The piston gauge’s fundamental operating principle and excellent long term stability have made it an indispensable tool in high accuracy pressure metrology. As accuracy levels increase, quality assurance requirements intensify and automation spreads through the workplace, the piston gauge must evolve to continue to fill its essential role in the measurement system. PG7000, the first truly new, high end piston gauge introduced since the late 1970s, responds to this challenge.
PG7000 development was undertaken with four main inter-related design objectives:
• Deliver real improvements in fundamental metrological performance to assure that today’s and tomorrow’s ever increasing accuracy requirements can be supported.
• Integrate automated monitoring of environmental and instrument operating conditions with extensive on-board intelligence to provide a modern stand alone instrument that outputs fully validated reference pressures, real time, through an intuitive operator interface.
• Reduce and when possible eliminate the influence of the operator on measurements to assure more consistent performance. 
• Improve piston gauge ergonomics for greater operator satisfaction and increased productivity.
The PG7000 design objectives were pursued with the refinement of existing techniques, innovative new design, process improvements and the thorough application of today's digital and information processing technologies. The result is a coherent line of piston gauges that sets a new standard for both performance and usability, redefining the state of the art in high end pressure metrology. 

PG7202 is a member of the PG7000 family of reference level piston gauges that covers the range of high pressure gas up to 110 MPa (16 000 psi). Operating directly in gas avoids difficult to use liquid/gas separators that add uncertainty to the measurement process. PG7202’s gas operated, liquid lubricated piston-cylinder design overcomes the disadvantages of lubricating the piston-cylinder with gas at high pressure. Performance is not affected by gas cleanliness and piston drop rates are even lower than those typically found in hydraulic piston gauges.
PG7202 is combined with the GPC1 gas pressure controller to configure a complete manually operated calibration system with effort free gas pressure setting and adjusting (see GPC1 brochure). By substituting a PPCH-G automated pressure controller for GPC1 and adding on AMH-100 automated mass handler, PG7202 operation can be fully automated.

Gas operated, liquid lubricated piston-cylinder
The principle of operation of the PG7202 piston-cylinder is simple but very effective. The measured gas pressure, Pg, is applied to the bottom of the piston and to the top of a liquid reservoir located around the cylinder. The reservoir is connected to the gap between the piston and the cylinder through lateral holes in the cylinder, allowing liquid from the reservoir to enter the gap.
The pressure of the liquid in the gap, Pl, is equal to the gas pressure, Pg, plus the pressure resulting from the liquid head, h. Therefore, regardless of the gas pressure value, the liquid pressure in the gap is always higher than the gas pressure by the amount of the liquid head. Since h is small and the space between the piston and cylinder is typically less than 1 micron, the bleed of liquid from the bottom of the cylinder towards the gas pressure is minute. By design, any liquid accumulation drops directly into a deadended sump. No significant contamination of the test system occurs. As the liquid reservoir is contained in the piston-cylinder module, the piston-cylinder can be removed and installed in the PG7202 piston gauge platform with no loss of liquid from the reservoir. 
All PG7202 piston-cylinder modules can be delivered using Krytox®, a fluorinated synthetic oil, to lubricate the piston-cylinder for applications in which the system must be free of hydrocarbons.

Hydraulic operation
The PG7202 platform can also be operated using oil as the pressurized medium in the range of 1 to 200 MPa (145 to 30 000 psi). To operate with oil, piston-cylinder modules PC-7300-1, -2, or -5 are used (see PG7000 brochure). A simple 10 minute switch over procedure, detailed in the product manual, is all that is needed to switch between pneumatic and hydraulic operation.

Integrated piston-cylinder metrological modules
Each PG7000 piston-cylinder is an integrated metrological assembly that includes the critical piston-cylinder mounting components. To change piston-cylinders, the complete module is installed and removed from the piston gauge mounting post. All of the mechanical parts that affect piston-cylinder metrology are associated with the individual piston-cylinder rather than being common parts of the piston gauge platform. This unique design provides many practical and metrological benefits.
Benefits:
• Change ranges (piston-cylinders) in seconds, without using tools.
• Handle and interchange piston-cylinders without exposing critical surfaces to contamination.
• Protect the piston-cylinder from damage due to accidental shock or impact when handling.
• Improve measurement reproducibility by avoiding frequent assembly/disassembly of mounting components and assuring that each piston-cylinder is always used with the same mounting hardware, even when used in a different platform.
• Improve piston-cylinder mounting design by allowing each mounting system to be optimized for a piston-cylinder size and range rather than accepting the compromises of a single, interchangeable mounting system.

