Heating Element for Tree Grafting

[Chuck H]

I need a solution for healing tree grafts. Fruit trees are often propagated in the spring by grafting (attaching) a piece of scion wood to rootstock. The graft must "heal" which means that the outer "cambium" layers of the two pieces join together. This process occurs around room temperature and takes 7-10 days. If the entire "tree" is exposed to room temperature, however, it may break out of winter dormancy and start to bud early (before the danger of frost has disappeared).

I envision a solution whereby a length of Ni-chrome wire is spiral wrapped around a .25" diameter size piece of wood for about 1" in length. Once connected to a power source, the grafted region can be heated to a "healing" temperature, while the rest of the tree is kept in a refrigerated environment.

I have several different gauges of Ni-chrome wire with which to experiment. I have a table that enables me to calculate power dissipation, but no way of knowing how much heat that will produce. Does anyone have an idea of a measuring device or datasheet that would enable me to measure the internal heat experienced from the spiral wire wrap?

Thanks for your help.

 

[dlgoff]

Welcome to PF Chuck H. I'm assuming you will be grafting more than one tree which may make you reconsider how you are going to build a "temperature controlled" system.

You might want to consider using "Heat Tracing" wire. I've used bulk Self Regulating tracing wire on several projects; easy to work with. But you will need to have some type of temperature controller. And if you are grafting many trees, cost can become a problem. i.e. if you have a controller for each tree.

Anyway, I did a quick google search and here are some examples to give you a starting point.

http://www.chromalox.com/productcat...-+Self+Regulating/product-details.aspx?p=355"

Overview

Chromalox SRL and SRM/E Self Regulating Heating Cables provide the most versatility in heat trace designs and applications. Self Regulating cables are flexible, can be cut-to-length in the field and can be single overlapped without fear of burnout in areas where complex piping and equipment require additional heat trace cable. Chromalox manufactures low temperature self-regulating heating cable for use on 120 and 208 to 277V. Equipped with a ground braid and optional TPR or FEP jacket, Chromalox self-regulating cables are third party tested and approved for use in harsh corrosive and hazardous applications.

http://www.chromalox.com/productcat...ntrols+and+Panels/product-details.aspx?p=309"

Overview

The DL Series Single Point On / Off Temperature Controls from Chromalox represent the state of the art in heat tracing and are available in five models to handle a broad range of applications. Models include two ambient sensing thermostats, two line sensing thermostats and a line sensing solid state controller. These high-quality models combine temperature control and power connection in a convenient, easy to use and economical package.

This is just one site. There are tons of places to get this good stuff.

http://www.chromalox.com/productcat...r.aspx?s=44&gclid=CLvI6feDqqcCFZFoKgodxjuYDQ"

 

[Chuck H]

Hi Don:

Thanks for the reply and research. My suspicion is that the temperatures produced by Chromalox are too high for my app. I estimate that I need to elevate the ambient temp by 10 deg. C. for a very small surface area--think of spiral wrapping 1 inch of pencil. If I need to produce more than .25 watts in a closed environment ( I will wrap the wire with insulation), I would be surprised. By the way, do you know if nichrome will short if it comes in contact with itself?Chuck H.

 

[dlgoff]

"By the way, do you know if nichrome will short if it comes in contact with itself?"

yes it will.

 

[vk6kro]

Nichrome wire is difficult to work with since you can't easily solder to it.

You could set up a test sample to determine the power you need.

You could use a stick thermometer. These are digital and have a rigid metal post which can be used to measure temperatures.

I would guess that you need 5 or 10 watts.

If you have a 12 volt power supply, calculate the size of resistor that will give you 5 watts dissipation with 12 volts across it.
Power = E²/_R
So, R = E²/_Power
R = 144 / 5 = 28.8 ohms

So, you could put two 15 ohm 5 watt resistors in series placed on opposite sides of the cutting. Wrap thermal insulation around them and the thermometer and apply power.

The current will be less than half an amp and the supply can be AC or DC.

Once you get some measured temperatures, you can look for cheaper ways of getting the best result.