NOTE This standard covers pressure measurement instruments installed to indicate, transmit, or switch on process pressure in construction and process-facility projects. (1.1)

NOTE It governs local-indicating pressure gauges, electronic pressure transmitters (gauge, absolute, differential, and multivariable), pressure switches, diaphragm seals, gauge snubbers, and the isolation and test valves immediately associated with each instrument. The pressure range of interest runs from high vacuum through 10,000 psi gauge and from high-vacuum to high-pressure absolute service. (1.2)

NOTE The pressure-measurement instrument family appears in nearly every facility type, and this standard applies across all of them. (1.3)

NOTE Industrial process plants, water and wastewater treatment works, HVAC and mechanical systems, fire-protection systems, and general commercial construction all rely on pressure indication and transmission. The selection logic -- range, accuracy, wetted material, connection, and signal -- is common across these settings even though the service pressures and chemistries differ widely. (1.4)

NOTE Flow inference from a differential-pressure element across an orifice plate or venturi is covered by Flow Measurement, not this standard, even though the sensing device is a DP transmitter. (1.7)

NOTE Hydrostatic or differential-pressure level measurement that infers liquid level in a vessel is covered by Level Sensors. (1.8)

NOTE Temperature measurement -- RTDs, thermocouples, and temperature transmitters -- is covered by Temperature Instrumentation; coordinate with that standard where a single multivariable transmitter reports both pressure and temperature. (1.9)

NOTE Pressure-regulating and pressure-relief valves with actuators are covered by Control Valves And Actuators; this standard covers only the measuring instrument, not the final control element. (1.10)

NOTE Fieldbus segment design, marshalling, and network topology for transmitter communications are covered by Process Control Networks; display, alarming, and historian configuration are covered by Scada And Hmi Systems. (1.11)

NOTE General instrument loop documentation, P&ID symbology beyond the pressure instrument, and control-panel wiring practice are covered by Process Instrumentation. (1.12)

NOTE Building-automation pressure points that are BAS-native sensors -- duct static pressure and chilled-water differential pressure for HVAC pump control -- are covered by Building Automation System. (1.13)

NOTE Where a pressure instrument is installed on a process covered by OSHA Process Safety Management (29 CFR 1910.119) because it handles a highly hazardous chemical above its threshold quantity, the instrument's mechanical-integrity, calibration, and documentation records are PSM process-safety information; this standard sets the instrument requirements, and the project PSM program governs the records retention. (1.14)

NOTE ANSI/ISA-51.1 remains the primary terminology authority for pressure instrument performance terms even though it predates current transmitter technology and is presently under revision; where its 1979 definitions of accuracy, span, range, hysteresis, and repeatability are cited on a datasheet, the cited terms carry the ISA-51.1 meaning. (2.3)

NOTE ASME B40.100-2022 consolidates the former B40.1, B40.2, B40.5, B40.6, and B40.7 documents and is the governing standard for analog dial-type gauges. Its accuracy grades -- Grade A at ± 1 % of span, Grade 2A at ± 0.5 %, and Grade B at ± 2 % -- are the values cited on gauge datasheets in this standard. (4.2)

NOTE Unless the zero and span are stated on the instrument datasheet or instrument index, transmitters ship at the factory default of 0 to 100 % of catalog span. A transmitter left at catalog default reads correctly only by coincidence, and the error is silent until a loop check catches it. (4.7)

NOTE Field-installed tags that do not match the P&ID generate commissioning RFIs and stall loop checks. Tag reconciliation is cheap during the construction-document phase and expensive at startup. (4.10)

NOTE IEC 61511 requires a documented SRS that defines the safety function, its safe state, and its target SIL before instrument selection. Calling out a "SIL 2 transmitter" with no SRS behind it is a contractual and safety risk -- the rating is meaningless without the function definition that sizes it. (4.12)

NOTE A bare "weatherproof" call-out has no defined meaning under NEMA 250 or IEC 60529. Outdoor, wash-down, and wet locations require an explicit NEMA 4X (or IP66/IP67) rating, and classified areas require an explicit NEMA 7 or intrinsically safe designation. (5.2)

NOTE The choice between explosion-proof housings and an intrinsically safe loop with a barrier is a project-wide convention, not an instrument-by-instrument decision; NEC 501.15 governs conduit seals and 501.105 governs instruments in Class I locations. (5.6)

