Author Topic: Water Heater Monitoring  (Read 11450 times)

Offline sparky961

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Water Heater Monitoring
« on: January 04, 2015, 08:58:48 PM »
Greetings all, and Happy New Year.  I spent two days of my New Year holidays replacing my recently deceased water heater, which included moving it to the other side of the house where it made more sense.

After all of this, I got to wondering how I could save some money in its operation (electric) and it seemed the first step would be to gather data on it's designed operation during normal household use over a period of time.  For this, I need to set up a data logger.

Using what I had at hand, I've tried to set up a current transformer so I can indirectly measure it's power usage using my DVOM that can output serial data.

I thought I had it nailed, but right at the end I got confused.  Perhaps someone can help me figure out this mystery?

1. Wind 100 turns magnet wire (perhaps 28ga or so?) onto small toroid
2. Insert 1 piece of 10ga solid copper wire with heavy insulation still attached
3. Characterize transformer using 60W and 100W incandescent light bulbs
  [Result: 60W -> 4.95mA, 100W -> 8.2mA]
4. Do some math, resulting in an average of 12.2 W/mA

Here's where things get strange.  Hooking my new current transformer up in series with one of the two wires connecting the water heater (240V single phase?) I took a reading from the meter... 102.8mA .  Assuming this was the same as the light bulb test, that would give me 1254.2W, but this water heater has two 3800W elements that should be operating one after the other.

Who wants to play teacher and educate me on my mistake and how to get a correct reading?  Or, maybe an easier (inexpensive) way?

Offline vtsteam

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Re: Water Heater Monitoring
« Reply #1 on: January 04, 2015, 11:17:05 PM »
Well it doesn't explain the whole discrepancy, but your lightbulb is on 110V and your water heater is almost certainly on 220V, so you can at least double your estimated wattage.
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Offline awemawson

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Re: Water Heater Monitoring
« Reply #2 on: January 05, 2015, 02:56:57 AM »
Double the voltage and quadruple the power as wattage is 'I squared R' R stays the same and I is proportional to V so the wattage on 240 is four time that on 120
Andrew Mawson
East Sussex

Offline RussellT

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Re: Water Heater Monitoring
« Reply #3 on: January 05, 2015, 05:46:18 AM »
I think we need more information about how the water heater is wired before we can sort out that calculation.  I don't understand what you mean by
 
Quote
two 3800W elements that should be operating one after the other

However I wonder if you actually need to measure current.  Water heaters are generally either on or off controlled by a thermostat.  Wouldn't it be adequate to measure how much time it's on?

Russell
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Offline Spurry

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Re: Water Heater Monitoring
« Reply #4 on: January 05, 2015, 07:57:46 AM »
I understand the joys of making something yourself, but would a unit like an Efergy provide the answer to the "current" problem?
http://efergy.com/
Pete

Offline vtsteam

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Re: Water Heater Monitoring
« Reply #5 on: January 05, 2015, 12:27:27 PM »
Double the voltage and quadruple the power as wattage is 'I squared R' R stays the same and I is proportional to V so the wattage on 240 is four time that on 120

 :scratch: Andrew, isn't Sparky already measuring (not calculating) the actual increased current through the 240V circuit? So power equals measured current times known voltage....yes?

Not sure I'm explaining this well  :(......Sparky's not trying to estimate what would happen if he doubled voltage through a circuit. He's trying to correct a mistaken assumption about what the voltage was in a circuit where the current is known.

(I hope I've got that right........tricky, this! :scratch:)

I love it when a Plan B comes together!
Steve
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Offline DavidA

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Re: Water Heater Monitoring
« Reply #6 on: January 05, 2015, 01:40:43 PM »
Sparky,

Perhaps you can provide a sketch of this set up.

At first I thought that you were using the coil as an inductive pick up with the the heater supply lead (one side only) running through it.

But now I'm not sure.

Dave.

Offline vtsteam

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Re: Water Heater Monitoring
« Reply #7 on: January 05, 2015, 02:14:53 PM »
Yes, David, and Sparky are you measuring short circuit current for your pickup loop without any resistor in series?
I love it when a Plan B comes together!
Steve
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Offline awemawson

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Re: Water Heater Monitoring
« Reply #8 on: January 05, 2015, 02:50:49 PM »
 :scratch: Andrew, isn't Sparky already measuring (not calculating) the actual increased current through the 240V circuit? So power equals measured current times known voltage....yes?

Steve, from my re-reading of his post no, he seems to be pro-rata'ing his 12.2 W/mA figure despite these being mA at twice the voltage. But we need him to clarify
Andrew Mawson
East Sussex

Offline sparky961

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Re: Water Heater Monitoring
« Reply #9 on: January 05, 2015, 05:22:38 PM »
Sorry for my lack of clarity, guys.  I was trying to give only facts to avoid skewing the discussion with any of my own mistaken assumptions.  Seems I was a bit lean on the details.

I'm using an inductive pickup, 100 turns of wire on a toroid to 1 turn carrying the current to the water heater.  The voltage is 220 or close thereto (I'll verify the exact number to fine tune my calculations later.)  I missed stating that in the original post, but I was thinking it would make sense to double the displayed value (problem #1?).   I figured if this got me close to the nameplate value it would validate my assumption.  It didn't.

My setup is the pickup loop with no resistor (problem #2?), connected to my DVOM on 200mA current setting.

I'm almost embarrassed to have to even ask this because I should be able to figure it out.  But AC and power has always been a tough area for me.

Offline vtsteam

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Re: Water Heater Monitoring
« Reply #10 on: January 05, 2015, 05:54:33 PM »
Hey don't be embarrassed Sparky! I don't know what's going on. either.  :)

I love it when a Plan B comes together!
Steve
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Offline sparky961

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Re: Water Heater Monitoring
« Reply #11 on: January 05, 2015, 07:27:35 PM »
 
Quote
two 3800W elements that should be operating one after the other

This is how most water heaters operate.  Only one element is on at a time.

