Proudly engineered in Switzerland

Viscosity sensor VLO-M2 for liquids

The VLO-M2 viscosity sensor measures the viscosity of a fluid in a microelectromechanical system (MEMS system). The medium is guided in the sensor via a pressure gradient to the omega chip, an omega-shaped microchannel. This vibronic measuring system generates the measured values by setting a silicon tube in the chip into resonant vibration and analysing this. This is because the vibration quality depends on the viscosity of the liquid in the microchannel. At the same time – and independent of the viscosity – the density of the medium can be determined via the frequency of the microchannel. Since temperature influences both viscosity and density, the temperature of the medium is also recorded in real time in the chip. In this way, the temperature effect can be compensated.

Thanks to the measurement in the MEMS system, the sensor is only 30 x 80 x 15 mm in size and can be accommodated even in tight spaces. The high-precision measurement results are immediately available, allowing continuous measurement during the process. The density of liquids depends on their temperature. To compensate for this effect, an integrated platinum resistor detects the temperature of the liquid.

The viscosity sensor sends the measured data to the readout system via the data line in Modbus RTU transmission mode.



Product description

Modbus RTU

Modbus RTU

  • Modbus RTU
  • RS-485 point-to-point connection
  • Modbus over serial line – Specification
Modbus RTU

Inline measurement

  • Directly in the running process
  • Values are output instantaneously
  • Quick change of media possible


  • Density value
  • Max. measurement deviation:
    ±0.2 kg/m³ (or 0.0075 x abs (T-25 °C)] kg/m³ if the value is >0.2 kg/m³)
  • Repeatability: ±0,1 kg/m³
Icon Viscosity Kinematic


  • Viscosity value: Dynamic & kinematic
  • Measurement accuracy:
    ±[0.2 mPa s + 5% of reading]
  • Repeatability: ±0.1 mPa s


  • Hydrocarbons
  • Aqueous media
  • and much more – Ask us
Icon Concentration

Concentration packages (option)

  • Sugar / water according to ICUMSA (%mass)
  • NaCl / water according to Laliberté / Cooper (%mass)

Continuation of concentration packages

  • High-fructose corn syrup HFCS42, HFCS55, HFCS90 (%mass / °Brix / °Plato / °Balling)

Continuation of concentration packages

  • Ethanol / water according to OIML IST-90
    (%mass / %vol@20 °C / ABF@20 °C)

Continuation of concentration packages

  • Methanol / water (%mass)
  • Ethylene glycol / water in (%mass)

Continuation of concentration packages

  • Mineral content in water according to Huber (mg/l)
  • Hydrogen peroxide in water (H2O2) (%mass)


Monitoring fuel concentrations

New cars must be thoroughly tested before they are registered. The engine is tested to the limits of its capabilities and often started “cold”. As a result, the fuel no longer burns completely and mixes with the engine oil. If there is too much fuel in the engine oil, this can destroy the engine. With the viscosity sensor, tests no longer need to be interrupted – you can monitor the viscosity of the oil directly in the process and only change it if necessary.

DLO-M1_Process monitoring

1. Test bench for testing engines. 2. Engine being tested. 3. Fuel used for the test. 4. Engine oil used in the engine. 5. The quality of the engine oil is checked directly in the process; damage due to possible contamination can be avoided.



The density sensor was designed to measure the density of fluids. This is done with a microelectromechanical system (MEMS) with an omega-shaped microchannel (omega chip) built into an internal bypass.

When a liquid flows through the density sensor, the bypass arrangement creates a pressure gradient across the microchannel, allowing the liquid to reach the omega chip. The liquid influences the physical properties of the excited sensor (resonance frequency and quality), these are digitalised and evaluated in the microcontroller. The measured values can be read out via the serial interface (RS-485, Modbus).

Thus, density measurements in the range 600…1,000 kg/m3 (for further options see product specifications) can be realised at a flow rate of 0…10 l/h. settings.

Omega chip

The Omega chip, a vibronic microsystem, is the heart of the measurement system and serves to generate sensor signals in the overall system. The essential component of this microsystem is a silicon tube (microchannel) that is vibrated electrostatically in a vacuum. To compensate for temperature effects, a platinum resistor is integrated, which allows local real-time temperature detection. The Omega chip consists essentially of crystalline silicon and glass.

Measuring principle Omega chip

Measuring principle (Omega chip)

Density measurement

For density measurement, the density sensor uses the Omega chip. The filled microchannel is set into resonant oscillation and analysed.

The resulting natural frequency of the microchannel depends on the mass and thus on the density of the medium in the microchannel: the greater the medium density, the lower the natural frequency. The natural frequency is thus a function of the density of the medium.


f = natural frequency, E ⋅ I = tube stiffness, ρTube = tube density, ATube = tube cross-section, ρFluid = fluid density, AFluid = fluid cross-section.




Density and variables derived from it (e.g. standard density, concentration, etc.)