NOTE Standard transmitters operate from -40 °F to +185 °F (-40 °C to +85 °C); extended-range units reach -67 °F to +221 °F. Cold-climate outdoor installations and hot process-adjacent mountings both push past the standard band, and the wrong rating produces drift or a dead display in the field. (5.9)

NOTE Every material in contact with drinking water must be NSF 61 certified, including the gauge socket, the transmitter diaphragm, and any seal fill fluid. AWWA M20 carries the same requirement for water-system instruments, and the certification must be stated explicitly -- it is not implied by a 316 stainless-steel call-out. (5.12)

NOTE A local gauge gives standalone visual indication with no power and no signal. A transmitter delivers a remote analog or digital signal to the control system. A pressure switch provides a discrete contact at a setpoint. The duplex arrangement pairs a transmitter for control with a local gauge for backup indication, which is the common pattern at field instruments where an operator needs to confirm the reading at the equipment. (6.2)

NOTE The 80 % case on US industrial and commercial projects is a two-wire 4-20 mA HART transmitter in a NEMA 4X enclosure, with a local gauge specified alongside for backup indication. (6.3)

NOTE Local gauges remain near-universal as backup even where a transmitter carries the signal, because an operator at the equipment must be able to read pressure with no power and no DCS. Treat the transmitter and the local gauge as a pair on most field instruments rather than an either/or choice. (6.4)

NOTE A gauge instrument reads relative to atmosphere; an absolute instrument reads relative to a vacuum reference; a differential instrument reads the pressure difference across two ports; a sealed-gauge instrument reads against a fixed sealed reference for cryogenic or high-vacuum service. Confusing gauge and absolute reference in vacuum or low-pressure service produces readings that are wrong by one atmosphere, so the reference is never left to be inferred. (6.6)

NOTE A gauge or transmitter is least accurate at the bottom of its scale. Selecting a 0-600 psi gauge for a 100 psi system parks the pointer in the low end where readability and accuracy are worst, while a range that sits the normal reading near mid-scale uses the instrument's best band. This 25-75 % rule from ASME B40.100 is the single most common range error and the easiest to avoid. (7.2)

NOTE HVAC and domestic water run 0-160 psi; chilled and hot water 0-100 psi; low-pressure steam 0-30 psi; high-pressure steam 0-300 psi; compressed air 0-200 psi; and natural-gas utility service 0-15 psi or 0-60 psi. These are starting points to be checked against the 25-75 % rule for the actual normal operating pressure, not substitutes for it. Coordinate gas-service ranges with Natural Gas Piping and hydronic ranges with Hydronic Piping. (7.6)

NOTE ASME B40.100 Grade A gauges are rated to withstand 130 % of full-scale without permanent error. Transmitters typically tolerate 2x to 3x their range without damage. For differential-pressure cells the static line pressure that the cell sees is a separate and often larger limit than the DP overpressure, and it must be specified alongside the DP range. (7.8)

NOTE A DP cell can be destroyed by static pressure on the high-side connection that exceeds the cell's static rating even when the differential is small. Ignoring static line pressure is a classic DP-cell failure, so the static rating is specified together with the DP range, never separately. (7.10)

NOTE Grade A delivers ± 1 % of span and is the field default; Grade 2A delivers ± 0.5 % for readings used in adjustment or acceptance; Grade B at ± 2 % is acceptable only for coarse utility indication. The grade is matched to how the reading is used, not chosen for its own sake. (8.2)

NOTE Mid-tier process transmitters specify a reference accuracy from ± 0.065 % to ± 0.25 % of calibrated span. Tighter accuracy carries cost and is justified by custody-transfer, performance-test, or safety-loop service rather than applied universally. (8.4)

NOTE The 1/2 in NPT connection is the workhorse for both gauges and transmitters; 1/4 in NPT appears on small or high-pressure instruments. NPT threads are the most common pressure-instrument connection and are governed by ASME B1.20.1. Flanged connections per ASME B16.5 are reserved for elevated temperature or high pressure. (9.2)

NOTE 316 SS covers most water, air, steam, and benign-process service. It is not suitable for high-chloride, strong-acid, or strong-caustic chemistry, where chloride stress-corrosion cracking and pitting attack the wetted parts. Hastelloy C-276, Monel, or PTFE-lined wetted parts must be specified explicitly for aggressive chemistry such as chlorine, acids, and caustics -- they are never assumed from a generic stainless call-out. (10.2)

