Water as part of a transformer? - Page 1
Forum section: electronics / general

author: Rick Law
author_rank: Super Contributor
timestamp: February 22, 2013, 11:25:12 pm
content: With the vast knowledge here, I think perhaps fellow EEvBloggers can help me understand this. I know AC or even DC that changes can induce current on a near by wire, that is how transformers work. I understand the induction current/voltage concept. However, I am seeing something that I am not sure is induction and it puzzles me.Here is the scenario: I have a fish tank, warmed by AC aquarium heater(s) – I am in the USA, so AC is 110v. I have 3 such heaters since I used to have multiple tanks. The heaters are encased in glass and the heat coil is not touching the glass. The heat coil wraps around a presumably-ceramic insulator and the insulator is the only thing touching the glass. Inside the glass is air. The submerged glass tube will flood if there is a leak, so an invisible leak in the glass tube is not possible.Since I have my new UT61E near by, just for fun, I wanted to see if there is a voltage in the fish tank. Not wanting to wet the probe, I use a wire clipped to the probe. The wire is 6-9 inches (15-23cm) long with about 2-3 inches (3-5cm) of wire submerged in the water. About 1cm (1/2 inch) of the submerged wire is stripped to give it electrical contact to the water. I clip the DMM's negative end to ground. I measured the AC Volts and I was surprised: It was around 12-14 volts AC. I was expecting fluctuations at below 1V.Seeing such high voltage, I want to investigate. I stick in another heater (of the same make and model) to see; I read 24-25volts. I put in the third one and I was reading over 33volts to as high as 38volts. I measured the current: it started in the sub 0.1mA range and decreasing – the meter settled at nano-amps range. Such low reading means the current I read could be just random fluctuations. The voltage is just about cumulative (1 heater @12v, 2 heaters@2x12v, 3 heaters@3x12v - just about).So, I am puzzled by such high voltage readings:(1) These three readings are consistent with inductive voltage. But I would not think it is possible to have inductive voltage with water as the “secondary coil” of the transformer. Perhaps it is?(2) The second possibility is the water is serving as the ferrite core of the transformer and the 2 inches of wire in the water is the secondary coil of the transformer. Could it induce that way?(3) Perhaps what I think of as the ceramic insulation is not really ceramic and it is not a good insulator so there is a leakage current into the water. The coil is about 300ohms. The ohm measurements show no shorts between the coil-wires to the glass nor the water. (Measuring ohms from either end of the heater AC plug to the glass or to the water reads over-range). Perhaps a short that only reacts when voltage exceeds certain break-down voltage?(4) Something I didn’t think of?What do you think?Rick

author: c4757p
author_rank: Super Contributor
timestamp: February 22, 2013, 11:38:57 pm
content: You are measuring the tiny capacitance between the water and ground, charged up probably through capacitive leakage out of the heater.

author: Rick Law
author_rank: Super Contributor
timestamp: February 22, 2013, 11:46:50 pm
content: Quote from: c4757p on February 22, 2013, 11:38:57 pmThat makes perfect sense to me!Also, the wire+water can be one side of the capacitor while the coil being the other side of the capacitor with the air in the glass as the insulator for the capacitor. A small amount of AC can of go though the capacitor no problem.Capacitance makes perfect sense. Geeze, all these times, my fish have been swimming inside a big capacitor. How about that!Thanks

author: c4757p
author_rank: Super Contributor
timestamp: February 23, 2013, 12:05:39 am
content: If the mains voltage flows right through the capacitance into your multimeter's 10M impedance, then the capacitance would only have to be:110V * 10M / (10M + ZC) = 12VZC= 81.66MZC= 1 / (2 * pi * C * 60 Hz)C = 32 pFI don't think it's too far-fetched to have 32 pF between the heater and the water! Of course, this neglects the fact that there is also capacitance between the water and the neutral wire, which will reabsorb some of the energy. It could be highly dependent on the position of the wires in the heater, as well as a number of other factors.

author: TerraHertz
author_rank: Super Contributor
timestamp: February 23, 2013, 03:11:30 am
content: I thought USA households used a balanced 110V AC system, with street transformers having a center-tapped 110V output winding with the center tap connected to ground? Thus the two supply lines are each really 55V AC relative to ground. Which is supposedly low enough to get away without a ground wire to equipment, hence the 2-pin American power plug.Sounds like that isn't the situation with the fish tank heater AC source, since if it was the capacitive coupling would cancel out? Oh wait... only if the heater coil was symmetrically arranged and the two ends coupled equally to the water.How is three phase power provided to USA premises? Is that case done with a grounded neutral and the three live phases directly connected, same as in Oz? Or still using an isolating street transformer?In Oz the street lines are three phase 415V plus a grounded Neutral. Premises are supplied directly via connections to Neutral and one or more of the street line phases (each of which is 240V AC relative to ground, 415V AC relative to the other phases.) So there are no 'street transformers' which are apparently common in the USA.

