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Calculating efficiency

  1. Apr 21, 2013 #1
    I have built a plumbing test rig for my project. I have a couple of electrical devices on this rig and need to find out the efficiency of them/the rig. I have looked online to find equations and have found all the relevant material except for the useful output energy. in all the examples they seem to just give out this information and not tell you how to obtain it.

    HOW DO YOU WORK OUT THE useful energy output?

    I have a 3 port valve with a power consumption of 6.5W and switching current of 2.2A

    There is almost a cylinder thermostat with a switching current of 3A

    I am also using a 20amp junction box (not sure if this effects anything) and a 3amp plug.

    using P= V x I i worked out the pwer for the wholse system to be 1.196 watts

    % energy efficiency = (Useful energy produced × 100) / Total energy used

  2. jcsd
  3. Apr 21, 2013 #2


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    It is not clear to me what efficiency you want to determine. Efficiency applies to transfer of power and energy not simply to its utilization. For example the valve simply uses energy it doesn't convert it to some other useful form. So in this sense it is zero percent efficient. But if does its job. A valve can in principle be arbitrarily "efficient" in that the theoretical limit of the power needed to open or close a valve is infinitesimal (unless you want to get into some deep quantum theory). But as a practical matter you can't find frictionless surfaces and zero mass perfectly rigid components. So for practical valves there is a tradeoff between the cost of making it use less energy and the reliability within the working parameters to the cost of the energy required to operate it.

    As far as the efficiency of your whole rig, you'll just have to add to your input power the average power consumption of the valve, (averaged over the time of a full operational cycle during which it may only sometimes be drawing power). The junction box should have little effect. The circuit resistance of it should be inconsequential if its within rated values (and if it isn't efficiency isn't your worry, safety is.)

    Do be careful with power for AC devices. RMS current time RMS voltage doesn't always give you power. If the voltage peaks at a different time in the AC cycle than the current then you are dealing with less power. (This mistake is BTW the basis for many a "free energy" device.)

    You say a plumbing test rig, what form does the output power take? If you're pumping fluid the power will be volumetric flow rate times pressure. That is the fluid analogue to the current times voltage.
  4. Apr 21, 2013 #3
    jambaugh thanks for your reply

    I cannot go into too much detail about the device/system but as the valve is motorised I though it would be possible to work out loss of energy through heat and voltage output ect and work out how much power is used/wasted

    As for the pressure it involves no pump as it would be plumbed into the pipes leading off a combi boiler to the taps in the kitchen. So the water pressure will be that of the boilers output

    To be honest im looking for equations and workings to put into my report. Though the efficiency would be an easy one!

    It has a 22mm copper pipe input to the valve and 2 15mm outputs so i guess i can do some fluid pressures for that
  5. Apr 22, 2013 #4


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    If this is the case you shouldn't use the term "efficiency". Just say "power utilization" in your report.
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