Typical media:

Particulate free (<30 μm) media such as

  • Gasoline, diesel, kerosene
  • OME (synthetic materials)
  • Oils and lubricants
  • Water-based media
  • Methanol, ethanol, isopropanol
  • LPG*
  • AdBlue®*
  • Glycol mixtures*

Other media can be used after individual clarification. *Optional

Concentration packages:

  • Various sugars in water
  • Invert sugar in water
  • High fructose corn syrup
  • Methanol in water
  • Ethanol in water
  • Salt in water
  • Minerals in water
  • Hydrogen peroxide in water
  • Ethylene glycol in water
  • Butane in propane

Note: User-specific concentration packages on request


Measurement performance

Max. measurement deviation:

Viscosity: ±[0,2 mPa s + 5% of reading]
Density: ±0.2 kg/m³ (or 0.0075 x abs (T-25 °C)] kg/m³ if the value is >0.2 kg/m³)
Temperature: ±0.15 °C (or ±[0.005 x abs(T-25 °C)] °C if the value is >0.15 °C)


Viscosity: ±0,1 mPa s
Density: ±0,1 kg/m³
Temperature: ±0,05 °C

Max. Messabweichung VLO-M2_ex

Temperature conditions

Permissible medium temperature:

-40 …..+60 °C

Permissible ambient temperature:

-40 …..+60 °C

Permissible storage temperature:

-40 …..+60 °C


Permissible density measuring range:

0…1600 kg/m³ 

Permissible viscosity range:

0,3…50 mPa s 

Permissible pressure of medium:

0…20 bar (abs)
Burst pressure: 80 bar (abs)

Permissible particle size:

Max. 30 μm

Permissible flow rate range:

0…10 l/h (water)


Vibrations (<20 kHz) have no influence on the measuring accuracy due to the high working frequency of the microchannel.

Inlet and outlet sections:

Inlet and outlet sections have no influence on the measuring accuracy.

Ambient conditions

Climate class:

Not yet defined

Electromagnetic compatibility:

EMC 2014/30/EU (EN 61326-1)

Vibration and shock resistance:

Not yet defined

Degree of protection:

IP54 (IEC 60529)



Stainless steel:

  • 1.4404 (316L)
  • 1.4542 (AISI/SUS 630)

In contact with media:

Stainless steel:

  • 1.4542 (AISI/SUS 630)

BOROFLOAT® 33 glass
Epoxy resin

Dimensions / Design
















Type: VLO-M2_ex (On the VLO-M2 (not Ex), the clamping bracket is omitted on the earthing plate with M3×8 TORX screws – marked orange in the diagram


30 mm x 66 mm x 15 mm (without cable and cable gland)


<200 g

Dimensions of measuring channel:

160 x 200 μm (500 nl)


Fluidic interfaces

Fluidic interfaces:

2 x M5 threaded holes at a 45° angle to the side and front surfaces

Electrical interface


Continuous, without the need for an external command.
On the hardware standard RS485.
Proprietary Modbus RTU communication protocol (see data sheet)

Cable design:

Permanently installed cable. Connection cable type KS-Li9YD11Y 4xAWG 28, manufacturer: Kabel Sterner

Cable length:

3 m (option up to 30 m)

Cable outer diameter:

2.3 mm

Wire diameter:

4 x AWG 28

Level control:

Digital communication lines and power supply in one common shielded cable, unidirectional, RS-485
Provide termination resistance of 330 Ω on the client side

Energy supply:

Maximum current consumption 26 mA
Maximum power consumption 350 mW

Supply: 5 V…13.3 V

Dielectric strength:

The reference potential (GND) is connected to the housing and the earth connection (see product structure). There is no electrical isolation between the supply circuits, the communication interface and GND

Data rate:

Response time: 100 ms

Cable assignment:

Wire colour assignment

  • yellow – RS485 B, D1
  • green – RS485 A, D0
  • brown – GND (signal ground), common
  • white – VDD (supply voltage)
  • blank – shielding
Certificates / Approvals

CE mark:

The density sensor meets the legal requirements of the EC directives. TrueDyne Sensors AG confirms successful testing of the density sensor with attachment of the CE mark.


  • LVD 2014/35/EU (L96/357)
  • EMC 2014/30/EU (L96/79)
  • RoHS 2011/65/EU (L174/88)


  • EN 61010-1:2010
  • EN 61326-1:2013
  • EN 61326-2-3:2013
  • EN 50581: 2012
Product structure

DLO-M2 – Product structure











Product structure:

  1. Density sensor VLO-M2(_ex)
  2. Mounting holes for mechanical fastening (6 x M3 threaded holes)
  3. Fluidic interface (2 x M5 threaded holes)
  4. Clamping bracket on earthing plate with screws M3×8 TORX
  5. Electronic interface for communication and power supply

For the VLO-M2 (not Ex), point 4 (clamping bracket on earthing plate with M3×8 TORX screws) is omitted.
Marked orange in the diagram.


1080x810px_DGF-M1_mechanical mounting_1901_TrueDyne_density sensor

Mechanical fastening: VLO-M2