Quote
However I wonder if you actually need to measure current.  Water heaters are generally either on or off controlled by a thermostat.  Wouldn't it be adequate to measure how much time it's on?

Quite true, this would work.  However I'd like to be able to measure the power from other things in the future as well.  This is just where I've started out.

Offline BillTodd

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Re: Water Heater Monitoring
« Reply #12 on: January 06, 2015, 04:58:33 AM »
if you want to make it more efficient, all you can really do is reduce heat loss with insulation. 

There is no saving in letting the water cool then reheating, unless the water is left to cool to ambient for some while.

Bill

Bill

Offline RussellT

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Re: Water Heater Monitoring
« Reply #13 on: January 06, 2015, 06:51:15 AM »
There is no saving in letting the water cool then reheating, unless the water is left to cool to ambient for some while.

I'm not so sure about that - if the water cools, then the heat loss will be reduced and because less heat has been lost the energy required to bring it back up to temperature will be less than would have been required to maintain the temperature.

Quote
This is how most water heaters operate.  Only one element is on at a time.

It isn't how they operate here.  I still don't understand.   :scratch:  Is it something to do with not having a proper 240V supply?  Do you have a circuit diagram?

Russell
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Offline awemawson

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Re: Water Heater Monitoring
« Reply #14 on: January 06, 2015, 08:08:06 AM »
Frequently there are thermostats fitted at different levels so you don't need to heat a whole tank, as the hot water rises and stays on top and is drawn from there. Perhaps this system is being confused with separate heaters?
Andrew Mawson
East Sussex

Offline DavidA

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Re: Water Heater Monitoring
« Reply #15 on: January 06, 2015, 08:16:27 AM »
It is fairly straightforward to work out the best route to take.

It takes 1 Watt to raise 1 CC of water by 1 degree Celcius.

So 1 Kw will raise 1 Litre by 1 degree in 1 second

So you need to know the volume of the cylinder,  the starting temperature of the water and the power rating of the heat coils.

From this you can calculate the time it will take to raise the water from ambient to the required temperature.

The better the insulation the longer it will take to fall back to ambient and thus the less time the heater will be on bringing it back up to heat.

It would also make sense to only have the tank as big as needed. Less water to heat up.

Dave.

Offline DavidA

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Re: Water Heater Monitoring
« Reply #16 on: January 06, 2015, 08:27:48 AM »
Back to the original problem.

I was wondering if you could leave out the toroid and wind the turns directly around one of the power leads.  Obviously not both as it wouldn't work.

If you then place a resistor between the ends of the coil you may be able to measure the voltage generated across the winding by the current induced into it from the current passing through the cable.
As the current through the cable varies so would the current in the coil and from this you would get a change in the voltage across the resistor.

Dave.

Offline Manxmodder

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Re: Water Heater Monitoring
« Reply #17 on: January 06, 2015, 08:40:06 AM »
Hi David, where you say "So 1 Kw will raise 1 Litre by 1 degree in 1 second"

Is that heat transfer rate constant throughout the range from ambient to boiling point,or does the transfer rate slow down as the water temp vs element temp differential becomes less?......OZ.
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Offline vtsteam

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Re: Water Heater Monitoring
« Reply #18 on: January 06, 2015, 08:58:52 AM »
There are 4 variable classes in your experiment and 3 data points. No good conclusions can be drawn about what the data means. If you want to know if it graphs as straight line (the original assumption) or a curve, or discontinuous straight lines, or discontinuous curves, you need more data.

Variable classes:

110 V
220V
light bulb (non-linear resistance)
water heater element (non-linear resistance)


Available Data Points:

4.9s ma x 110 V ......60 watt bulb
8.2ma x 110V ..........100 watt bulb
102.8 ma x 220 V ....3700 watt water heater (single element assumed)

Do a 3rd bulb test at 110v to characterize that curve,

And do a 2 element 220V water heater test to determine if those points appear to lie on the same curve as the 110V points. You still need another data point @220V to determine if that curve is a straight line or not. Which, unfortunately you can't get (at least with the water heater.) But you should be able to tell if those points lie on the extended 110V curve.

If they do, you just need to write your formula, and you're good to go with your new induced wattmeter.


I love it when a Plan B comes together!
Steve
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Offline David Jupp

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Re: Water Heater Monitoring
« Reply #19 on: January 06, 2015, 09:16:24 AM »


It takes 1 Watt to raise 1 CC of water by 1 degree Celcius.

So 1 Kw will raise 1 Litre by 1 degree in 1 second


I think there is a factor of 4.2 missing - specific heat capacity of water = 4.2 J/K/g   

4.2 kJ required to heat 1kg (1 ltr) of water by 1K (or C) -> to raise 1 ltr of water by 1 K in one second would require power of 4.2kW.

Offline Will_D

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Re: Water Heater Monitoring
« Reply #20 on: January 06, 2015, 09:27:10 AM »
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
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Offline vtsteam

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Re: Water Heater Monitoring
« Reply #21 on: January 06, 2015, 09:54:32 AM »
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.
I love it when a Plan B comes together!
Steve
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Offline DavidA

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Re: Water Heater Monitoring
« Reply #22 on: January 06, 2015, 10:20:36 AM »
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.

Offline David Jupp

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Re: Water Heater Monitoring
« Reply #23 on: January 06, 2015, 10:39:17 AM »
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).

Offline awemawson

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Re: Water Heater Monitoring
« Reply #24 on: January 06, 2015, 10:56:28 AM »
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.
Andrew Mawson
East Sussex