What is Vacuum Hose Conductance?

You may have heard or read the term “conductance” in reference to vacuum hoses and fittings. According to VAC AERO, conductance is volumetric flow rate divided by pressure drop, expressed as liters per second. Simplified, the conductance of a vacuum hose means its ability to allow gas to flow through it. The really important point is that no matter how big your vacuum pump is, it cannot move gas through a hose any faster than that hose’s conductance. Vac Aero

Hose conductance is not fixed, but varies with the type of gas, pressure, temperature, geometry of the passageway, hose diameter, and hose length. Duniway Stockroom Corp offers this formula: Conductance = 75 x Diameter³/Length. Note this formula calculates the conductance through a smooth tube in liters per second for dry air at 75°F and very low vacuums (under 50 microns). It won’t really calculate the conductance of a hose removing water vapor or refrigerant at atmospheric pressure. However, we can use it to get some general idea of the effect of diameter and length on hose conductance. Duniway

For example, the conductance of a ¼” diameter, 60” hose would be 75 x 0.25³ /60 = 75 x .015625/60 = 0.0195 liters per second. Turning that into cubic feet per minute (CFM) we get 0.0414 CFM. Suppose we shorten the hose to 36 inches. Now the conductance is 75 x 0.15625/36 = 0.03255 liters per second. That translates to 0.069 CFM. More than a 50% increase just by switching from a 60” hose to a 36” hose. What about changing the diameter? Using a 3/8” hose that is 60 inches long, the conductance becomes 75 x 0.375³/60 = 75 x 0.0527/60 = 0.066 liters per second. Translated into CFM, that is 0.14 CFM. We get over three times the conductance by increasing the diameter to 3/8”. Using similar calculations for ½” and ¾” we get 0.33 CFM and 1.12 CFM respectfully. So comparing different diameter hoses using this formula we see that a 3/8” hose has over three times the conductance of a ¼” hose, a ½” hose has more than twice the conductance of a 3/8” hose, and a ¾” hose has more than three times the conductance of a ½” hose. All together, a ¾” hose has 27 times the conductance of a ¼” hose. Large diameter hoses really do make a difference in the time it takes t pull a vacuum. However, there are other restrictions that must be addressed before the hose size matters: the Schrader valve cores. We’ll talk about them next time.