Selection, wiring and fault finding — engineering notes

4-20 mA Loop Fault Finding: A Six-Fault Order

Field order for 4-20 mA loops: live zero, voltage headroom at 20 mA, ground loops and shield bonding, plus the one measurement that splits each fault class.

Loop supply

24 V DC

Receiver input resistor

250 ohm

Cable loop resistance, illustrative run

35 ohm

IP67 and IP69K: Test Method, Not Protection

Why IP67 and IP69K are not a ranking: the water test behind each digit, and what CIP and SIP cycles really do to housings, gaskets and cable entries.

EPDM, hot water and steam

good

EPDM, mineral oil and grease

poor

FKM, caustic at elevated temperature

variable

Laser Triangulation vs Time-of-Flight

How laser triangulation and time-of-flight sensors fail on shiny, dark and transparent targets, and which mechanism decides between them.

Required resolution

decides the mechanism first

Standoff available

decides the second

Target optical behaviour

decides the model within the mechanism

Turbidity Sensor Selection: Four Parameters That Decide It

How to pick a turbidity sensor: measuring band, nephelometric versus transmitted-light geometry, window fouling and the sample plumbing that sets the zero.

Nephelometric, 90 degree scatter

0.02 - 40 NTU

Ratio, scattered over transmitted

1 - 400 NTU

Transmitted light, attenuation

40 - 4000 NTU

What an Online Turbidity Reading Does Not Tell You

An inline turbidity value tracks what passes the optical window. It cannot report particle shape, colour or dissolved colour.