Many new aircraft parts are made from carbon fiber composite materials. These parts must be cured at precise temperatures and partial pressures to achieve the required structural strength. Controlling the vacuum in a curing oven depends on the accurate measurement of the partial pressure. A standard 4-20 mA control signal must be sent to the pump controller over a pressure range of 0 to –2 psi.

Accurate measurement of partial pressure requires that the difference between the local atmospheric pressure and the pressure inside of the curing oven be sensed. This requires a differential pressure transmitter. The - port of the transmitter is connected to the oven chamber. The + port of the transmitter is left open to the ambient atmosphere. The calibration of the transmitter is 0 to 2 psid so that 4 mA is equal to 0 differential pressure and 20 mA is equal to 2 psid.

When the pump lowers the pressure in the oven chamber, the pressure on the + port of the transmitter is higher than the on the – port. The signal moves from 4 mA towards 20 mA. The controller will turn the pump on or off depending on the actual partial pressure measured by the transmitter.

The atmosphere can vary with the weather. A low pressure front can depress the barometric pressure by 1 psi from normal. In addition, increasing elevation will lower the barometric pressure. The normal atmosphere in Denver is about 12 psi; the normal atmosphere at sea level is 14.7 psi.

By using a differential transmitter to control the pump, the atmospheric changes due to weather and elevation will not affect the measurement of the correct partial pressure.