To accurately identify the substance in the tube, the user must first determine the current when there is no tube present. The other signals are recognized by determining their contrast ratio to the reference current. The contrast ratio is used because the absolute voltage or current can be subject to a number of factors and will vary from unit to unit; however the ratios will remain relatively stable. The no-tube signal is the most stable reference point, because it is not influenced by such factors as minute differences in tubing, minute differences in the liquid inside the tube, and so on.