This application for heater furnace pass on cokers is done quite successfully with Flexim’s ultrasonic non-intrusive clamp on technology. There are many installed worldwide with great success. The major benefits to this technique is that one does not have to interrupt the process flow and the measurement is accurate to within 1 to 2%. The transducer design is patented and be used on applications up to 800 F. If you haven’t tried this Flexim product, then you need to contact them and ask about it. If you doubt it, then have them bring their portable demo to your site and test it for a day.