Recharging and melting salt crystals on demand#

This section outlines the second goal of the project -- how can we recharge and melt sodium acetate trihydrate (SAT) salt crystals on demand (after they've formed)?

Experiments in recharging and melting sodium acetate trihydrate (SAT) crystals#

Let me preface the next few subsections by saying that my experimental set-ups for trying diffrent ways to melt the solution is very crude. The two attempts I've made are probably possible if the set-up was designed well for optimal heat transfer of heating element to salt, or optimizing the ratio of salt volume to heating element volume -- which is why I spent a great deal of the last month thinking about how to translate my research into a wearable design. I believe that once you know how you envision your product on the system scale, it clarifies the research performed on the component level.

Are heated textile pads enough to heat and melt SAT crystals?#

No, but it can if designed differently

The heating pad used in this section is from Adafruit. The image below is extracted from the tech sheet. Knowing that:

the input voltages required can range from 7.4-12V, to get the heating pad hot enough to melt the SAT crystals.

Initially, I had hooked up the heating pad to a 9V battery with alligator clips to the pad. The SAT solution (with the crystals formed) was contained in a round bottom flask (see following image). The result? The heating pad didn't get as hot (I could comfortably touch it with my bare hands). Thinking that there wasn't enough heat transfer between the heating pad and the contained salt solution, I changed up my set-up completely.

  1. Instead of using a battery, I used a DC power supply
  2. Instead of containing the salt solution in solid glass, I tried seeing if I could melt it in a bag (thinner walls than the thick walls of the glass)

Below are some snapshots of creating intimate contact between heating pad (connected to DC power supply [GW Instek GPD-3303D]) and handwarmer bag. I played around with the voltage and current with the DC power supply, until I got a reading of >58 degrees C, and then pressed the handwarmer bag to the heating pad. Unfortunately, the handwarmer refused to melt.

Can we use resistance wire to melt SAT crystals?#

Because the handwarmer refused to melt the SAT crystals even when connected to a larger power supply, I looked toward resistance wire. I wanted to use a heating element that didn't require so much power since I wanted to make something wearable. Resistance wire, as its name indicates, has high resistance and when electrically charged, will dissipate the electricity it can't conduct as heat to its surroundings. Doing some research, I came across materials like Nichrome and Kanthal wire - metal alloys that have resistance to oxidation (think, resistance to rusting) - which I thought would be useful for heating up my SAT salt crystals, especially if I dip the heating element into solution. Nichrome wire is a heating element that can be found in toaster ovens, hair dryers - really anything that uses electricity for heat. A few Google searches later, I found out that these wires are used in vaping pens (think: electronic cigarettes), and I ended up going to a vape store to acquire some wire. (Note: I do not vape, so it was an interesting experience asking for some sample wire and explain what I needed it for).

My most "successful" attempt at melting salt using resistance wire involved using Kanthal wire that was wound into a coil already. Since you can't solder onto resistance wire directly, just like making an electrical connection to conductive thread, I used a crimp bead to wrap one end of the resistance wire to another end of electrical wire (See image below):

Connecting this set-up to a DC power supply as in the previous section, submerging the wire coil into a solution of crystallized SAT (the solution here was extracted from a HotSnapz handwarmer), and setting the DC power supply to a voltage = 2V and current = 3.2A seemed to get the coil hot enough to melt the SAT. Below is a timelapse of the coil melting the crystals. The yellow wire outside of the jar moves as a result of the coil being submerged deeper into the crystal solution.

Although the coil was hot enough for the SAT crystal to melt, it was localized to the volume that the coil was sitting in. In the future, I think a design where the heating element was distributed all throughout the salt solution would be sufficient for melting a bulk of SAT crystals.