Mass loading concentricity
Piston gauge performance is greatly affected by piston-cylinder verticality and mass loading concentricity. For optimum performance, the piston-cylinder axis must be aligned as well as possible with the acceleration due to gravity and the mass load must be concentric with the piston-cylinder axis.
Assuring piston-cylinder verticality and mass loading quality were key PG7000 design objectives. Piston-cylinder module alignment is established by the large diameter mating surface between the mounting post and the piston-cylinder module. The number of independent parts between the piston and mass load has been reduced to two (the piston cap and the mass loading bell). The piston head is effectively made part of the piston by machining it after installation concentrical to the piston within ± 20 microns. PG7000 piston-cylinders have excellent mechanical characteristics but realizing their exceptional sensitivity and rotation times would not be possible without the special attention paid to alignment and mass loading quality.

Integrated electronics, software and remote interfacing
PG7000 is a modern, digital instrument taking full advantage of today’s sensor and data processing technologies supported by embedded software, an integrated local user interface and standard remote interfaces. Full feature operation is provided directly by the instrument itself without retrofits, add-on modules or external computers and software.
The electronics necessary for monitoring all ambient and instrument conditions and functions
are integrated into the PG7000 platform. The display and keypad for the local operator are located on a compact terminal with the look and feel of other DHI products. The system power supplies are also contained in the terminal to remove their heat source from the PG platform.
Embedded software supports extensive onboard functions: displaying individual ambient condition variables and PG operating parameters-storing and recalling piston-cylinder and mass set metrological data calculating fully compensated pressure to mass and mass to pressure values including all influences and head corrections-providing an objective, performance based go/no go indication of conditions for valid measurement.
Local operator interaction with the PG7000, as with other modern test instruments, is through an integrated keypad and alphanumeric display not requiring an external computer or software. The keypad is function driven allowing rapid, intuitive operation. In addition, both RS-232 and IEEE 488 interfaces with extensive, fully documented, ASCII string commands are included for remote communication.

On-board measurement of operating conditions
PG7000 includes integrated on-board measurement of all the ambient and operating conditions needed to calculate pressure within tolerance.
These include:
• Relative humidity: capacitance sensor on PG platform rear panel.
• Barometric pressure: internal piezoresistive sensor or by remote interfacing with any RS-232 barometer.
• Ambient temperature: platinum resistance thermometer mounted on PG platform rear panel.
• Piston-cylinder temperature: platinum resistance thermometer embedded in piston-cylinder mounting post.
• Reference vacuum (PG7601 only): piranni gauge integrated directly under bell jar vacuum plate.
The individual measurements can be observed real time, both locally through the PG Terminal and remotely over the RS-232 or IEEE 488 interface.
Provision for verification and recalibration of the on-board sensors is included and supported by PG7000’s embedded software.

Monitoring piston behavior
Precise information on piston behavior is indispensable to achieving best performance with a high accuracy piston gauge. Piston position and fall rate are important to assure the piston is at the proper point in its stroke and falling at its natural rate. Piston rotation rate monitoring, though frequently ignored, is also very important to assure consistent operating conditions and detect possible piston-cylinder contamination.
PG7000 measures and provides real time indication of:
• Piston position
• Piston drop rate
• Piston rotation rate
• Piston rotation deceleration
Piston position is measured on the LVDT principle with a ring on the inside of the mass loading bell acting as the armature. Rotation rate is measured optically using a sensor in the mounting post which detects the movement of a notched ring on the inside of the mass loading bell. Both measurement systems are completely non-interfering. They have no influence on the free movement of the piston in any axis.

Ready/not ready indication
PG7000 simplifies operation with a “ready/not ready” indication to provide the operator with a clear “go/no go” indication of when an intolerance measurement can be made.
The ready/not ready indication is based on testing for a variety of operating conditions including piston position, piston fall rate, piston rotation rate, piston rotation deceleration, piston temperature rate of change and vacuum reference (when applicable). A “ready” condition is indicated when all conditions fall within specific limits. If any condition is outside of limits, not ready is indicated with the responsible condition identified. The limits for the various “ready/not ready” criteria can be customized by the user if desired.
The “ready/not ready” feature assures that measurements are made under consistent conditions without requiring the operator to monitor multiple variables independently and/or make subjective judgements and decisions.

