Author Topic: Water Heater Monitoring  (Read 11266 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
Common sense is unfortunately not as common as its name suggests.

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:)

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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?
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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.  :)

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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

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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.
Helixes aren't always downward spirals,sometimes they're screwed up

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.


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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.
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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

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Re: Water Heater Monitoring
« Reply #25 on: January 06, 2015, 11:09:01 AM »
Can I butt in on this

Excuse the safety warning but be extremely careful with CT,s

They must always be 1 connected to the ammeter or 2 fitted with a short across the out put terminals

This is the case In circuit or out

The short is the same as the meter and no harm will come to the CT

Reason they are capable of producing a high voltage on the output terminals

It's the turns ratio one turn on the primary and many on the secondary eg it a step up transformer
please be careful this does not apply to VT or PT both are 110vac and a short on these will kill them

Stuart

Offline awemawson

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Re: Water Heater Monitoring
« Reply #26 on: January 06, 2015, 11:20:04 AM »

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.

 :lol: The above is of course on reflection a load of old balony  :lol:

He is measuring current, which he has previously related to power at 110 at 12.2 watts per  milliamp.

So measuring the heater running off 240 his 1 mA reading is now (240 / 110) * 12.2 or 26.62 watts per milliamp. So 102.8 mA represents 2.7 kW

Again probably within the measurement accuracy  :clap:
Andrew Mawson
East Sussex

Offline vtsteam

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Re: Water Heater Monitoring
« Reply #27 on: January 06, 2015, 12:47:04 PM »
Total agreement with you on that last, Andrew :thumbup:

Or a manufacturer rated an  2500 watt element as a 3700 watt.

Nah, impossible....... :)

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Offline vtsteam

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Re: Water Heater Monitoring
« Reply #28 on: January 06, 2015, 01:21:12 PM »
Seriously, though....

element ages, resistance wire thins, resistance increases, wire heats hotter
hotter wire increases resistance
element ages, scale builds up on shield
scale builds up on shield, element is insulated, temp builds up inside
temp builds up, resistance wire thins further
wire thins, resistance increases, wire heats hotter
hotter wire increases resistance
etc.
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Offline BillTodd

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Re: Water Heater Monitoring
« Reply #29 on: January 06, 2015, 02:22:05 PM »
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.


Russell

OK not absolutely 'no saving' , but the saving are vanishingly small if the quantity of water is large and the insulation is good.

Essentially the money is better spent on insulation than on trying to match the use to the household . Unless the water drops to ambient for sometime, there is no serious power saving (don't forget to include losses in the heating process as you're trying to get it hot again in a hurry).

Bill

Offline RussellT

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Re: Water Heater Monitoring
« Reply #30 on: January 06, 2015, 03:36:01 PM »
I agree that you will make best savings by adding insulation, but a timeswitch will also produce significant savings.  In a lot of households water only needs to be heated for a couple of hours a day if the tank is well enough insulated.

Quote
saving are vanishingly small

I'm not sure I'd agree with that either.  I was making the point about a tank with no water drawn off to clarify that there must be a saving but in practice things are more complicated.  In most domestic hot water systems with a storage tank the hot water sits on top of the incoming cold, so it's quite possible to have the bottom of the tank warming up as the incoming water is cooler than the air surrounding the tank while the top half is losing heat.  Interestingly in that situation it's possible to reduce the water temperature at the tap by putting the heater on because the heater starts convection currents and destratifies and mixes the contents of the tank.

There are lots more factors at work here too - for example where is the thermostat?  Most gas heating systems have a thermostat about two thirds of the way down the tank and so won't heat more water until a third of the hot water has been drawn.  In most electric systems the thermostat is combined with the heater so won't come on until most of the hot water has been used.

As far as losses in the heating process go they are practically zero for electric heating as all the electricity is turned into heat and is surrounded by water - the heat has to go into the water. (I'm assuming cables are correctly sized etc).

In the case of gas heating if you are heating a cold tank the boiler efficiency will be improved compared to a tank that is already hot because the water returned to the boiler will be cooler and heat transfer in the boiler will be more efficient.

