Micro Motion FDM Fork Density Meter

Direct-insertion density and concentration measurement

The Micro Motion Fork Density Meter (FDM) uses vibrating fork technology to measure liquid density directly in pipelines, bypass loops and tanks. The datasheet quotes ±1 kg/m3 density accuracy and concentration accuracy of up to ±0.1 percent, with stem lengths up to 13 ft (4 m) and a choice of corrosion-resistant materials. It is used where density is the primary control parameter or an indicator of solids or concentration.

Technical specifications

Applications and replacement

Outputs include density, referred density, specific gravity, concentration, °API and alcohol by volume, with HART inputs for external temperature, pressure and flow to improve readings. Full backwards compatibility gives the same form and fit as the Micro Motion 7826 and 7828 direct insertion density meters. Known Density Verification supports meter health checks. For a meter that also reads viscosity see the Micro Motion fork viscosity meter, and for mass flow with density see the Micro Motion ELITE CMF400.

Viscosity matters for accuracy. For the standard long-tine FDM1 there is no viscosity effect from 0 to 50 cP, and between 200 and 500 cP the effect grows to a maximum of ±19 kg/m3 unless an on-site calibration is made. The short-tine FDM2 has no effect from 0 to 100 cP and a reduced effect from 100 to 500 cP. The meter also contains a class B 100 ohm RTD on the fork, and the transmitter uses that reading to optimize performance over the process temperature range.

What to confirm before ordering

• Liquid, density window, viscosity and operating temperature and pressure
• Stem length, wetted material and tine finish, with the process connection and rating
• Transmitter output, display requirement and hazardous area approval
• Whether the fluid sticks or forms deposits, which affects tine coating choice

How the Micro Motion FDM works

A tuning fork with two tines sits in the liquid. A drive coil makes the tines vibrate at their natural frequency. When the liquid is denser, more liquid moves with the tines and the frequency falls. The meter reads the frequency, corrects it for temperature and gives the density.

Where the FDM is used

• Pipelines and bypass loops
• Process tanks
• Concentration control
• Solids content monitoring
• Alcohol by volume measurement

Selection checklist for the FDM

• FDM1 or FDM2 model
• Stem length
• Wetted material
• Process temperature
• Process pressure
• Fluid viscosity
• Tine finish
• Output and housing type

Installation, commissioning and maintenance notes for the FDM

Insert the fork so the tines sit in the flow and the liquid drains freely. Fit the process connection and tighten it as the manual says. Connect the 24 Vdc supply and the output wiring. Enter the calibration data and check the density against a known sample. Inspect the tines for build-up at regular intervals.

Requesting a quotation for the Micro Motion FDM

Please send the fluid, the temperature, the pressure and the stem length needed. Seven Star LLC serves industrial customers in Oman and the GCC and replies with a quotation based on the information provided.

FDM density accuracy

The density accuracy is plus or minus 1 kg/m3.

FDM calibrated range

The calibrated range is 600 to 1,250 kg/m3.

FDM operating density range

The operating range is 0 to 3,000 kg/m3.

FDM repeatability

The repeatability is plus or minus 0.1 kg/m3.

FDM viscosity limit

Viscosity can be up to 500 cP.

FDM stem length

Stem lengths run up to 13 ft or 4 m.

FDM wetted materials

Options include 304L and 316L stainless steel, alloy C22, titanium and zirconium.

FDM temperature range

The short stem works from minus 50 to plus 200 degrees C.

FDM pressure rating

The short stem works up to 207 bar.

FDM power supply

The supply is 24 Vdc at 0.65 W typical.

FDM outputs

Outputs include density, specific gravity, concentration, degrees API and alcohol by volume.

Density records for the FDM

Record the calibration data, the sample checks and the date of each tine inspection.

Choosing the right FDM stem

Choose a stem length that puts the tines clear of the pipe wall and in the flowing liquid.

Documents and quotation

The Micro Motion Fork Density Meters datasheet (PS-001485 Rev K, February 2024) is available from the product documents on this page. Seven Star LLC is an independent supplier in Muscat; send the complete order code from the nameplate or your specification and we will check it against the manufacturer documentation before quoting. Browse more flowmeters in the catalogue.