Piston near-float detection
Traditionally, one of the most tedious aspects of operating a piston gauge is adjusting pressure to float the piston. The procedure is delicate because the piston will lift, suddenly and without warning, only at the exact pressure corresponding to the mass loaded on the piston. Finding that point without overshooting requires slow and very cautious pressure control.
PG7000 makes it easier to float the piston (whether controlling pressure manually or automatically) with a piston preloading system that provides advance warning that the pressure is near the point where the piston will leave the end of stroke. The preload is applied by spring loaded end of stroke stops. The spring force is equivalent to about a 2 kg (4.4 lb) load on the piston. The preload causes the piston to begin moving away from the end of stroke before the pressure under the piston is equivalent to the mass loaded on the piston. The early movement of the piston is detected by PG7000 and visual and audible warnings that the piston is about to leave the end of stroke are provided. The preloading system only affects the piston when it is at the end of stroke, it does not interfere with the free movement of the piston when it is floating.
The piston preload system provides advance warning of piston float making it easier to
operate a PG7000. The spring loaded stops also provide cushioning to dampen the impact when the piston reaches the end of stroke to reduce wear and tear on the instrument.

Intelligent piston rotation
In high performance piston-cylinders, the piston must be rotated to operate properly. The rotation sets up centering forces that keep the piston aligned in the cylinder assuring a uniform annular gap and the piston’s freedom and mobility. For the most repeatable pressure measurements, it is important that the rotation rate be consistent. In addition, when measurements are made, no drive system can be engaged as it would contribute parasitic forces and unquantifiable errors.
PG7000 is the first commercially available piston gauge to provide monitoring of rotation rate and decay in rotation rate. These measurements, coupled with PG7000’s internal logic, are used to assure that pressure readings are always made within rotation rate and decay in rotation rate limits. This relieves the operator of rotation rate monitoring responsibility and replaces subjective operator judgement with objective measurement.
In PG7000 platforms, the rotation stimulus may be provided either manually or by an optional motor drive. In most cases, manual rotation is adequate as PG7000 piston-cylinders require only occasional rotational acceleration and the rate of rotation monitoring system assures that measurements will be made within proper limits.
Motor driven rotation, when needed, is provided by a patented system that engages and disengages intelligently, depending on current rate of rotation, to maintain the rotation rate within limits. Unlike conventional piston drive systems, drive contact never occurs randomly or without warning. The rotation system can engage with the piston in any position and does not significantly affect its position or the set pressure. A warning is provided when the rotation system is about to engage and while it is running. An override is available so that engagement will occur only on operator demand.

Advanced pressure generation and control components
In day to day operation, the operator’s main interaction with a piston gauge based pressure calibration system is with the pressure generation and control components. These are used constantly, both for the large pressure changes between increments and to perform the fine adjustments needed to float and refloat the piston. Over the years, the ease of use and reliability of these components will weigh heavily in the system’s value as a calibration tool.
The central role of pressure generation and control components in meeting PG7000’s objectives was recognized at the start of the PG7000 development process and stayed at the forefront. All PG7000 pressure accessories were designed specifically for their application, searching out ways to increase the efficiency and ergonomics of piston gauge operation.
PG7000 introduced fully automated gas pressure control in a bench top piston gauge system which, coupled with PG7000’s other intelligent features, leaves only loading and unloading masses in response to system prompts to the operator.
A manual gas pressure controller is also available. It is compact in size to facilitate system setup and features custom, soft grip valve knobs—not just a detail if you are the one using them day in and day out.
OPG1, the hydraulic pressure generator/controller for PG7302, breaks new ground in oil piston gauge operation. With a perfect balance between the benefits of automation and the practicality of direct operator control, it sets pressures effortlessly, precisely and very quickly, eliminating the pumping and screw press cranking for which oil piston gauges were once notorious.

COMPASS® for Pressure calibration assistance software
COMPASS for Pressure calibration assistance software takes PG7000 to the next step in automating calibrations.
Since PG7000 on-board electronics and software handle all the basic metrological and system monitoring aspects of PG7000 operation, COMPASS is not required to operate the system and make accurate measurements. COMPASS concentrates on optimizing the broader application of PG7000 in a calibration laboratory. COMPASS sets up device under test (DUT) records, defines and associates test procedures with DUTs, runs tests, acquires reference and test data, produces standard and custom calibration reports. All instrument, DUT and test data is collected and stored in standard delimited files that can be easily downloaded to other applications.