Ok. I'll get off my hobby horse now.

Russell

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Offline Manxmodder

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Re: Water Heater Monitoring
« Reply #31 on: January 06, 2015, 05:52:37 PM »
Cheers Russell,that last part answers my earlier query about heat transfer efficiency between the heat source and the heated liquid becoming less effective as the 2 temperatures become closer.

That is how I have always understood it to be......OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline awemawson

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Re: Water Heater Monitoring
« Reply #32 on: January 06, 2015, 06:12:09 PM »
"Rate of heat transfer is proportional to the absolute temperature difference"

That was the answer to the ONLY technical question I was asked in the interview for University  applied physics course :bugeye:

There were lots of other none technical ones and after all they had my exam results but I was rather shocked to say the least - perhaps they were surprised that I got it right  :lol:
Andrew Mawson
East Sussex

Offline Manxmodder

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Re: Water Heater Monitoring
« Reply #33 on: January 06, 2015, 06:22:23 PM »
Cheers Andrew,this is evidenced by simple observation of how the cooling system on a car engine works more effectively in cold weather conditions vs heatwave conditions. The only thing that has changed is the heat differential between the engine coolant and ambient air temperature.

It therefore is also clear that a good deal of energy can be saved with a water heating device by having the upper limit on the thermostat set at a temperature that isn't heating the water to a higher temp than needed.
.....OZ. 
Helixes aren't always downward spirals,sometimes they're screwed up

Offline sparky961

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Re: Water Heater Monitoring
« Reply #34 on: January 06, 2015, 07:39:11 PM »
Who knew such a mundane topic could elicit such spirited response? I'm thrilled. :)

So, I stuck it with a meter tonight just to make sure what I was working with.  I currently get 121V and 243V, which I'm sure will vary by a few volts depending on the current grid load.  If I go with Andrew's updated math it still gets me back to my original number (which I had yet to double).  I'm favouring the hypotheses that the element isn't accurately rated or my makeshift setup isn't accurate enough, but I still have to wonder if it worked out ok for 60W (I tried 3 different bulbs) and 100W why it wouldn't for 3800W.  True that incandescent bulbs offer non-linear resistance, but the current is inversely proportional to temperature and certainly stable once warmed up.  I was indirectly measuring the current through whatever effective "on" resistance was there at 120V.  It would be better to characterize my setup using a known resistance closer to that which I'd like to measure, and some points in between but I don't have that many 100W light bulbs to connect in parallel** to get a few KW....... or do I....? :)

(** Boy, just about put "series" there.  That would have caused a RIOT!)

The side debate that's been going on regarding whether putting a timer on it can make it run more efficiently (to really distill it down) is essentially what I'm trying to prove or disprove to myself.  The most accurate way I can come up with is to actually measure the power it uses and log the data over time, giving me kWH to compare timer vs. no timer.  True, there are commercial meters that do just this but they cost money (why not buy a steam engine kit instead of making all from stratch? - age old question, no good answer) and more importantly I don't like the interface.  A simple display doesn't let me process the data as I will inevitably want to.

Bottom line?  Geez, I don't know. ;)

Offline sparky961

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Re: Water Heater Monitoring
« Reply #35 on: January 06, 2015, 07:41:26 PM »
Excuse the safety warning but be extremely careful with CT,s

The short is the same as the meter and no harm will come to the CT

Reason they are capable of producing a high voltage on the output terminals

Thanks, I did see that when doing some preliminary research.  You've reminded me of the importance.

Offline sparky961

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Re: Water Heater Monitoring
« Reply #36 on: January 06, 2015, 07:47:48 PM »
He is measuring current, which he has previously related to power at 110 at 12.2 watts per  milliamp.

So measuring the heater running off 240 his 1 mA reading is now (240 / 110) * 12.2 or 26.62 watts per milliamp. So 102.8 mA represents 2.7 kW

Again probably within the measurement accuracy  :clap:

I did notice that on different DVOM ranges that the reading changed fairly significantly (10's of mA).  My assumption (that bad word again) was that it had to do with the meter's internal resistance so I thought I could negate that effect by using that range for all my measurements.  Do you think I could improve accuracy by using an external resistance in the CT loop?  It did nag at me a bit that the measurement was inconsistent on those different ranges.  I suppose the way to answer the question is by collecting more data from known sources.

