Okay, dragging out the last bits of my ECE know-how, this is because pumps have electric motors, which are inherently inductive, and measuring Vrms and Irms would yield volt-amps, which be slightly more than the watts.

Is that anywhere close?

Alchemy

redleader, try it with an actual flow of 3 gpm. Let us know.

Yes.

Here's some stuff I posted in a thread a while back:

"I measured simultaneously:
The voltage applied to the pump.
The current through the pump.
And the power consumed by the pump.

I had previously measured the winding resistance of the pump at 14.1 Ohms.

I got the following results.

In the unrestricted case with 123 Volts connected to the pump:
The current was 0.801 Amps.
The power consumption was 35.3 Watts.

In the restricted case with 123 Volts connected to the pump:
The current was 0.757 Amps.
The power consumption was 24.6 Watts."

This was with a Danner Mag5. (Spec'd at 45 Watts.)

As you can see, in the AC case, Volts X Amps doesn't necessarily equal Watts.

And yes, it's because the motor windings are inductive.

BTW, the larger Iwaki's (MD-20 and up) seem to have a power factor correction capacitor to reduce the difference between VA and Watts.

I thought those were startup caps, to prevent a large power draw, when you turn it on?

Wouldn't a meter compensate for the inductive load? I mean, if it measures current, and the current is cyclic (i.e. AC), then shouldn't it still give off a good reading?

From what I remember in electronics courses, P=VI, but in AC circuits, you have to use the RMS values: 127 Vac, is actually rms, aka averaged, as the peak is somewhere around 170 volts. RMS is the actual average value over the sine wave in the AC signal.

I don't see the numbers being off by too much: if the pump was almost completely obstructed, you should get the max power rating of 45 Watts, no?

response yo an old post
 

bah
Cathar, you are confusing flow rate with 'residence time'

if the internal volume of the wb is 10 cc, and that of the rad is 40 cc; at a flow rate of 10cc/sec:
what is the time that 1 cc will be exposed to the effects of each ?
(in the wb as compared to the rad)

you guys are confusing yourselves
look at some of my old data showing T/W values
the deltaT is much smaller in the rad, and the h values lower
-> takes longer to 'do the work'
or less time can be allowed for a higher coolant equilibrium temp

Re: response yo an old post
 

I know what I'm doing/saying on this one.

What I'm addressing is the "argument" that many people throw up, that higher flow rates means that the coolant has less time to pick up the heat, when really it has the same amount of time. ie. this is the "residence time", and it doesn't alter w.r.t. flow rate.

What's then left is the effects of turbulence of the flow and how that affects the heat transfer efficiency, which is what you've been measuring.

i.e. of all the factors that influence cooling efficiency for items, "residence time" is an invariable, but it is the argument that many people use as an explanation for why they think (without using any emperical evidence) that heat transfer will be less.

Their use is twofold, yes they help reduce peak current during startup, but they also help smooth out the inductive loading.

No a RMS AC meter will only be accurate with a pure sine wave. The more inductive the load the more it will be off. ie the wave form looks less and less like a sine wave.

Look at Since87's numbers. Unrestricted measured about 35W but the V x A = 98W. BIG difference.

Since87 some help explaining this, please?

slow down Cathar
I am not concerned with the 'arguments' of others
read the above, it is true only if the wb and rad have the same internal volume (which seldom occurs)

rad size (= volume also) is a variable which can be manipulated in several ways

Matter of poor grammar on my part Bill. I meant the same time within the water-block which is unique to that waterblock and the same time within a radiator unique to that radiator respectively, not that the water and radiator share the same residence time.

Reading back on it, I suppose the word "or" should've been used instead of "and". Slightly confusing to read maybe, but I wasn't confused when I wrote it.

C'mon, I'm sure you knew what I meant, or do you really think I was silly enough to be suggesting that radiators and waterblocks have the same volume?

Bill,

I'm with Cathar on this one, and I think it's purely a matter of how you've interpreted his words. I'll offer up my take on what he said. Cathar, please jump in if you see something amiss.

When he said "the same time in the waterblock and radiator" what he likely meant was that the relative percentage is unaffected by flow rate. If the block's volume is 1/10th the radiator's then irrespective of flow rate the water is in the radiator ten times longer than in the block.

A residence time within the radiator is definitely a requirement, but simply slowing flow down generally does not improve radiator efficiency. The "right" way to increase residence time is to (duh) have a bigger radiator. I believe we're all in agreement that infinite residence time in an infinitesimal radiator is bad. For radiators, size rules, particularly the surface area for convection to air.

All the other observations about total flow rate and pump power are pretty much spot on the mark. Folks looking to achieve a "real 3 gpm" typically don't have any idea of what that truly takes or how little (if any) the benefit will be. What's really comical is when they wish to combine this with 7V fans for their radiator. Can anyone say "counter-productive"?

{edit}Damn, I'm too slow. . .

Ok, as a newbit to this...sounds like to really enjoy the benifits of more flow, your gonna need a rad big enough to make benifit of it...right?

For pure sinusoidal AC signals:

P = V * I * cos(phase angle)

In a resistive (or power factor corrected) load, the current is in phase with the voltage. (phase angle = 0)

In a purely inductive load, the current "lags" the voltage by 90 degrees. (phase angle = -90 degrees)

In a real world load like a pump, there is both a resistive and an inductive component to the load, so the current will "lag" the voltage by some angle between 0 and 90 degrees. The yellow line in the picture below shows current lagging the voltage by 60 degrees.

http://uffish-thought.net/wc-gifs/phase.gif

The power consumed in the out of phase case shown, is half as much as the in phase case, even though the amplitude of the current is the same in both cases.

