Level measurement accuracy

One of the hassles with top down level measurement systems, for liquids or solids, is that the user measures tank contents from the bottom up, so when the reading is zero, the tank is empty. This would be fine, except this is the extreme range for the top down measurement system, and likely to be where the reading errors are most significant, in absolute terms of cms of error. Not too good if the user is measuring the flow through a V-notch weir, with an ultrasonic system, or boiler water level above the fire tubes, with a GWR (Guided Wave Radar) system: the measurement error becomes a significant zero error.

Having asked the question, “What effect do the vapour layers in the ullage space have on system accuracy?” of most suppliers of GWR systems, or “What is the effect of the top layer in an interface measurement system?” – the normal answer is that vapours, or foam, or light hydrocarbons, have minimal effect. I think this is wrong: the effect is proportional to the relative permittivity (and also permeability) of the vapour through which the GWR or radar pulse travels. So the error depends on the actual site conditions and vapours involved. Have you asked your supplier for any figures relevant to your application, to see that this is indeed a negligible effect on your tank?

Rosemount have launched a GWR probe system that uses the classic built-in reference reflector system to calibrate out the vapour effects of the ullage space, that they realize are particularly significant in steam drums, for example in boiler and de-aerator applications: see the Top Ten story from Mobrey this week (Link). Here Rosemount suggest a final achievable accuracy figure of 2%, compared to the possible errors from standard systems of 20%: while these figures need to be seen in the actual tank and application context, they are far larger then the normal accuracies quoted for general GWR systems.

When a supplier does submit a good technical article discussing higher temperature GWR measurement accuracy, through vapours, I will be delighted to highlight it. Meanwhile, why not ask the manufacturers showing GWR systems at Offshore Europe, what additional error is introduced in a GWR pressurized tank level measurement where for example an aqueous liquid is covered by 2 metres of a saturated vapour at 60, 80 and 100C. (Then tell me the answers!).

Another interesting product launch this week comes from Flir Systems, with their new radiometric versions of some special purpose IR cameras and video recorders. The GF series use Sterling cooled detectors: the GF309 can see through flames in furnaces and boilers, to inspect the internal surfaces, tubes and refractories at up to 1500C: the GF320 wavelength sensitivity is configured to detect the gas leaks that are of relevance to petrochemical installations. Both cameras also offer the best features needed for radiometric condition monitoring duties, including GPS and colour imaging, 25mK sensitivity and 1C accuracy, to make them true dual or multi-purpose equipments for use in preventive maintenance (Link).


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