Density Measurement with Uni-Probe LB 491

A universal field device for a variety of applications

The measuring system Uni-Probe LB 491 is used for non-contact density and concentration measurement on liquids and bulk solids. The density meter has a current input for a speed signal and can therefore also be used for mass flow measurement on pipelines.

Funktion of density measurement

The Uni-Probe LB 491 is a compact field device, combining both detector and evaluation in one single instrument. It provides interfaces for

  • HART
  • Profibus PA
  • Foundation Fieldbus

Even if the Uni-Probe LB 491 is used in Profibus or Fieldbus systems, it is always possible to switch to HART communication if required. In addition the separate 4-20mA current output can always be used as alternative signal output.

Uni-Probe LB 491 - your advantages at a glance:

• Versatile detector for various applications
• Compact field device with integrated transmitter
• Communication via HART, Foundation Fieldbus or Profibus PA
• Communication can be switched from Bus to HART at any time
• Solid system for standard applications
• Can also be used for mass flow measurements




CrystalSENS Detector

Point detector with high-sensitive, high-quality Sodium Iodide crystal for best long-term stability.

SuperSENS Detector

Best sensitivity on the market, is able to work with smallest source activities.
LB 491
Detector operating data 
Power supply100 ... 240 VAC, ±10%, 50 ... 60 Hz, 15 VA
24 VDC (18 ... 32 VDC), 15 W; 24 VAC + 10%/-15%,
50 ... 60 Hz, 15 VA

Cable connections

4 cable entries 3/4 inch, NPT, closed with blind plug
Option: metric adapters and cable glands upon request

Maximum cable length

3300 m (120 Ω), 1600 m (250 Ω), 800 m (500 Ω)

Wire cross section

0,5 ... 1,5 mm2

Housing materialStainless steel ISO 1.4301 / AISI 304
Water coolingOption, max. 6 bar
CascadingUp to 8 detectors
 Scintillator size      Weight   Weight with
Ø x lenght [mm]     [kg]       cooling system   
(point detectors)
50 x 50 (NaI/TI)        22.5        24                       Standard
SuperSENS150 x 150 (polymer) 52           62                       Standard
Ambient temperature
(Operation and storage)
-40 ... +60°C (-40 ... +140°F) for CrystalSENS and / or
-40 ... + 55°C (-40 ... +131°F) for UniSENS
Observe possible temperature restrictions for Ex-protection!
for 100...240 VAC version, operation only up to max. 50°C
Temperature stability≤0.002%/°C (-40 ... +50°C) for CrystalSENS and/ or
≤0.01%/°C (-40 ... +50°C) for UniSENS
Detector certificates
& tests

IP protection

IP65 / IP66 + Nema 4X

Explosion protection


II 2 GD EEx d IIB T5 IP66 T80°C                      -40 ... +80°C
II 2 GD EEx d IIC T6 IP66 T80°C                      -40 ... +60°C
(...+50°C for LB 490 TowerSENS and SuperSENS)
II 2 GD EEx d [ia] IIC T6 IP66 T80°C                -20 ... +50°C

Class I Division 1 Group A,B,C,D
Class II Division 1 Group E,F,G                          -40 ... +50°C

Other certificatesNepsi, IECEx, Kosha, CCOE, others upon request
Signal inputs and outputs 

Signal output

HART 4 ... 20 mA potential-free, active or passive
Max. impedance: 500 Ω (active)
Voltage supply: 12 V ... 24 V (passive)
Max. impedance at 12 V: 250 Ω and/or 24 V: 500 Ω (passive)
Option: intrinsically safe HART current output 4 ... 20 mA, potential-free,
Voltage supply: 12 ... 30 V, voltage drop <3,5 V, 20 m signal cable (blue),
Exi IIB: Lo=14.78 mH; Co=679 nF / Exi IIC: Lo=2,18 mH; Co=84 nF

Bus output - option

Bus interface: Profibus PA or Foundation Fieldbus
Bus powered, typical 13 mA with 2xAI function blocks
Option:intrinsically safe bus interface, 20 m signal cable (blue),
pre-assembled Approval according to ATEX and FISCO

Digital inputsDig In 1: Hold input, Dig In 2: Empty adjustment
Analogue inputPt100 for temperature compensation

0/4 ... 20 mA for speed signal for mass flow in the pipeline

Digital outputs1 relay (SPDT) for collective fault message
3 relays (SPDT) alternatively for: Hold signal, min./max. alarm,
Detector temperature, radiation interference detection

Permissible load at ohmic load: max. 5 A at 250 VAC or 30 VDC


RS 232 for software update

Data backup

in non-volatile memory



For HART, Foundation Fieldbus and Profibus
X-ray interference protection (XIP)
Event log
Continuously monitored current output


Format Size
Densidad y Concentracion solidos Spanish language en PDF 2.65 MB
Density Measurement Russian language en PDF 2.51 MB
Density measurement - system overview en PDF 1.57 MB
Medicao de Densidade e Concentracao Portuguese language en PDF 2.81 MB
Uni-Probe LB 491 en PDF 1.51 MB
Whitepaper Radiometric Measurements Accurarcy, repeatability and errors en PDF 739.65 KB
Format Size
ATEX LB 490-TS, LB 491 SuperSENS en PDF 1.89 MB
CCOE LB 490-TS, LB 491 SuperSENS en PDF 243.11 KB
CCOE Zertifikat LB 49x en PDF 243.11 KB
CSA Zertifikat LB 49x en PDF 383.09 KB
FM LB 490-TS, LB 491 SuperSENS en PDF 244.5 KB
Uni-Probe ATEX Certificat LB49x en PDF 1.63 MB
Uni-Probe FM Certificat LB49x en PDF 63.96 KB
Operation Manual
Format Size
LB 491 Uni-Probe en PDF 31.36 MB
Product Information
Format Size
Level & Density Measurement for -50°C (-58°F) de, en PDF 2.26 MB
Uni-Probe User Interfaces en PPT 511 KB
Software Tool
Format Size
LB 490 & LB 491 DTM for HART de, en ZIP 18.78 MB
LB 491 device description for Foundation Fieldbus with capability file de, en ZIP 79.94 KB
LB 491 device description for HART for Siemens Simatic PDM Version 6.0 de, en RAR 55.09 KB
LB 491 device description for HART-communicator de, en ZIP 84.23 KB
LB 491 device description for Profibus PA with GSD file de, en ZIP 116.76 KB
Technical Information
Format Size
LB 491 Uni-Probe de, en PDF 1.07 MB