There are several simpler methods:

For known viscosity and density you could measure a pressure difference across an orifice or a (long) piece of a straight duct  and thus compute the mass flow.

You also could employ a balance and measure the increase of weight with time in a open system (possibly not the best idea for a hot fluid).

You could employ a venturi nozzle, however this needs careful calibration.

Be creative

Best regards

Bastian

Ultrasonic measurement is easy, clean a non intrusive. GE-Panametrics is very useful. I had experience. The parameter that should be used to calibrate it is temperature, pipe material and thickness, fluid and density relationship.

I too believe, ultrasonic measurement is clean, accurate and repeatable. There are several technologies. I especially like the cross correlation  methods that measure downstream and upstream time of arrival differences. An interesting R&D project to dive deeper into fluid characteristics is look at spectral changes in the cross flow signal measurements

There are many things to consider.

Is it a one or a two phase flow? How accurate you have to determin the flow? Is it a stationary flow or transient? How much space do you have? Is it a closed loop? Is insulation an issue?

You see there is no simple answer.

If you have a single flow, you can use a balance and measure the increase of weight vesus time. As you have a hot fluid, you must make a closed and isolated systeme.

I have used hot-film anemometry to measure the velocity of single phase and two phase flow. for this method, you need to calibrate the probe using a simple standard method, such as Pitot tube to find acorrelation between the velocity and the output voltage from the probe. then use the output from the probe to find the velocity. the following papers may help:

1. 1. Hamad, F. A., Khan, M. K., Bruun H. H., Experimental study of kerosene–water two-phase flow in a vertical pipe using hot-film and dual-optical probe bend. The Canadian Journal of Chemical Engineering, Vol 91,7 p 1296-1311, 2013.  

2. Hamad F. A., He S., Evaluation of hot-film, dual optical and Pitot tube probes for liquid-liquid two-phase flow measurements, Flow Measurement and Instrumentation Journal, vol. 21(3), pp. 302- 311, 2010.

3. Hamad F. A., Pierscionek B. K, Bruun H. H, A Dual Optical Probe for Volume Fraction, Drop Velocity and Drop Size Measurements in Liquid-Liquid Two-phase Flow, Meas. Sci. Technol., Vol. 11, August, 2000.

4.. Hamad F. A., Bruun H. H., Evaluation of Bubble/Drop Velocity By A Single Normal Hot-Film Probe Placed in a Two-Phase Flow, Meas. Sci. Technol., Vol. 11, pp. 11-19, 2000.

Methods suggested by Batian are the simplest and can be setup very easily. However, since the temperature of the oil is quite high care has to be taken that pressure measuring devices can withstand the temperature and that proper temperature compensation is done (which may be necessary in certain pressure transducers).

Another method is to use variable area rotatmeters. These are readily available and though many of them may be calibrated for air or water, to derive the flow rate for other working fluids is simple enough. Again care should be taken that the device used can withstand the temperature.

@ Klaus - Thanks for your interest. 1. Single phase flow 2. Accuracy  +/- 0.01 m3/s. 3. Transient flow 4. Less space only available 5. Closed loop 6. Insulation is not a big issue.

I think in that case ultrasonic measurements are really the easiest and most convenient way to measure your mass flows. I am using Bronckhorst flow meters and they do their job well!