Select a gauge whose full scale reading
approximates 130 percent to 160 percent of the maximum operating pressure of
the system. Following this guideline, a gauge with a full scale reading of 4,000 or
5,000 psi would be satisfactory for installation in a system with a maximum operating
pressure of 3,000 psi
Selecting gauge accuracy and precision should be based on the type of system and
how the gauge will be used. For example, a high level of precision is not required
on air bank pressure gauges where only relative values are necessary to determine
how much air is left in the bank or when to shut down the charging compressor.
However, considerable accuracy (¼ of 1 percent of full scale for saturation diving
operations and 1 percent of full scale for surface supplied operations) is required
for gauges that read diver depth (pneumofathometers and chamber depth gauges).
Depth gauge accuracy is critical to selecting the proper decompression or treatment
table.
Many gauges are provided with a case blowout plug on the rear surface. The
blowout plug protects the operator in the event of Bourdon tube failure, when case
overpressurization could otherwise result in explosion of the gauge lens. The plug
must not be obstructed by brackets or other hardware.
All diving system gauges should be provided with gauge isolation valves and calibration
fittings. If a gauge fails during an operation, the isolation valve closes to
prevent loss of system pressure.
All installed gauges and portable gauges
(tank pressure gauges, submersible tank pressure gauges, and gauges in small
portable test sets) in use must be calibrated or compared in accordance with Planned Maintenance System schedule unless a malfunction requires repair and
calibration sooner. Programs such as the Shipboard Gauge Calibration Program as
outlined in the NAVSEA Instruction 4734.1 (series) provide authority for a
command to calibrate its own gauges. Calibrated gauges not in use should be kept
in a clean, dry, vibration-free environment. The Meteorology Requirements List,
NAVSEA OD-45845, should be consulted to determine storage times not considered
part of the calibration interval.
Calibration and comparison data must include the date of the last satisfactory
check, the date the next calibration is due, and the activity accomplishing the calibration.
Labels attached to gauge lens are satisfactory for recording this data.
When oxygen systems are being cleaned, gauge lines should be removed and
cleaned separately, after first cleaning the system with gauge lines attached. This
will ensure that the gauge lines are thoroughly flushed. All gauges should be
removed from the system prior to the cleaning process to avoid dead ends in the
system and damage to the gauges from the cleaning solution.
Gauges are delicate instruments and can be damaged by vibration, shock, or
impact. They should be mounted in locations that minimize these factors and
should always be mounted to gauge boards, panels, or brackets. The piping
connection should not be the sole support for the gauge. A gauge can be severely
damaged by rapid pulsations of the system when the fluid pressure is being
measured. When this condition exists, a gauge snubber should be installed
between the isolation valve and the gauge to protect the instrument. Most gauges
are not waterproof and are not designed for use in a marine environment. Enclosures
of transparent acrylic plastic, such as lucite, can be used to protect the
gauges from water and salt spray. However, the enclosure must have vent
passages to allow the atmospheric pressure to act on the gauge sensing element.
Manufacturers make two basic types of helical
Bourdon tube gauges for use on recompression chambers and for surface-supplied
diving systems. One is a caisson gauge with two ports on the back. The reference
port, which is capped, is sealed with ambient air pressure or is piped to the exterior
of the pressure chamber. The sensing port is left open to interior pressure. The
other gauge is the standard exterior gauge.
Both are direct-drive instruments employing a helical Bourdon tube as the sensing
element. The gauges are accurate to ¼ of 1 percent of full scale pressure at all dial
points. With no gears or linkages, the movement is unaffected by wear, and accuracy
and initial calibration remains permanent.
A comparative check in lieu of recalibration should be made in accordance with
the Planned Maintenance System. A dial adjustment screw on the front face of the
gauge provides for zero-point adjustment and special set pressure. Dial readout
units of measure can be in pounds per square inch (psi) and/or feet of seawater
(fsw).
