Pressure transducers have been employed in a myriad of applications. One such transducer is the capacitance manometer which provides very precise and accurate measurements of pressure of a gas, vapor or other fluid. Applications include precise pressure measurement and high-precision gas and vapor delivery systems, which have become very important in many industrial applications, for example in the semiconductor industry for wafer and chip fabrication, although other applications are known. Such fluid delivery systems typically also include, but are not limited to, devices such as mass flow controllers (MFCs) and mass flow verifiers (MFVs) to regulate and/or monitor the flow of gases and vapors.
Capacitance manometers typically use a flexible diaphragm forming or including an electrode structure and a fixed electrode structure spaced from the diaphragm so as to establish a capacitance there between. Variation in pressure on one side of the diaphragm relative to the pressure on the opposite side of the diaphragm causes the diaphragm to flex so that the capacitance between the electrode structure of the diaphragm and the fixed electrode structure varies as a function of this differential pressure. Usually, the gas or vapor on one side of the diaphragm is at the pressure being measured, while the gas or vapor on the opposite side of the diaphragm is at a known reference value, whether at atmosphere, or some fixed high or low (vacuum) pressure, so that the pressure on the measuring side of the diaphragm can be determined as a function of the capacitance measurement.
When a capacitance manometer is used to measure pressure and the diaphragm flexes to provide a capacitance change, it is expected that when the pressure on the measuring side of the diaphragm returns to the same pressure as the reference side (the “zero state”), the instrument will indicate a “zero” reading. However, over time, for various reasons, the reading will drift from zero when the manometer returns to a zero state. Accordingly, the manometer reading must be zeroed and calibrated from time to time to return the reading to zero for the zero state. For prior art capacitance manometers, determination of whether the manometer needs to be zeroed (e.g., have its zero-reading calibrated) has typically been decided on a time related basis, e.g., every six months, or on a routine basis, e.g., every preventative maintenance cycle (PM), or never. The choice of how often to zero the manometer is typically left to the discretion of an operator or user (e.g., a semiconductor process engineer). Such a decision has typically been based on a human inspection of previous drifts.
Any recording of accumulated drift for prior art capacitance manometers, has typically been accomplished by measuring such drift manually during routine zeroing. A judgment by the user then would then need to be made about the zeroing frequency, or whether the manometer has reached the end of its useful life.
In practice, users have typically not made such judgments due to various reasons including lack of time and/or complexity of factors involved. Instead, users have typically decided to zero or replace a manometer in reaction to an alarm on the related process tool or system or when they notice the manometer can no longer be zeroed (out of adjustment). In either case, the replacement or zeroing of the manometer causes an unexpected equipment failure problem (down time of the tool or system). Such unscheduled maintenance or replacements are something that must be dealt with outside of routine maintenance schedules, which is costly in terms of efficient use of the overall tool or system.
Thus, users have had to make a priori judgments about the frequency of zeroing. Determination of whether a manometer needs routine maintenance such as recalibration has generally not been done. Any recording of the manometer cumulative zero changes are typically also performed manually, an inconvenient process. In fact, the user generally only knows when a problem exists when there is a tool or system alarm.
What is desirable, therefore, are systems, methods, and apparatus that address the limitations noted for the prior art by predicting and/or indicating when a capacitance manometer, such as those used for high-precision fluid delivery systems, requires zeroing, maintenance, and/or replacement, and automatically calibrating the system to correct for drift when possible.