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ControlAir offers a negative bias option for the Type 600, Type 6100, and Type 6200 volume boosters. We often get questions around the purpose of the negative bias and how it works. This blog post will dive deeper into how this negative bias option works and what is the purpose behind it.
Volume boosters are devices that offer wide fluid paths to allow large amounts of flow while being controlled by a pneumatic signal. Volume boosters typically offer wide output ranges such a 0-150 PSI. The unit is controlled by a pneumatic pilot signal, most commonly from an I/P transducer or air pressure regulator, that applies a downward force on the signal diaphragm assembly. This downward force is eventually transferred and used to open the supply valve a specific amount. Below this force is shown acting on the supply diaphragm assembly of the 1:6 ratio Type 600 volume booster.
When the unit is built with the negative bias option, a small bias spring is placed between the baffle plate and the control diaphragm. This spring applies about 4 PSI of force to the control diaphragm and acts in the direction opposite to the force from the pilot signal. With the bias spring in place this creates a situation where the output from the volume booster is always behind the input signal by approximately 4 PSI. In other words, if the signal pressure is 10 PSIG then the output from the volume booster would be approximately 6 PSIG.
So what is the real purpose of the negative bias option? The most common application is when a volume booster is used in conjunction with an I/P transducer whose minimum output is 3 PSIG, for instance a 3-15 PSIG I/P transducer. The negative bias option on the volume booster allows the end user to “zero out” the I/P transducer. In other words, as the I/P transducer goes from 3 to 15 PSIG the volume booster would produce 0 to 12 PSIG output when calibrated appropriately. This allows the end user to have an electronically controlled output that is zero-based and will allow for large amounts of flow. The negative bias option also represents a cost savings for the end user as a zero-based I/P transducer is typically more expensive than adding a negative bias option to a volume booster.