Ashcroft A4A Precision Pressure Test Gauge, 6 inch dial size

The Ashcroft A4A Precision Dial Pressure Gauge is known for high accuracy and performance. Ideal for test, calibration, and validation applications. The A4A is the new standard for precision test gauges. It yields consistent, reliable accurate results through the use of state-of-the-art precision machining and the world's most refined Bourdon Tube technology. This eliminates the need for power source with digital gauges. It also avoids other issues with digital gauges such as susceptibility to electrical noise, power outage, or fire hazards in hazardous areas.

The main differences between an Ashcroft A4A gauge and other “precision pressure gauges” can be found in the Bourdon tube and the quality of the other components used in the gauge. The process of making a Bourdon tube for the A4A gauge takes 15 days to complete and produces the most highly developed pressure sensing element of its type. In the A4A gauge this tube is combined with a movement which incorporates precision brass gears and miniature stainless steel ball bearings to provide the most accurate conversion of tip travel to pointer movement possible.

These components are all mounted in a heavy duty cast aluminum case which provides a rigid base. As a result of the rigid mounting platform the A4A is immune to inaccuracy caused by pipe strain and case flexing. In many other gauges these are common problems often caused by stresses generated during the installation process.

Simple to operate, easy to troubleshoot, and can be readily flushed or purged to remove foreign matter or trapped gas.

The Ashcroft A4A precision pressure gauge consists of a Bourdon tube elastic element, a movement, case, dial and pointer. The accuracy of a pressure gauge is contingent on the performance capabilities of the sensing element, movement and the accuracy of the scale graduations which are imprinted on the dial.

A mechanical pressure gauge measures pressure by converting the movement of the elastic element into the movement of a pointer across a dial.

The Ashcroft A4A precision pressure gauge incorporates a highly engineered Bourdon tube as a pressure sensing element. A Bourdon tube is a piece of tubing that has been bent into a circular shape. Depending on the pressure range the Ashcroft A4A gauge may include a single or multi-coil Bourdon tube.

During the measurement process pressure is introduced to the inside of the Bourdon tube via the externally accessible process connection. When pressure is applied to the Bourdon tube it straightens out a small amount (vacuum actually pulls the free end of the coil in tighter). Since the process connection is locked into position by the mounting of the Bourdon tube into the gauge case, the movement caused by the application of pressure or vacuum is limited to the loose end or the tip of the tube. This tube movement is commonly called deflection or tip travel.

The tip travel is converted to a pressure measurement via a system of gears that drive a pointer. The gear system is commonly referred to as the gauge movement.

The pointer moves with the application of pressure or vacuum over a dial imprinted with information including pressure range, engineering units, graduations and other pertinent information.

The Bourdon System

The Ashcroft A4A Bourdon tube is a product of more than 40 years of intensive, continuous research. The “unitized” one-piece tube system is an outstanding example of modern technology.

Through exclusive Ashcroft A4A Bourdon tube designs and manufacturing techniques, uniform stress distribution is provided in a wide range of tubes of varying cross sectional configurations. All A4A tubes are formed from a single piece of seamless tube material.
Tubes of 403 stainless steel are standard for ranges of 50 psi and above. Beryllium Copper may be supplied for low pressures and special applications.

The Bourdon tube used in the Ashcroft A4A Gauge is a “Unitized” single or multi-coil element which is approximately three times the length of tubes used in competitive gauges. The greater length results in a much lower deflection rate, minimizes stress and virtually eliminates hysteresis, creep and fatigue effect.

The unitized tube design eliminates soldered, welded or threaded joints which further provides for controlled distribution of stress during the tube forming operations. Internal traps in which foreign matter may accumulate are also eliminated. The interior of the Ashcroft A4A tube may be easily cleaned by positive flow through flushing through the standard tip flushing (bleeder cap) connector.

All Ashcroft A4A Bourdon tubes are heat treated in high vacuum to maintain internal cleanliness and optimize mechanical properties. Ashcroft laboratories pioneered and continues to lead in the development of precision Bourdon tubes for ultra-high pressure gauges in ranges to 100,000 psi.

Dial and Pointer

The dial is supported at the center by a dial pivot which passes through the solid front gauge case and extends into an aligning recess in the base casting of the movement. This provides concentricity of the dial and pointer shaft of the movement, assuring maximum accuracy.

Black dial graduations on a white background are clearly defined and highly legible assuring the resolution necessary to obtain accurate measurements. All dials are generated on a state of the art computer based design system. All Ashcroft gauges include a mirror band which allows for the elimination of parallax reading errors. Each gauge carries an individual serial number on the dial to facilitate identification.

The reading end of the knife-edge pointer provides a plane which is perpendicular to the mirror band on the dial face aiding in the elimination of parallax errors.

A zero adjustment knob on the front of the gauge case allows for easy zeroing of the gauge. This same adjustment system allows an operator to set the gauge to display barometric pressure, when setting up an absolute A4A gauge or to set the dial to some other predetermined pressure pre-load. One knob rotates the dial, not the pointer, through approximately 30 degrees of arc; the other knob locks the bezel. A friction brake, acting on the dial, prevents accidental rotation.

Where a constant value is to be carried through a series of readings, such as a tare*, this adjustment eliminates the need for computing values at each reading.

*A tare value is a pressure pre-load which may not need to be reflected in the measured value. For example, in weighing operations the pressure generated by the weighing platform is often tared or zeroed out prior to beginning the measurement process.

Optional Slotted Link

The slotted link option is designed to protect gauges from movement damage which could result in applications that produce sudden, violent release of system pressure. When gauges are used for burst or tensile testing, comprehensive strength testing or similar applications, a sudden release of pressure is the norm. In these types of applications a slotted link prevents violent flutter of the pointer and possible damage to the gauge movement.

Optional Peak Load Indicator

It is often necessary to retain the highest or lowest measured value during a pressure cycle or test. The peak load indicator is available to simplify the recording of this information. The peak load indicator is a red tracking pointer, often referred to as a lazy hand. Movement of the pointer actuates the peak load indicator in either a positive or negative direction. The red pointer will remain at the highest or lowest measured value during the pressure cycle. An exterior knob permits convenient resetting for subsequent pressure cycles. The peak load indicator has been carefully engineered so that it will not diminish the ±0.1% accuracy of the A4A gauge.

Optional Thermal Compensator (Patented)

Holds the gauge in calibration to the ±0.1% accuracy rating through ambient temperature ranges from -25°F to +125°F. Recommended for precision applications where ambient temperature varies by more than ± 10°F. The thermal compensator is comprised of a bimetallic bar which forms part of the linkage between the tip of the Bourdon tube and the gear segment slide of the gauge movement. Deflection of the bimetallic bar, resulting from a change in ambient temperature, causes the point at which motion is imparted to the gear segment slide, to move in an arc. This results in a change in both the angular position and effective length of the slide. The length of the bimetallic bar is trimmed specifically to offset the thermal characteristics of the Bourdon tube with which it is used.

Correction for ambient temperature change is completely automatic and requires no adjustment of the compensator, pointer or dial.