There are any number of indirect measurements that can be considered.
Indirect methods exploit the correlation between the changes in concentration with changes is some other property that might, in some way, be more effectively measured.
For example, it may be that pH sensors or conductivity probes are less sensitive in some critical aspect of the operation. Ultrasonics can be used where the change in Velocity of sound can be as good or better to measure because the changes in velocity of sound with concentration are more easily measured or resolved.
Alternatively density is often used as an effective measurement of concentration. In some applications, viscosity might be used.
When trying to not only measure concentration but also to verify the fluid is what it is supposed to be, such as when receiving chemicals at a tank farm and to be sure that caustic is not loaded into the nitric tank, a combination of different properties might be measured.
Ultrasonics and density can be measured without the need for intensive maintenance or calibration.
When dealing with strong acids or bases and especially when diluting them from the strength at which they are stored to the strength at which they are required, the other factors that must be considered include temperature. Caustic dilution schemes, for example, require that the sensor is able to compensate for the changes in the property measured with both temperature and concentration.
Examples of sensors used for nitric and other acids and bases are the Canongate range of ultrasonic sensors (
the Emerson Micromotion tube or fork density meters (
and Ultrasonic (
and a useful comparison of pH with ultrasound is found here (
Of course, if looking at density or some other property, the relationship is not necessarily linear and may even reverse. Density of acids may peak at a particular concentration and then reduce again so that a particular density could correspond to two different concentrations.
The choice of instrument depends on the operational range of conditions, the chemical compatibility of the sensor with the fluid and of course, the balance of cost (cost of ownership as well as capital cost) and performance.
The density techniques I have indicated (Emerson) are vibrating element but you might also consider radiation absorption devices.
A caution, never neglect temperature.
Not all sensors are suitable even though they may appear at first glance to be equivalent (most manufacturers quote performance based on claibration conditions at a set of reference conditions e.g. at 20degC. It is important to know how they behave if there are significant temperature changes associated with quality changes as, for example, in dilution schemes).
For example, modern coriolis meters may appear to measure density with the same precision as a dedicated density transducer but coriolis meters often have complex tube shapes and may be very temperature sensitive as temperature changes introduce significant changes in the stresses. The Emerson tube density meters are signals straight tube devices with isolation bellows and are particularly stable with temperature and the fork sensors have only the Young's Modulus effects of temperature to be compensated for.
SO while in some applications there is little to choose between coriolis and density in others it can be very important.
JMW
