Thurning Instruments' radiometric systems measure in transmission mode. For fluid density measurements the instrument consists of a source of radiation (X-ray or gamma), a pipe containing the working fluid, collimators to define the radiation beam and one or more Thurning detectors (see Figure 1).

Figure 1.

The following notes give details of the performance that would be expected for three densitometers based on the RMS1000/SZ70 detector. The performance of the instruments is quoted as 2-sigma uncertainty, in units of g/cc, for water densities in the range 0 to 1 g/cc.

For radiometric measurement the principal source of measurement uncertainty is the statistics of radiation emission. If you use more photons to make a measurement (by increasing the source intensity or by measuring for longer/integrating readings), you get smaller uncertainty and better resolution.

Thurning's RMS instruments integrate the radiation signal digitally, so if no analogue filtering is applied the maximum time for the output to respond to a step-input is twice the integration time. At the end of the second integration period, the output will reach 100% of the new reading (not 63% which would be the case if an analogue or exponential filtering method were used).

Physical characteristics:

Figure 2.

The use of an isotope source gives excellent source stability and high-energy radiation for a reasonably economic price, but it limits the photon rate that can be achieved. A long integration period must be used with this instrument to obtain good resolution.

Physical characteristics:

This instrument has a lower resolution (Figure 3) than the previous example for three reasons:

The integration period is shorter to make the instrument respond more quickly,

The ratio of steel to water in the radiation beam is worse than for Instrument 1,

The radiation path length through the water is low for such high-energy radiation (662keV).

However the compromise of 0.5 % resolution and 100 ms measurement interval is satisfactory for many applications.

Figure 3.

The photograph below shows a three-beam densitometer for nuclear safety research. It uses a large 10Ci Cs¹³⁷ source to measure steam water two-phase flows in a 50 mm pipe. The instrument is being supplied to KFKI in Budapest for measuring steam-water mass flow on a reactor safety experimental rig. The project is part of the EC's PHARE programme.

Three-beam densitometer for KFKI in Budapest.

The amplifiers, interlock panel and warning sign for the KFKI densitometer.

This densitometer will perform much as Instrument 2. The detectors and amplifiers are the fully analogue predecessors to the RMS1000. The water-cooled shielding body of the instrument was designed and manufactured by Konstandin & Partner GmbH, of Pfinztal in Germany with the collaboration of Thurning Instruments and Siemens KWU, Erlangen. The densitometer was delivered to Budapest in December 1999.

Physical characteristics:

Figure 4.

Slower instruments can introduce measurement errors (in slug flow for example) because they average the radiation transmission, not the fluid density. As the relationship between density and radiation transmission is non-linear, averaging the radiation transmission leads to an error in the calculated density. A fast RMS1000 based system overcomes this problem.

These three examples show the sort of system that Thurning Instruments Ltd can produce, and the performance that can be expected. We use special purpose software to design and predict the performance of our radiometric measurement systems. This allows us to rapidly investigate various options for each measurement and to optimise a system to suit each customer's requirement.