VIBRATING FORK SENSOR

  f  <  100 Hz       →  FAILURE  (wire break or sensor fault)
  f  ∈  500–900 Hz   →  COVERED  (liquid present, LSHH alarm active)
  f  ∈  1100–1300 Hz →  FREE     (no liquid, normal level)

  Variables :
    f   measured vibration frequency (Hz)

  Rationale : the vibrating fork oscillates at its natural resonance
  frequency (~ 1200 Hz). When a liquid covers it, the added mass
  lowers the frequency to ~ 700 Hz. A frequency below 100 Hz means
  no signal at all — automatic fault diagnostic. Reference
  calibration : Endress+Hauser Liquiphant FTL.


NAMUR NE 43 — 4-20 mA LOOP DIAGNOSTIC

  I  <  3.6 mA         →  Fault low     (broken wire, sensor dead)
  I  ∈  3.6–3.8 mA     →  Under-range   (measurement below low limit)
  I  ∈  3.8–20.5 mA    →  Normal usable range
  I  ∈  20.5–21.0 mA   →  Over-range    (measurement above high limit)
  I  >  21 mA          →  Fault high    (short circuit, sensor dead)

  Variables :
    I   4-20 mA loop current (mA)

  Rationale : the NAMUR NE 43 standard (1994, revised 2003)
  standardises the current ranges meaning "normal measurement" vs.
  "sensor diagnostic". Before it, each manufacturer had its own
  convention. Today it is required by all SIL systems and pharma
  (21 CFR Part 11), because it enables automatic diagnostic without
  an additional channel.