author: c4757p
author_rank: Super Contributor
timestamp: February 23, 2013, 03:30:04 am
content: We have a balanced 240 (I've heard 220 and 230 as well, but every time I've measured it it was 120/240). Most of the house connects between ground and only one phase, and large appliances connect across both phases. We've omitted ground for so many years not because we only have 55V relative, but because we're reckless idiots.As far as street transformers being "common" - that is an understatement. I don't pay enough attention to the power systems to tell you what it's like in the city, but here in the woods of upstate New York, where it's sparse enough that everyone has an acre or more (I don't know how you measure land in Australia...), every house has its own small transformer all to itself.

author: Rick Law
author_rank: Super Contributor
timestamp: March 26, 2013, 02:07:20 am
content: Quote from: c4757p on February 23, 2013, 12:05:39 amWhile I know C4757 was right that it was the capacitance, I have been curious as to exactly how much that would be. Finally, I got around to another fish tank cleaning and I have access to all the three heaters again. I measured their capacitance.The three heaters were at 80nF, 90nF, and 120nF. I averaged about 5 reading for each heater, and subtracted the meter’s internal capacitance. Their position and the position of the other wire affect the capacitance significantly.So, hat off to you C4757. You spot it right off. At the get-go, I was thinking was it inductance? Was it leaking current because there is some kind of a hole in the wiring? … When you said capacitance, I though “dumb me, why didn’t I think of that.” The capacitance is measurable and significantly higher than I though it would be.Interesting puzzles make life interesting. Off to another puzzle.Rick

author: c4757p
author_rank: Super Contributor
timestamp: March 26, 2013, 02:27:22 am
content: How exactly did you measure the capacitance? 120 nF is a lot. One more equation:Simplifying the capacitance between the water and the heater to a basic parallel plate capacitor. Assumptions: 1mm thick glass, 0.0012 m2contact area, no space between the heater and the glass, dielectric constant of glass = 4.7, e0= 8.85 pF/m.C = ke0A/d = (4.7)(8.85 pF/m)(0.0012 m2)/(1mm) = 49.9 pFObviously at least two of the numbers were pulled out of my ass, but that's pretty close to a best-case value. All the "improvements" I can think of to the estimates make the capacitance even smaller.Edit: Dammit, the forum doesn't like epsilon. Worse, it shows up fine in the preview. Assume e = epsilon...

author: Rick Law
author_rank: Super Contributor
timestamp: March 26, 2013, 03:38:02 am
content: I have a small bucket about 2 quarts (2Liters approx) of water. I used alligator clip wires from my DMM leads. One DMM lead clipped to the bare wire into the water. The other lead clipped to the "RETURN/GROUND" of the water heater. I reason that since there is temperature on/off control, they would likely be hooked up to the "HOT" side of the wiring. I don't recall my polarity setup whether I clipped the + of the DMM to the bare wire or the water heater. The maximum distance between barewire and the heater is only about 3-4 inches.I use the UT61E. I have tested real capacitors with the 61E and the measurements usually come in about 10% of nominal value of the real capacitors (in the 100 to 2200 microFarads range). The 61E itself has around 60nF internal capacitance which is not important for mF capacitors. So I know while not an LCR, the 61E is measuring capacitance in the right range for microFarad capacitors. I don't have any small (below 1mF) capacitors, so I cannot verify the UT's accuracy when it comes to tiny capacitance in the range of it's own internal capacitance.

author: c4757p
author_rank: Super Contributor
timestamp: March 26, 2013, 03:54:35 am
content: Quote from: Rick Law on March 26, 2013, 03:38:02 am60nF? Holy crap! I'd say it's probably faulty then - it's supposed to have a 22nF range! How can it measure under its own offset?Edit: The specification on the 22 nF range is 3.0% + 5 counts, which allows for an error from 50pF to 710pF. On the 220 nF range, 500pF to 7.1nF. The test leads can add some error but not almost 60nF!

author: Rick Law
author_rank: Super Contributor
timestamp: March 26, 2013, 05:11:54 am
content: Quote from: c4757p on March 26, 2013, 03:54:35 amHoly cow! My fault. I reviewed my notes - it was pF, not uF.The first row, I wrote 0.0060uF / 0.134 -> 74 (dropping the 0.0 from 0.074), all subsequent rows, I merely wrote 149, 130 instead of 0.149 and 0.130 so on. I converted uF to pF but didn't note it right!So the heaters are in the 80,90 and 120pF, not uF! The internal capacitance of the meter is 60pF, not 60uF. Geez. What a dummy.(From the excuse department - When you have fish of all sort yelling at you saying "get me back into my tank," it is easy to make mistakes. Besides, I never make a mistake! Last time I thought I made a mistake, I was wrong.)

author: c4757p
author_rank: Super Contributor
timestamp: March 26, 2013, 01:04:23 pm
content: I wondered if you did that. 80-120pF is much more realistic

author: Rick Law
author_rank: Super Contributor
timestamp: March 26, 2013, 01:56:46 pm
content: Quote from: c4757p on March 26, 2013, 01:04:23 pmAh ha, now we have confirmation of something we all suspected: "The LAW can do stupid things!"Rick LAW