Semi-Disposable Package Heater

Sending things to places is at an all-time popularity high. Many things, like pencils, can be confidently shipped with minimal regard for any potential temperature swings*. Other products, such as perishable food or lab supplies are shipped cold, packaged in an insulated box with plenty of dry ice. So long as your package arrives with any unsublimated CO2, you know it has been at -78C for the entire journey**. Dry ice is a robust solution that maintains temperature with 100% reliability and no mess.

There are, however, situations where minimum temperatures must be maintained. For example, if you wanted some pretty blue shrimp sent to you and the 1000 miles between origin and destination averages -15C. Here, the thermal buffering capacity of water and styrofoam insulation help, but often not enough. Some mitigate the problem by adding small disposable, chemical heating pouches designed for gloves etc. but these are unregulated and success is guesswork.

So, a niche has been identified. What we need is temperature regulation. So let's start by solving the problem with the Mk1 bs0 Sure-Temp Package Cosy.

Simply take a PID temperature controller, an RTD sensor, a 25W silicone-encased heating element and a 3-cell Li-Ion pack of about 20 AmpHr capacity. Link it all up with a voltage-regulator & charging circuit, a microcontroller with logging, possibly wireless data. Done. That was easy! I reckon you could mass produce that for less than $150!

Ok, so accounts & sales aren't happy and FedEx has vetoed the extra lithium cells. Let's iterate with a little value engineering. Take 2 alkaline cells. A positive temperature coefficient (PTC) heater and a switch. Done! That get's you ~20 hours of 2W heating, which inside insulation should be good. Getting PTC at the right range accurately isn't easy, or at least easy and cheap at the same time... but we're on the right track with the rest.

We can lower cost with a dumb heater if we get the sensing/control done reliably. For this I'm going with a wax-motor thermistor type doodad. Wax is reliable, like dry ice, you're relying on a fundamental physical property of wax&temperature. Wax expands, moves potentiometer, current to heater changes. The heater is just the cheapest wire available. Then we need a switch***, a low voltage LED and a bit of silicone casing. Done.

You can get fancy, color code various models for 20C/25C/30C and so on, there's applications outside living things, chemical solutions that precipitate etc. For safety, maybe add a tiny PTC thermistor to the heater to prevent thermal runaway, which isn't spectacular with standard cells. Your package recipient will know by looking at the LED to see if the cells are almost completely done. The key is to keep it cheap, so even the voltage indicators on the side of some cells could replace the LED.

There, someone build and mass-produce this so I don't have to wait for spring.

*keep below 250C or package in nitrogen **if no dry ice remains, add some and leave it for someone more junior to test. *** think a bit of removable plastic between two contacts.