Level Instruments

Chapter from the Instrumentation & Control Design Guide.

LEVEL INSTRUMENT SELECTION & INSTALLATION

Level Instrument Type Comparison

Type	Principle	Range	Accuracy	Best Application	Limitations	Install Notes
DP Transmitter	Hydrostatic pressure	Any	±0.5-1%	Most common, reliable	Requires specific gravity input	Wet legs or capillary for HP/HT
Displacer (Fisher)	Buoyancy on displacer rod	0-60" typical	±0.5%	Interface level, narrow range	Limited range, moving parts	Level bridle/cage mounting
Guided Wave Radar	TDR pulse on probe	Up to 20 ft	±0.1"	Foam, turbulent, boiling service	Probe coating in sticky service	Top-mounted, avoid nozzle restrictions
Non-Contact Radar	Microwave reflection	Up to 100+ ft	±0.1"	Large tanks, clean vapor space	Foam/turbulence can absorb signal	Top-mounted, horn antenna typical
Magnetic Level Gauge	Float in external chamber	Up to 20 ft	Visual ±1/4"	Visual indication + switches	External chamber fouling	Side-mounted bridle, heat trace if needed
Float Switch	Mechanical float on arm	Point level only	±0.5"	High/low level alarms	Moving parts, sticking in dirty service	Simple, reliable for alarm-only
Capacitance Probe	Dielectric change	0-20 ft	±1%	Interface detection	Coating changes dielectric	Requires calibration per fluid
Nuclear/Gamma	Radiation attenuation	Any	±1%	Extreme service (lethal, plugging)	Radiation source licensing	Non-intrusive, through vessel wall

Level Nozzle Spacing Guidelines

Standard Level Nozzle Arrangement (from bottom up):
• LSLL (Low-Low Level Switch): bottom tangent line + 6" minimum
• LT connection (lower): LSLL + 6" (or bottom of range)
• Normal operating level: per process design
• LT connection (upper): top of range (min 6" below HLL nozzle)
• LSHH (High-High Level Switch): operating level + margin (depends on vessel)
• LAH (High Level Alarm): between normal and LSHH
• LAL (Low Level Alarm): between normal and LSLL

General Rules:
• Minimum nozzle spacing: 6" center-to-center (mechanical clearance)
• DP transmitter range: set for LSLL to LSHH + 10% margin each end
• All level instruments on same side of vessel (consistent reference)
• Isolation valves on all process connections (for maintenance)
• Drain valve at bottom of each bridle/cage

Source: Instrumentation_Control_Design_Guide_v1.xlsx · sheet “Level Instruments”

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