Pool boiling experiment

The voltage over the heater is the voltage of your power supply - the voltage drop in your copper wires. The more current you send, the more voltage is dropped in the copper leads, and the less voltage will be available at your steel wire ends.

Can you show an example of what you are measuring? A graph maybe of one of the experiments?

A photo of the measuring points / connections

The diameter and length of the copper wires

And if available, how stable the supply voltage is between 0 and 250 A?

Also put a voltmeter over each of the copper wires. See how much voltage you are losing there.

Also, the current measurement may become inaccurate over time* (this is a guess). It might be better to find an alternate way to measure current, e.g. measuring the voltage across the wires, and the voltage across the heating element, if you know the resistance of the wires (and how that will change as they warm up).

*I'm thinking of drift, since it's DC you're measuring with the clamp meter.

Jan,

Thanks for your help. I will be in the lab tomorrow and I can take some pictures of our setup. Basically we have two 8 AWG stranded copper wires connected to the positive and negative sides of the power supply. I welded two copper clamps (designed for 100A) on the other extremities to make the connection with our heater easier and more reliable).

We also developed a structure to make our heating element more stable. It consists of a stainless steel squared base connected to two stainless steel squared pillars. On each pillar, a teflon squared bar is attached and then a copper bar is attached on the teflon. Teflon was implemented for insulation purposes between the copper squared bar and the stainless steel structure. The heating element is placed between the copper bar and the teflon bar connecting the two sides of the structure. I'm gonna send you a picture of this structure tomorrow as well.

Let's call the above described structure as test section. So, we attached a small length of the 8 AWG stranded copper wires to the test section to make it easier to connect with the clamps (those connected to the power supply as described in the first paragraph).

That was actually our initial approach which included so many connections in our system. Like I said, we did not get good measurements.

In a second approach, we tried to make a straight connection between the test section and the power supply. However, our measurements were still not good.  

Shabaz,

Thanks for the help. I will keep that in mind. We fist tried to use a shunt resistor in our setup in order to measure the current of the system instead of using the clamp ammeter. Then, I decided to take it out of the system. Then you are saying I should put it back and get the system current based on the voltage drop across it, right? Does a shunt resistor have a fixed resistance or not?

This is our shunt model by the way:

www.ohmite.com/.../SH2-200R0005DE

Hi,

Nothing has a fixed resistance, there will be a change with temperature, and that can be caused by the amount of power loss through the material. The 8 AWG wire will have more resistance than your shunt resistor. You'll need to calculate how much power loss there is in the wire, and see if that's sufficient to raise the temperature enough to make a material difference to the resistance or not. Just 1 metre of 8 AWG copper wire will have 2 mOhm of resistance (if I've calculated that correctly) so you can't ignore the voltage across the wire, as Jan mentions. You don't need the shunt resistor if you consider the wire to be a resistor anyway (which it is), so maybe I'm missing something, but I don't see why you're adding the 0.5 mOhm shunt to the circuit if the wire has comparable resistance.

You could try to use many wires in parallel to reduce the resistance compared to the shunt, but you should still measure across the wires and the shunt, since the wire resistance and connections may still not be insignificant (I don't know). 

Shunts are typically used because they have a lower change with temperature than wire - generally assumed to be fixed value. Yours changes no more than 30ppm/C.

When you say you have bad readings what does that mean? They are fluctuating or just appear to be inaccurate?

Dougw,

Thanks for your help. In general, when I increase the power, the resistance decreases. I'm wondering if this is happening because somehow the resistance x temperature relation for the stainless steel wire is being affected by the same relation for the system copper wires.

Also, do you think I should add the shunt back to the system in order to obtain a reliable system current measurement?

It would help if you show the formula you use, and the input values and measurements for that formula. For several measurements in your range.

We may spot something then?

Think you need to draw a diagram showing precisely where your wires are connected to which test equipment. Your wire resistance is swamping the shunt. Unless you take the wire resistance into account, it's pointless measuring just across a shunt.