How much backpressure is my tubing generating?

Tubing in (U)HPLC instruments and other precision fluidic systems will cause a pressure drop when fluids are pumped through them. This backpressure is proportional to the volumetric flow rate and the viscosity of the pumped liquids, plus the physical dimensions of the tubing, i.e., its length and inner diameter (ID). According to the Hagen–Poiseuille equation, backpressure will increase linearly with volumetric flow rate, tubing length, and dynamic liquid viscosity, but by the power of four with decreasing tubing radii.

In a strive to minimize peak dispersion in liquid chromatography systems, there is thus a risk of installing a too narrow tubing ID, which could give rise to significant backpressure and possibly even exceed the pressure limits of some fluid path components. Conversely, installing a too large tubing ID would add unnecessary volumes into the flow path that would risk influencing the shape and width of the chromatographic peaks.

To facilitate the selection of a suitable tubing ID, we have assembled a table of common dimensions used in liquid chromatography, including their colour codes when acquired from Biotech Fluidics. The outer diameter of these tubing is typically 1/16” or 1/32”, although that is irrelevant to the calculations below. The calculated data include volume per decimetre of tubing (1 dm = 3.9 inch), and estimated pressure drop for the same length at a volumetric flow rate of one millilitre per minute. The backpressure values were estimated at room temperature (20 °C) for pure water and pure isopropanol, plus 50:50 mixtures of acetonitrile/water and methanol/water, since such compositions are commonly used in various modes of liquid chromatography.

Browse the tubing product pages linked below or contact Biotech Fluidics to learn more about the different tubing materials and dimensions available for precision fluidic systems.