Is anyone able to provide a description of instruments used for the measurement of process fluid density. Nucleonic comes to mind. Ultra sonic, tuning fork perhaps? Anyone?

 

Coriolis flow meters are expensive, but measure fluid density as well as mass flow rate.

Dick Caro

 

I'll take a shot at the vibrating element sensors which are amongst the most accurate density sensors for both lab and process.

Tuning forks, vibrating tubes and cylinders vibrate at their resonant frequency.

The resonant frequency is a function of the effective mass of the system.

The effective mass is the fixed mass of the sensor plus the effective mass of the fluid. As the density of the fluid changes so too does its effective mass. This then causes the resonant frequency to change.
(http://www.viscoanalyser.co.uk/Digital Density DG.pdf)
Not all vibrating element sensors, including resonant frequency sensors, can measure density.

For example, vibrating element sensors are used to measure viscosity. Some use the torsional twisting approach which does not displace the fluid. Therefore the effective mass of these sensors is the fixed mass of the sensor only.

Note that the resonant frequency is also a function of the stiffness of the material. SO as temperature changes, the Young's modulus changes and must be compensated for. Those sensors which contain the fluid under pressure (coriolis meters, tube type density meters) are also affected by pressure. Pressure and temperature can also cause the sensor to be stressed by the constraint of the connection to the process fluid pipework.

In a dedicated density meter such as the Emerson 7835 the tube is connected to the process pipework via triple ply stainless steel bellows minimising on of the effects of temperature and the tubes are oval in cross section minimises one of the effects of pressure.
Temperature and pressure both cause the volume of fluid to increase because the geometry changes and this also has to be accounted for.

All are affected to some degree or other by viscosity.

Velocity of sound effects may also play a part. With single phase fluids this is apparent with very low density liquids but it also affects two phase flows.

Tuning fork, vibrating rods, and open cylinders (using hoop mode vibration) are fully immersed in the fluid, are constrained only at one end and do not retain pressure and therefore Young's modulus is the only significant temperature effect and there is no pressure effect. They are generally far more sensitive to viscosity.

Note: While we can refer to coriolis meters as being able, to measure density it is not correct to refer to it as "coriolis density" measurement. The coriolis effect has nothing to do with the measurement of density. (Indeed, it may not have anything to do with mass flow either (http://www.flowcontrolnetwork.com/measurement/flowmeters/article/the-coriolis-effect Plus a good discussion of this in one of the Linkedin groups).

In the "coriolis" meter the tube is vibrated and the twisting effect across the length of the tube is a function of the mass flow of fluid.

In the early days the tubes were vibrated at mains frequency. They could not measure density.

It is only when they shifted to operating at the resonant frequency that the density effect became apparent and was later exploited.
Not all coriolis meters measure density very well.

The temperature density relationship of hydrocarbons was initially created using hydrometers. When the tables were later extended they used the "digital density meter". This is a glass U tube vibrating element sensor which has very high precision. (see Anton Paar)

Good sites to visit:
Anton Paar: http://www.anton-paar.com/Density-Meter/59_Corporate_en?productgroup_id=2&Density-Meter

Emerson: http://www2.emersonprocess.com/en-US/brands/micromotion/density-viscosity-meters/Pages/index.aspx

LEMIS: http://www.lemis-baltic.com/pages/index.php?mid=8

RMI: www.viscoanalyser.com

 

PS I forgot to mention the Gravitrol.

Originally manufactured by Hallikainen, the Gravitrol is a U shaped tube where the legs of the U are connected to the process fluid by means of bellows.

The U tube is arranged horizontally flat so that it is connected only by the bellows and the tube will thus tend to droop if unsupported.
The tube is maintained horizontal by a restoring force. The force required is a function of the weight of the tube which is the weight of the tube itself and the fluid it contains.

SO as the density of the fluid changes so too does the restoring force.

Note that this is also temperature and pressure affected and the density is derived from the weight rather than the mass.
http://www.google.co.uk/url?sa=t&rc...iYDQAQ&usg=AFQjCNEDhqSSfSgwMrk8hAlUaVGxeTeUEQ