[nkurz]

I'd be interested to hear more about your technique. What were the issues with the dehydrators you tried? Are you pre-boiling to peel skins or do you have some other way? Are you scooping and discarding the "guts" or are your tomatoes dry enough to do whole? Are you blowing air through the heater on the way in to the plywood box, or do you have the heater in the box? What temperature are you drying at, and where are you measuring it?

[Syzygies]

I've been planning to put up a web site with plans, as most DIY plans out there are rather sketchy. For most DIY engineering projects I have a pretty good idea of the practical physics, but this challenged me and involved some false steps on the way to a working design. What heat and airflow did I need? That was an empirical question, I couldn't find equations that were relevant to practice in this instance.

I have to go to work shortly, I'll try to update this if you check back.

My heat source is a Cadet Com-Pak 1500-Watt 120-Volt Fan Heater, suspended by metal angle irons in a cradle underneath the unit, intake filtered by a generic 12" air filter. I convinced myself that the wood never got hot because of the gap and airflow, but the auto shutoff in the heater itself was too sensitive for me to drive the dehydrator hard. I shorted its thermostat and added my own from Digikey. This allows me to aim for temps up to 145 F without the heater cutting out.

I regulate the temperature using a Ranco Electronic Temperature Control from a beer making supply company.

I regulate airflow using a AIRTITAN T8-N, CRAWL SPACE AND BASEMENT VENTILATOR FAN. Expensive but slick.

The dehydrator operates very effectively if one has outdoor barbecue experience. While I managed to weave the airflow tray to tray, one needs to tend the trays every few hours, and there can be front or back edge effects from a leaky airflow. This is nothing new to anyone comfortable with fires, but frustrating if one expects a perfect abstraction. Believe me, I've thought about redesigns with perfect air mixing, and they all double the size of the unit, which already looks like a washing machine.

The Italian grandmothers making estrattu on tables in the Sicilian sun have to worry about which tables first get afternoon shade. Nothing new here, the issues just take a different form.

This relates to your temperature measurement question. I choose middle locations for both thermostat probes, and I also have two TelTrue analog thermometers. Like BBQ, the idea of a single temperature is misleading. One flies this ship somehow, with better results than I know how to obtain any other way.

[nkurz]

Thanks, sounds like a great project. Please add a link here to your web site if you ever make it. I try to check my old comments occasionally, so will probably notice eventually.