The Shop > Electronics & IC Programing
Water Heater Monitoring
Will_D:
If using a current transformer (torroid with 100 turns on it and one of the wires to be measured passing though it) then you measure the Voltage induced with a DVOM not the current. The current flowing in the main circuit induces a voltage in the coil on the torroid.
The voltage will be proportional to the current
vtsteam:
So then the wattage would change as the square of the reading. Which is similar to what Andrew was saying in results.
To check:
The difference between 60 watts and 100 watts (the two bulbs) should be proportional to the square of the change in meter reading of 3.3 ma.
So (8.2/4.9) ^2 = 2.8
2.8 x 60 = 168 watts
Doesn't look right.......
Okay check if it's linear:
8.2/4.9 = 1.67
1.67 * 60 = 100.2 watts
Yup. Checks out. His meter is working as a linear wattmeter for these two data points.
DavidA:
David.
I just checked another site and found the following.
http://hyperphysics.phy-astr.gsu.edu/hbase/thermo/spht.html
I am going to run a test on my electric kettle using my power meter and a thermo couple temperature meter to try for a practical reading.
You may be right, but I never found the need to allow for specific heat. Off to find my ice pack and some black coffee.
Dave.
David Jupp:
It's worth remembering that a lot of DVMs may not display true RMS values when working with AC - you can get some rather weird results (Just to add further complication).
awemawson:
A 'standard' current transformer is made to be exactly what it says, a current transformer. I have several of them monitoring 3 phase loads, and although the ratios are all different, the 'output current' is 5 amps full scale in each case so they interface with a 5 amp fsd device. Could be a multi-meter or (as in my case) an 'electricity meter' as per provided by your electricity provider.
Now if the original poster measures current using his device, and sources that current from 110 volts (as he did) and gets a correlation between wattages (VxA) and indication of 12.2 watts per milliamp, what I am saying is if he sources his power from 220 volts each milliamp no long represents 12.2 Watts, it now represents 4 x 12.2 = 48.8 Watts per milliamp.
His reading of 102.8 mA on the actual heater equates to 48.8 x 102.8 = 5kW which is rather high as the heater is rated at 3.8 kW but is probably within the (in)accuracy of the equipment being used to measure it.
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