Selecting a Level Instrument

Quick pick (TL;DR)

Most clean/most liquids → guided-wave radar (GWR): unaffected by density, pressure & temperature changes.
Open, large or dusty (silos) → non-contact radar or ultrasonic.
Density & pressure stable, or interface/sealed systems → DP (hydrostatic).
Foam, coating or low dielectric? GWR/capacitance handle these; ultrasonic does not.
Always confirm range, accuracy and a non-contact option for sticky/corrosive fluids.

Step 1 — Select by situation

Situation	Best technology	Why
Most liquids	GWR	P/T independent
Open / dusty	Non-contact radar	No contact
Stable density	DP (hydrostatic)	Cheap, interface
Simple / budget	Ultrasonic	Cheap

Step 2 — DP level example

DP level: pressure → height

Principle P = ρ · g · h (head pressure) Given range h = 3.0 m , ρ = 1000 kg/m³ g = 9.81 m/s² Step 1 — Full-range ΔP P = 1000 × 9.81 × 3.0 = 29,430 Pa = 294 mbar (span of the DP cell) Step 2 — Density effect if ρ drops 5% → reading drops 5% → DP needs stable density Conclusion stable ρ → DP ok; variable ρ → use GWR

DP infers height from head pressure assuming fixed density — radar measures distance directly.

Step 3 — Watch-outs

Foam: GWR ok, ultrasonic fails.
Coating: non-contact radar.
Low dielectric: check GWR reflectivity.

DP level calculator

Hydrostatic head → DP span

P = ρ·g·h → DP span, plus density sensitivity.

ρ (kg/m³)

Level range h (m)

Density drop (%)

DP assumes fixed density — if it varies, use guided-wave radar.

Common mistakes

DP on variable-density fluid.
Ultrasonic with foam/vapour.
No independent overfill switch.

Requisition & datasheet

Application, fluid, vessel, range, P, T.
Technology, accuracy, output, materials, Ex.

Frequently asked questions

What should I know about Selecting a Level Instrument?

Most clean/most liquids → guided-wave radar (GWR): unaffected by density, pressure & temperature changes. Open, large or dusty (silos) → non-contact radar or ultrasonic. Density & pressure stable, or interface/sealed systems → DP (hydrostatic). Foam, coating or low dielectric? GWR/capacitance handle these; ultrasonic does not. Always confirm range, accuracy and a non-contact option for sticky/corrosive fluids.

Which standards or codes apply?

IEC 61298 (instrument performance), API 2350 (overfill protection), ISA-RP for level installation.

Standards — IEC 61298; API 2350; ISA-RP.