Corrosion Resistant Micro-Bath Heater

This instructable will show you how to make a miniature heating element which is resistant to corrosion. I'm hoping to use it in conjunction with cupric chloride for etching printed circuit boards. You can either screw it into a side-hole of an HDPE container, with teflon tape acting as a washer, or you can lower it into the bath.

I decided to use very small containers, which I found in the form of powdery makeup for little girls at a local dollar store. If you want to go smaller, perfume sample vials may be used. If you want to go larger, baby food jars may work.

As a minimum, you will need an assortment of NiChrome wire, which can be purchased online from various vendors since it is the primary mechanism for converting electrical energy into heat. Everything else assists in getting power to the container and in distributing the heat.

In this step, you will need to find a small container and a dowel with a diameter smaller than the mouth of the container. It is important that:

1. The container be made of glass so that it's resistant to corrosive liquid -AND- has good thermal conductivity.
2. The dowel is made of hardwood since lots of tension will be present with the nichrome wire.

After, saw a piece off the dowel which will fit nicely into the container.

In this step, create a template for drawing side notches on the dowel.

1. Trace the dowel along its circumference.
2. Use a right-angle to draw a tangent on two edges of a circle.
3. Use a right-angle to complete the square around the circle.
4. Draw a line from opposite corners of the square.

In this step, you will use the pattern in the previous step to create drill points:

1. Place the dowel on the pattern.
2. Mark opposing ends of the dowel.
3. Draw across opposing ends of the dowel.
4. Lay the dowel flat next to a raised surface.
5. Draw across the raised surface to create a drill point.
6. Indent the drill point to allow the drill pit to align properly using a wife's sewing pin.

In this step, machine the dowel and carbonize it so that it doesn't release air or smoke:

1. Use a wire-thin drill bit in a drill press to drill through the dowel. Use tape if the chuck is too large.
2. Saw both ends of the dowel where the holes are to create a notch.
3. From 250°F - 500°F, boil the dowel in laxative mineral oil to carbonize.

In this step, prepare one leg/electrode of the heating element:

1. Insert a long nichrome wire through a drill hole.
2. Bend the nichrome wire outwardly within the notch.
3. Pull a bent copper wire through a drill hole.
4. Bend the copper wire outwardly within the notch.
5. Clip excess length.

In this step, calibrate the resistance and complete the other leg/electrode of the heating element:

1. Using a low-voltage / high-current power supply, measure the current going into the completed leg.
2. With the remaining wire, slide an electrode along the wire.
3. Cut the nichrome wire with excess length at the desired resistance / voltage / current.
4. Wrap the nichrome around the dowel with even spacing VERY tightly (it will expand somewhat when heated).
5. Insert the other leg.
6. Verify the resistance. Close enough will generally be OKAY.
7. Redo if necessary.

In this step, fill the container with a coarse, absorbent medium and laxative mineral oil after inserting the dowel and attach a thermistor. 

1. Use a coarse medium. It should retain mineral oil easily, but be somewhat porous to allow air/gas to escape. I used Silica Gel. Grind it up if necessary using a mortar / pestle.
2. Pour a thin layer of medium into the container.
3. Insert the dowel.
4. Cover the dowel completely.
5. Pour in laxative mineral oil.
6. Create an indent for the thermistor.
7. Insert the thermistor.
8. Cover the thermistor.

In this step, prepare the lid.

1. Trace the lid.
2. Add a square of tangents.
3. Draw diagonal lines.
4. Mark the center point with a pin.
5. Attach the lid with mounting putty.
6. Poke through the lid or mark it.
7. Use an ice-pick to create a deeper indent.
8. Drill the lid to allow clearance for the leads.

In this step, create a lead spacer out of plastic or wood. I used HDPE (Number 2) plastic.

1. Using a compass, draw the outermost circle, which should fit perfectly into the lid.
2. Draw the innermost circle, which should allow clearance between the lid and leads.
3. Draw a tangent square with diagonals.
4. Mark the center, and inner-circle intersections with a pin.
5. Drill the pin-marked items.
6. Sand the edges if the outermost circle is too big.

You can use this heating element as follows:

1. You can mount it through a side-hole of an HDPE container using teflon tape as a washer between the glass and lid.
2. You can lower it into the chemical bath.

With that said, it's time to consider how much energy you'll likely need. Water has a VERY high heat capacity. Specifically 4.1855 J/(gºC). Assuming the following:

• 1 cup of water (water soluble chemical) = 263 g

It will take about 1100 Joules to heat up the water by 1 ºC.
To heat the bath to 40 ºC from 25 ºC it will take about 16500 Joules.

As the bath gets hotter, you will end up losing more energy to the environment, but assuming an ideal system with no external losses. Let's assume that we have a 5W, 10W, and 20W heating element. Note that a Watt is defined as a Joule / Second.
As such, to supply the 16500 Joules, This is how much time it would ideally take:

• 5W = 3300 Seconds ~ 55 minutes
• 10W = 1650 Seconds ~ 27 minutes
• 20W = 825 Seconds ~ 13 minutes

Compare this to a typical electric burner of 1500W with a 40% efficiency, it would only take 27 seconds.

As such, you may be better of NOT building it.