Is meg-meter flow meter, a good choice for sludge flow measurement? what about ultrasonic one
Yes a mag meter is a good choice for sludge flow. The choice of the model will depend on the sludge concentration.
As you can see on this schematic of a dewatering process, the inlet is measured with a model but the dewatered sludge is measured with a different model because of it's higher concentration.
Give us more detaisl about your specific application and I'll give you a more specific answer.
Also, as shown, an alternative to a magmeter could be a Coriolis meter if you'd like to get more info than just flow, like density.
Ultrasonic Flow meter wouldn't be a good choice if your concentration is above 5-7%. Remember, Ultrasonic is not Doppler meter.
It sends a sound wave through the pipe, that bounces on the other side and comes back. The time of travel is calculated and proportional to the velocity. If this sound wave hits particules, it will either be absorb and won't come back or it might bounce on the solids and get back quicker which would be interpreted as a higher velocity and give you high flow readings.