Tubing, Piping, and Wiring/Termination
Tubing and piping
Instrument tubing is sized by outside diameter (1/4, 3/8, 1/2 in) and wall thickness; pipe is sized nominally. Cut tube square, deburr, and insert it fully into the fitting. Two-ferrule compression fittings are made up hand-tight plus the manufacturer's specified turns (often about 1-1/4), and brands should not be mixed. Respect minimum bend radius and support the run. Threaded joints are NPT: apply sealant to the male threads only, leaving the first thread or two bare, and use a backup wrench. Keep impulse-line volume small for pressure and DP work, heat trace or purge lines that can freeze or plug, and install root, vent, and drain valves.
Wiring and termination
A two-wire 4-20 mA loop uses a DC supply, typically 24 V, with the transmitter modulating current and a receiver developing the voltage. Polarity matters, and total loop resistance must stay within the transmitter's drive capability. Use shielded twisted pair with the drain grounded at one point only, normally the control room, to avoid ground loops. Intrinsically safe circuits are usually blue-jacketed and must be separated from non-IS wiring by spacing or a barrier. In hazardous locations, conduit seals are installed within the code-required distance of the enclosure to stop gas migration. Terminate with the correct ferrule or lug, torque to specification, and tag both ends.
Traps
Shield grounded at both ends, sealant on the first threads, over-tightened fittings, and signal cable run parallel to VFD power.
Rule of thumb
Keep signal wiring at least 12 in from AC power and cross at 90 degrees.
Worked example
With a 250-ohm sense resistor, 4 mA gives 1 V and 20 mA gives 5 V; a 1-5 V input card plus 250 ohms sets the loop burden.