Roarty, Francis X wrote:
I agree the energy utilized should be subtracted from the output but how much of the pressure or flow rate is actually removed from the system?
No measurable amount is removed. I guarantee that.
-- the differential measurements are only for temp but you should also quantify the pressure/flow rate into the reactor and the pressure/flow rate out of the reactor if you want to determine if any energy was added or subtracted -- otherwise the same pressure and flow are still potential -- available for use downstream. I am sure the portion of pressure/flow removed from the system is only a small fraction. Maybe put the exiting wat er after measurement into same diameter pipe as the source and measure with an identical flow rate meter?
Most flowmeters slow down the water and distort the numbers far more than the flow water through the pipes would. Especially rotary flowmeters take more energy out of the system than friction would. That is to say, they take out the energy to turn the rotor. Displacement ones also take energy. The ones that add a pulse of heat to detect it downstream add a tiny amount of energy. (But it is OUTSIDE THE SYSTEM!!! Doesn't count!)
Anyway, there is no such thing as an "identical flowmeter." Get two of the same make and model and you will find they each have a personality and a mind of their own. The variations between the flowmeters will be far greater than the energy losses from water friction. A couple of grains of sand in rotors or bearings will make a difference big enough to detect. I have put two or three flowmeters in line at about 100 ml/min. These were expensive ones. They produced different answers, for reasons beyond the scope of the discussion. The differences are too small to affect these conclusions but they are always present.
Anyway, at 1 L/s you can put a dozen flowmeters in line above and below the inlet and it will not cause or detect any measurable change to the temperature or flow. Every one of them will register a different flow -- of that you can be sure -- but the variations will be caused by the instruments themselves and they will far exceed water friction.
Ditto thermocouples, by the way. Install a dozen of them and you will get a dozen answers, with variations far larger than water friction can cause. They will have a different bias, speed, range, accuracy and precision. Gene and I installed multiple pairs of these as well, backed up with mercury thermometers. Although they misbehave and drift less than flowmeters do. Once you calibrate them, they are highly reliable in this range of temperatures. And contrary to assertions made here, any one of them can measure 5°C with confidence. Actually, they can measure 0.1°C, but real fluctuations in the water temperature are as large as that, for various reasons.
Rest assured, Levi, Kullander and people like that know how flow meters and thermocouples work. Way more than I do.
If were trying to measure the effect of friction from water flow in pipes, equipment such as the best $2000 flow meter and a top-notch RTD thermocouple would be ridiculously crude. That would be like trying to see a virus with a plastic magnifying glass. Someone like Rob Duncan has micro-calorimeters that could measure this friction easily, but they cannot measure a power level higher than a few milliwatts. Here is one with 0.002 micro-watt resolution. It does not say what the max power is, but it can't be much:
http://www.stats-reports.com/webstat/dlcount.php?id=41061&url=http://www.setaram.com/traitement/export_doc.php?doc=../files/documents/MICRODSC3-EVO.pdf - Jed
