Temperature’s Rising

Temperature’s Rising was Originally Posted on June 15, 2005 by

I bought a wireless thermometer today. It took longer than I expected and the whole time I kept my cool (so to speak).

Ace Hardware had a wireless thermometer on sale and I had been looking for a new one. The price was right and I took it to the checkout counter. I got carried away with the new screens they had on the register. It looks to be a nice computer system. As I clicked OK on the electronic signature pad it dawned on me that the price was NOT the sale price! If I had been paying with cash I would have noticed the problem.

The cashier went and verified the sale price and came back to the register. It seems the system are so new that there are still “cheat sheets” with step by step instructions on how to perform tasks. Unfortunately, the steps for cancelling a transaction were incorrect. After 2 phone calls and 20 minutes, the cashier was able to finally get the charge reversed and re-entered. I kept my cool because 1) I was not in a hury and 2) What good would it be for me to be in a hurry because me being in a hurry wouldn’t have sped up the fix anyway…

So I got my thermometer home and proceeded to play with it. I verified that the remote sensor and the local sensor read the same value when placed next to each other. Then I looked for something to measure.

Living so far away from Kona (where I usually can find excellent bargains of certain frozen foods), I used to have problems getting them home before defrosting took place. I tried the ice chests with those frozen gel things and paper bags and so on. Finally I found a solution. I bought a relatively inexpensive electric cooler (called a HandyKool) made by Igloo. It will hold perhaps 15 frozen dinners or maybe 48 cans of soda. The cost was about $87 at K-Mart.

The unit is a Thermoelectric unit and used what I think is a Peltier Junction. Years ago someone discovered that certain transistors could cool things by transferring heat from one side of a junction to another. One company describes them here http://www.tetech.com/techinfo/

So this unit has a junction and only 1 moving part (a very small fan) to move the air around. The unit is very light and when empty, I can pick it up with 1 finger!

The plus for this unit is that besides running directly off a car battery, there is a power supply with it that lets me run it in the house also; thus I can plug it in and let it cool down, then put it into the car for the trip to get frozen foods. One must be somewhat careful because it does draw some current from the car battery and you would not want to leave it plugged in for more than a few hours with the engine turned off. Still, it is quite handy in the truck and fits behind the passengers seat. I have “suicide doors” on the truck and position the door of the “fridge” so I can put things in easily when standing outside the truck.

The unit does not freeze things but it will cool them down considerably. The specs say that you should plug it in for about 3 hours before using it and that the inside temperature will be about 40 degrees cooler than outside. In fact, their example shows 80 degrees outside and 41 degrees inside. I decided that with my new wireless thermometer I would check it out and see how fast it cools down and how long it would stay cool. The instruction booklet says give it time to cool down and also that it works best when food put into it is already cold. That makes sense.

I tried to make this as accurate as possible, realizing that the ambient temperature in the house was dropping somewhat as the sun was already down; thus I captured the room temperature and the temperature inside the cooler. This was easy since the unit is plastic and thermometer is wireless (using a radio signal to send data).

My testing started at exactly 7pm and I took readings every 5 minutes. Since the room temperature only changed about 2.5 degrees, I’ll assume that that did not effect the readings much at all.

What I found was the unit took a reading or two to actually change temperature, but then I saw quite a bit of consistancy. As expected the temperature started getting cooler and as time went by the rate of change got less and less. It became a game of me guessing what the next reading would be, and I was right most of the time.

At the beginning the temperature dropped about a degree and a half every 5 mintes. The unit went from 79.2 degrees to about 44.4 degrees at 11pm when I unplugged it. It was still slowly getting colder but I tired waiting and the change was .1 to .2 degrees every 5 minutes.

If there is sufficient interest, I could graph the thing and post the results… but here is the simple data:

7pm 79
8pm 62
9pm 51.6
10pm 46.6
11pm 44.4

So after 3 hours it really was pretty much as cool as it was going to get.

At 11pm when I unplugged the unit I started to note the temperature as it warmed back up. That rate was NOT the curved slope the cooling down acquired but rather looked to be a straight line change and I fell asleep it was so constant! :)

The rate of warming back up appears to be just slowly leaking cold air out or the transfer of heat back into the unit throught the plastic case. The warming happened alot faster then the cooling. The rate of warmth looked to be about 1 degree every 5 minutes; thus after unplugging the unit at 44.4 degrees, it was up to 52.3 degrees after only 40 minutes. This time would be extended quite a bit if the cooler was full of food I expect.

In any event, I like the cooler and it does what it was supposed to do. The company’s specs seem to be correct and science wins in the end!

Remember that there is always more than one way to look at things. You don’t add cold to things, you are actually removing heat. This juncture transfers heat from one side to the other thus moving the heat from inside the unit to the outside.

That makes is unit really “Cool”! :)

Leave a Reply