Optical density instrument and systems and methods using the same

An optical density testing system, the system comprising:

• a sample tube;
• and
• an apparatus comprising:
• a housing defining an opening and a cavity, wherein the housing is configured to receive at least a portion of the sample tube in the cavity via the opening, and wherein the opening is configured to allow a user to view contents of the sample tube during testing;
• an emitter disposed in the housing, wherein the emitter is configured to emit light towards the sample tube;
• at least one sensor disposed in the housing and configured to receive the emitted light from the emitter and generate a signal indicative of an intensity of the received light;
• an illumination light disposed in the cavity and configured to illuminate the sample tube to allow the user to view contents of the sample tube during testing;
• and
• a spring configured to engage the sample tube in an operational position, the spring defining a first leg and a second leg, wherein the first leg and the second leg are configured to apply a force on the sample tube towards a point between the first leg and the second leg.