Offline vtsteam

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Re: Water Heater Monitoring
« Reply #37 on: January 06, 2015, 09:43:16 PM »
Sparky, all theories about why, and projections from them aside, as I said earlier, you can simply characterize your meter by a few more data points, and that's all that is really needed.

1.) do a reading for a 200 watt bulb (or so)
2.) do a reading for the water tank with both elements on (probably when it is run out of hot water completly)

That will allow you to do a graph of 5 points instead of 3. Much more meaningful. Then we can see what's going on rather than guess what's going on.
I love it when a Plan B comes together!
Steve
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Offline vtsteam

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Re: Water Heater Monitoring
« Reply #38 on: January 06, 2015, 09:50:53 PM »
And yes, you are depending on meter resistance, which is itself variable, not fixed. It's usually reckoned in ohms per volt.

But this is again just one more possible theory, not a replacement for data, which would show you what is going on before attempting to explain why it is going on, So forget about meter resistance.

Get the data first, and then you will likely find the answer out of all the theories, because it will point to one or the other. The curve(s) will suggest the answer.
I love it when a Plan B comes together!
Steve
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lordedmond

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Re: Water Heater Monitoring
« Reply #39 on: January 07, 2015, 03:34:43 AM »
I am getting confused here not an unusual thing

Are we talking about measuring AC current still
The term ohms per volt refers to the meter when used in volts , current is measured by referencing the volt drop across a shunt ,which is very low and the meter resistance will have very little impact being in parallel with the said shunt

As to using lamps are they 100 watt ? Have you proved that because it seems like they are being used as a standard

If you want to be sure get a clamp on amp meter and a decent volt meter and go from there , then if you need to remotely measure use the figure produced to calibrate you gizmo

Normal V I calc will be ok as we are dealing with a resitive load and power factor is unity so has no effect on the result

Stuart

Offline vtsteam

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Re: Water Heater Monitoring
« Reply #40 on: January 07, 2015, 08:36:24 AM »
Okay, my turn to be confused (or perhaps, confusing).

Stuart, isn't he actually showing a few miliamps across a shorted secondary of a very, very inefficient 110 V step up transformer?

Inefficient as the transformer is, meter resistance must be the big factor in limiting current. A true short would theoretically pass infinite current no matter how low the voltage, unless there was some resistance in the circuit. That resistance is significant no matter how low, if the voltage is correspondingly low.

And I don't think the meter resistance in this case is likely to be a constant for two different primary voltages.
I love it when a Plan B comes together!
Steve
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Offline Baron

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Re: Water Heater Monitoring
« Reply #41 on: January 07, 2015, 04:04:43 PM »
Hi Guys,

A number of posts have touched on, but not actually said that a burden resistor is needed on the output of a current transformer used to measure AC current.  The burden resistor is needed to firstly prevent dangerous voltages being produced by the large turns ratio and secondly to provide a reference for the current being measured.  The meter should be set to read voltage and should have, ideally, infinite impedance.  Once these conditions are met the calculations for power are straight forward.

Best Regards:
                     Baron

Offline DavidA

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Re: Water Heater Monitoring
« Reply #42 on: January 08, 2015, 07:37:50 AM »
As promised above,  here is the result of my water raising experiment.

Equipment.

2,200 Watt electric kettle.
Workzone watt meter.
Therma 3 thermocouple thermometer.

Procedure.

1 litre of tap water added to kettle and left to stand for a while to allow temperature to settle.
Thermo couple placed in kettle.
Kettle plugged into powermeter.

The powermeter also displays voltage and current as well as wattage. It has a second counter on it.

Power supply to meter/kettle turned on. turned off after 120 second.

water in kettle stirred around.
Temperature taken

Result.

2,200 cc of water raised from 10 C to 37 C in 120 second.  = 27 degree raise.

Dave.