1. Field of the Exemplary Embodiments
The exemplary embodiments disclosed herein relate to measurement instruments and, more particularly, to measurement instruments having multiple sensors.
2. Brief Description of Related Developments
Many different types of sensors have been used to measure various physical quantities, for example pressure or density of a gas. As different types of sensors may have different operating ranges, it has been desired to combine different types of sensors into a single pressure instrument, with an extended operating range. For example, as the pressure of a gas is pumped down to vacuum, the output of the instrument may first correspond to a reading from one of the sensors. Then, when the output reaches a threshold value, the output may be switched to correspond to a reading from another sensor having better accuracy at the lower pressures. While this type of arrangement has an advantage in extending the pressure or density range over which the instrument can operate with suitable accuracy, there are also drawbacks. Most notably, a problem may arise in switching between the two sensors. If the two sensors do not produce identical readings at the threshold value, there may be an abrupt change in the output of the instrument when the instrument switches between sensors. Even if the difference in readings between the two sensors is small, the abrupt change can cause undesirable hysteresis effects. For example, problems may arise if the output is used as part of a feedback loop designed to control pressure. The difficulties may be more pronounced if a derivative of the output is used as a feedback signal in a feedback loop, because the derivative will be very high at the transition threshold. Therefore it may be desired to provide a pressure instrument that combine readings from two or more sensors and allow for smooth transitioning between the readings.