FPG8601 Automated Calibration System

Operating Principles

Measurement

FPG8601 operates on the well known piston gauge principle in which pressure applied to the effective area of a piston-cylinder is transformed into a proportional force. However, rather than being balanced against masses accelerated by gravity, the force resulting from pressure is measured by a force balanced load cell to which the piston is attached. As the pistoncylinder is fitted with pressure chambers at both its top and bottom, the reference pressure against which pressure is defined can be changed (atmosphere for gauge, vacuum for absolute). Zeroing the load cell with the high and low chambers at a common pressure, tares out the mass of the piston and any other forces not due to measured pressure, allowing pressure measurement to start from zero. The value of the differential pressure applied is calculated from the effective area of the piston-cylinder and the net force value measured by the load cell.

To avoid the instability and mechanical complexity of centering the piston by rotating it in the cylinder, the FPG piston is centered by the flow of an independent lubricating gas pressure in the piston-cylinder gap. The gap is conical, narrowing symmetrically toward the piston's extremities. The piston-cylinder gap is 1 to 6 μm and the lubricating pressure is 40 kPa greater than the reference pressure, keeping flow into the measurement chambers very low (less than 1 sccm total). The force across the piston is transmitted to the load cell through a gimbal coupling system that holds the piston at its center of gravity. The connecting system passage is also used to supply the piston-cylinder lubricating gas. The load cell is enclosed in a hermetic chamber through which the lubricating gas flows. The design of the load cell chamber contributes to maintaining its temperature constant and the lubricating gas is conditioned to assure relative humidity.

Control

The FPG8601 pressure controller operates by the adjustment of flow across a flow restriction. The upstream side of the restriction is connected to the upper FPG pressure chamber and the downstream side is connected to the lower FPG pressure chamber and to atmosphere or to an independent vacuum source for absolute mode operation. Several flow restrictions of different conductance are included in the controller and the one appropriate for the pressure range is selected automatically. Two mass flow controllers (MFC), one for coarse pressure control and the other, lower range, for fine pressure control, are used in parallel to adjust the flow in a feedback loop. Control is based on the difference between the pressure set point and the FPG pressure measurement. A two stage pressure regulator whose second stage is referenced to the downstream side of the flow restrictions supplies a stable input pressure to the mass flow controllers.

Automated Operation

The FPG8601 includes a Windows® based system controller. The system controller communicates with the FPG8601 hardware and devices under test. FPG Tools™ software monitors and controls FPG operation and supports a wide variety of high level functions, including running fully automated, unattended test sequences with device under test data acquistion. All FPG and device under test data are recorded in delimited data files that are easily downloaded to other applications for analysis. Some of the functions of FPG Tools software include:

• Measurement mode (gauge/absolute) changes
• Auto zero and auto span the load cell
• Data point averaging time adjust
• Excessive change in operating conditions alarms
• Thermal transpiration corrections in low absolute
• Device under test setup
• Test procedure definition and storage
• Fully automated test execution
• Real time test result plotting

Gauge, Absolute and Absolute Differential Measurement Modes

FPG8601 supports three different measurement modes:

• Gauge mode: The lower chamber of the FPG and the "low" side of the test are connected together and left open to atmosphere.
• Absolute mode: The lower chamber of the FPG is evacuated and the residual vacuum is measured by a vacuum gauge. This mode is used to calibrate sealed absolute DUTs.
• Absolute differential mode: The lower chamber of the FPG and the "low" side of the test are connected together and evacuated. This mode is used to calibrate differential DUTs relative to vacuum.

Bench Top System without Special Environmental Requirements

FPG8601 can be installed on a high quality, 2 m (6 ft.) laboratory bench. The pressure controller is often mounted under the bench. Location of the vacuum pumps needs to be considered when absolute pressure capability is included. A custom mounting bench can be supplied with the system if desired.

The environmental requirements for FPG8601 operation are those typically found in a high end metrology laboratory.

Installation and Training Service

Three to five days of installation and training service is recommended with a new FPG8601 installation. This service includes setup and testing of the FPG8601 system as well as training of users on operation and maintenance, including running typical tests. Training can be supplied at Fluke Calibration's Phoenix, Arizona facility prior to delivery and/or at the installation location following delivery.