No, a "True RMS" meter ideally is unaffected by distortion. (There are practical limits with regard to the range of frequencies over which an actual meter will be accurate.)

An inductive load does not distort the shape of the waveform, it only shifts the phase of the current with respect to the voltage.

I agree aswell.

I think that when I first began contributing to this thread people were under the impression I was gunning for some rediculously high flow rate.

I have never denied that the higher the flow rate, the more significant the pump becomes as a heat source. Most of the people I try and convince that more flow would be an advantage are running the kind of system that would have a flow rate most likely below 1gpm. For these people, trying to increase the flow to say 1.5 gpm from aroun 0.5gpm would be a real advantage.

8-ball

Ben: Sorry but I don't have it anymore :( If I get another chance I will though.

At the time it didn't occur to me that draw would vary much, which is unfortunate because I took apart my rig then anyway.

Some quick real world experience relating back to the original topic.

I had a discussion with someone over at bit-tech about using dual pumps, whether to go for serial/parallel and so on.

Here is the thread

He recently posted his results, and although he was having major issues with his motherboard (I think it didn't support his cpu) and having no air flow around the socket messing up the temp diode on the board, there was something interesting.

I had convinced him to buy another Via Aqua 1300 to go with the one he already had, instead of ditching it and buying an eheim 1250.

Having set it up, he ran the system with one pump, then after switching the other one on, his measured cpu temps dropped by 10 degrees.

Now granted he was having issues with his board, but that is still pretty convincing, and I think that still falls into the range of flow rates I was discussing.

8-ball

a whopper of a thread!!! think its taken me nearly 2hrs to read, not sure how much i've digested but its certainly dispelled some myths i'd viewed as facts so thanks on that one!
No where near as far up the thermo-understanding ladder as most of you guys but be able to add my 2 cents to the digressed discussion of measuring ac motor power, there are tools to measure ac power e.g modern flukes with clamp on p.f. meters (almost accurate lol), standard kilowatt hour meters, watt-meter or a combination of normal voltmeters, ammeters, and power factor meters. There are 3 types of AC power, we're interested in 'real power = v x a x p.f' - (the average of the instaneous product of voltage & current), the P=VI furher up the thread is true for d.c pwoer, in a.c. it gives 'apparent power - the product of RMS volts & RMS amps - (iirc if you dont have an RMS meter you can muliply by 0.7071)', finally theres 'reactive power' - (the time average of the instaneous product of voltagte & current with current phase shifted 90 degrees -VAr).
From the specs of the pump you should find a p.f rating. with this you can then use a dvm to measure voltage & current & use 'Real' power = V x A x p.f, not 100% accurate .... although for torque & slip calcs p.f. is assumed as constant, its not!, but its often quoted at particluar / expected duty levels so can be treat as constant, certainly within the bounds of accuracy of the measurement kit i expect to be employed anyway.
p.s not sure of eheim, hydor etc p.f rating but i'd expect for them all to be around 0.8?
p.p.s think i noted someone had a watt meter further up the thread, if they measure true/real power then divide that by apparent power(VA) that will give the power factor for others to use (if not quoted in specs).

I haven't seen a power factor spec for any of the pumps in common use for watercooling, including the 'high end' Iwaki's. Some of the bigger Iwakis (MD-20 and higher) have a power factor correction capacitor.

i wasnt sure (never looked) if they'd be quoted hence by pps, maybe alittle closer to unity?

I'm not sure I understand the question.

The attached image shows my test results for a very unrestricted Danner Mag5.

As you can see, (by calculating W / VA ) the power factor is about 0.35. The power factor was about 0.26 with the pump substantially more restricted. I doubt Eheims and Hydors get above 0.5 PF, but unless you have so many pumps that you are blowing circuit breakers, it's not much of an issue in any case.

sorry wasnt very clear, the iwaki's with the PFCC, i assumed them to be much closer to unity.
0.35 - 0.26, i had i no idea the little pumps where so grossly inefficient! Not familiar with your meter i assume it has a clip on? what its accuracy at such low current?
No, no big deal just means your electricity supplier makes uneccessary profit from you, out with the caps :D

The meter's error is 0.02% worst case at those currents. Here's a spec sheet. These are most often used as transfer standards for calibration purposes. The manufacturer happens to be my employer.

I wasn't using a clamp-on. The current is directly connected through a compensated transformer internal to the meter.

sorry you've lost me re: the current measurement, you placed the meter in series?

Yes.

I've always preferred bong coolers because the go slightly below ambient. Skip down if you don't want to read about my current bong. I have a closed loop to gather heat from the system, it them runs through a spiral of tubes in the bottom of my bong. A eperate pump cools the bong water. Same principal as a nuclear reactor. Since I use aditives in my coolant i don't have to worry about them evaperating. Having 2 seperate circuits has worked well for me so far.

In my opinion faster is better in both block and Rad, in a very fast system the water may only heat up a few C so from entry to exit theres little variation, also if you circulate this into another cooler IE GPU or HD those are cooler. Same fast flow rate goes for teh core as well.

If you needed fast to gain heat and slow to release it. You could have a resevor with dual pumps. One high flow one to get heat and a second slow one to pump it to the radiator. The mixing of the water in the Res should keep the water pretty equal in temp. Similar to the circulatory system in the body. Not all the blood goes the kidneys but because it continually mixed its all constantly cleaned.