter; a first annular array of slots (108a-108n) to receive a first reaction vessel; a first track of rotation about the first carousel having a first diameter; and a second track of rotation about the first carousel having a second diameter smaller than the first diameter; a first diverter to move the first reaction vessel from the first track to the second track; a second carousel coaxial with the first carousel, the second carousel comprising a second annular array of slots (110a-110n) to receive a second reaction vessel; a first pipetting mechanism disposed outside of a first outer circumference of the first carousel and outside of a second outer circumference of the second carousel; a memory (1013, 1014, 1016) including coded instructions; and a processor circuity configured to execute the coded instructions to cause the first pipetting mechanism to: aspirate a sample from a first container disposed outside of the first and second outer circumferences and dispense the sample into the second reaction vessel on the second carousel; and after the sample mixes with a reagent in the second reaction vessel, aspirate a mixture of the sample and the reagent from the second reaction vessel and dispense the mixture into the first reaction vessel on the first carousel. aspirate a sample from a first container disposed outside of the first and second outer circumferences and dispense the sample into the second reaction vessel on the second carousel; and after the sample mixes with a reagent in the second reaction vessel, aspirate a mixture of the sample and the reagent from the second reaction vessel and dispense the mixture into the first reaction vessel on the first carousel.