NOTE A direct-connected gauge or transmitter on a viscous, slurry, or crystallizing process plugs at the connection and stops reading. A flush-diaphragm seal isolates the instrument behind a process-wetted diaphragm and a captive fill fluid, keeping the process out of the sensing element. Sanitary and hygienic processes require a flush seal with a tri-clamp connection and a 3-A certified or NSF-listed assembly. (11.2)

NOTE Glycerin is the standard fill for general and vibrating service; silicone oil suits high-temperature ambients above 140 °F; food-grade white mineral oil or food-grade silicone is required for sanitary and NSF service. The fill fluid is a process-contact material through the diaphragm, so its food or potable rating matters as much as the diaphragm metal. (11.4)

NOTE The Bourdon-tube dial gauge is the standard local indicator for most pressure service. A digital gauge with an LCD display suits readings taken close-up where resolution matters, and a differential-pressure dial gauge (diaphragm or piston type) suits filter-monitoring and other low-DP applications where a Bourdon tube cannot resolve the small difference. (12.2)

NOTE Dial size is driven by viewing distance and available space. A 4 in dial reads cleanly at the equipment, a 6 in dial reads across a control room or panel front, and a 2.5 in dial fits a crowded manifold. Oversizing wastes space and undersizing hurts readability. (12.4)

NOTE The two-wire 4-20 mA loop with HART revision 7 is the 80 % default for US industrial projects: it powers and signals on one pair, and HART carries digital configuration and diagnostics over the analog loop. Foundation Fieldbus H1, PROFIBUS PA, and WirelessHART (ISA100/IEC 62591) are specified where the project network architecture in Process Control Networks calls for them. (13.2)

NOTE HART 5 and HART 7 are not fully interoperable, and a transmitter that defaults to HART 5 may not present its full diagnostics to a current HART multiplexer or DCS. Specifying HART revision 7 minimum avoids a commissioning surprise where the device communicates but the host cannot read everything it expects. (13.4)

NOTE A DP transmitter cannot be zeroed, calibrated, or removed in service without an isolation manifold. The two-valve manifold isolates a single-port instrument; the three-valve manifold equalizes a DP cell for zero check; the five-valve manifold adds bleed/vent ports for full block-and-bleed on a DP transmitter. The manifold is part of the DP installation, not an accessory. (13.8)

NOTE A pressure switch closes or opens a discrete contact at a setpoint to start a pump, trip an alarm, or interlock equipment. The single-pole double-throw snap-action switch is the common form. Where the switch performs a safety function, a transmitter with a programmable trip in the logic solver is generally preferred over a mechanical switch because it is testable and diagnosable per Process Instrumentation. (14.2)

NOTE Pressure pulses from reciprocating equipment fatigue a Bourdon tube or sensor in weeks. A snubber -- sintered-metal or adjustable piston type -- damps the pulses before they reach the sensing element. On a pulsating-service gauge the snubber and a glycerin case fill are specified together, because each addresses a different part of the same problem. (15.2)

NOTE A 1/2 in NPT needle valve (gauge saver) isolates a single-port instrument. A two-valve block-and-bleed manifold serves a single-port transmitter, and a five-valve manifold serves a differential-pressure transmitter. Without isolation, calibration and replacement require a line shutdown, which is rarely acceptable in service. (15.4)

NOTE Overpressure-protection and loss-of-containment safety loops require a SIL 2 or SIL 3 transmitter certified per IEC 61508 for hardware fault tolerance and average probability of failure on demand. The instrument's certified SIL is a property of the device; the SIL of the safety function is established by the SRS and verified for the whole loop, including the logic solver and final element in Control Valves And Actuators. (16.2)

NOTE Safety-instrumented-function and critical instruments are recalibrated annually; general process transmitters carry a maximum interval of five years under typical plant standards. The interval belongs in the operation and maintenance manual and feeds the facility's mechanical-integrity program. (17.6)

NOTE In gas service the impulse line slopes back toward the process so condensate drains away from the instrument; in liquid service it slopes so gas vents back to the line. A line sloped the wrong way traps liquid or gas in the impulse leg and produces a standing offset in the reading that no calibration can remove. (18.3)

NOTE A practical spares package covers the wear and consumable items: gauge isolation/needle valves, snubbers, transmitter LCD modules, manifold seal kits, and at least one complete spare of each critical transmitter range used in the project. Stocking a spare of each critical range avoids a long lead time when a safety or production loop loses its instrument. (21.2)