The principal options would appear to be:
[ul][li]ultrasonic[/li]
[li]vibrating element[/li]
[li]nucleonic[/li][/ul]
I can't advise on ultrasonic and I do understand that many users prefer not to use nucleonic due to the gravation associated with the Health and Safety aspects though others are very happy with them.
I can assist with some info on the vibrating element types:
Vibrating element sensors include all sorts of rod, fork, tube and crystal types. Coriolis meters are simply a mass meter which, because it operates at the resonant frequency, can also measure density. Some are very good at density measurement but any advantage may be lost once in a slip-stream or by-pass sample loop and beome very expensive in the main line.
Tube types are often slip-stream or sample loop installed and care has to be taken to ensure that there is sufficient flow to prevent blockages but not so much that erossion occurs. They may also require frost protection in some climates. Periodically they may require cleaning either by roding them with a "bottle brush" or using a water flush.
However, they have a very successful applications history with slurries going back over 40 years and provide accurate measurement (0.2kg/m3 typical).
Typical applications include china clays, calcium carbonate, calsicum silicates, and so on. Some Tube types have an advantage that they can be used with aerated slurries though because they operate in a different harmonic, the accuracy is then less.
The obvious requirement of tube types for slurries is a single straight tube device but in many applications where a lesser accuracy can be tolerated, tuning forks are used in both pipelines and tanks (+/-1.0kg.m3)
These include the above applications but also chalk slurries (cement industry: the slurry is transferred from the quarry to the cement works and solids content is maximied by dosing with sodium lignosulphonate to reduce the viscosity) which contain silicates, flints and the odd bit of machinery. They are also now used downhole for density and viscosity and in towed arrays for measuring river mud for navigation purposes.
Tuning forks can be installed in the main pipeline and can be installed in tanks. The most flexible approach is to top mount using long stem versions (up to 4 metres) which allows the operator to adjust the depth to find the optimum measurement location.
Most density meters should include temperature correction to determine the density at a reference temperature from which the percent mass or volume is then determined.
Much depends on what you want to do with the measurement, what accuracy you can tolerate and how maintenance free you need it. A search will find a variety of manufacturers, types and performances.
E.g. Controlotron, Mobrey, Dynatrol, ThermoElectron etc.
PS to see what a fork looks like when badly installed in a chalk slurry application, see here:
You can, periodically checkinmg the air point (frequency in air) determine if erossion or deposition is occuring and compensate the calibration very simply with an offset value.
JMW
