I was an electrician in commercial construction for 20 yrs. But, I went to Nursing school 30 years ago and worked in the OR until I retired a couple of years ago. I'm purely a hobbyist. I am currently (sorry not intended pun) helping my son build a heated water bath for his dark room. It needs to maintain 101 degrees precisely.
It also needs to control a mechanism that rolls a cylinder for 5-8 sec, reverses 5-8 sec and repeats continuously.
I have made some rough drawing to illustrate if anyone wishes to see them.
Thanx,
Will

 

This is the very rough drawing. One picture got repeated, sorry.

 

"It needs to maintain 101 degrees precisely."

How precisely is 'precisely'? More precisely means more costly.
What is the ambient temperature in the dark room?
How many gallons in the in the heated bath? Is it water or gasoline ;-)?
Is the water still or moving?
Is the bath insulated to aid in maintaining constant temperature?
Is the temperature bath physically or otherwise tied into the rolling mechanism or are they two completely operations?

 

The middle 4 questions all address the required heating wattage, and heater configuration options. Do you already know these parameters and/or already have the heater?

Can we assume you'd rather use off-the-shelf solutions rather than build your own? (The available products are likely cheaper than what you can build yourself, but some folks enjoy building anyway.)

What powers the rollers?

 

"It needs to maintain 101 degrees precisely."

How precisely is 'precisely'? More precisely means more costly.
What is the ambient temperature in the dark room?
How many gallons in the in the heated bath? Is it water or gasoline ;-)?
Is the water still or moving?
Is the bath insulated to aid in maintaining constant temperature?
Is the temperature bath physically or otherwise tied into the rolling mechanism or are they two completely operations?

These are all sound and valid questions.

 

These are all sound and valid questions.

tHANK YOU FOR RESPONDING, ESP WITH GOOD POINTED QUESTIONS.
1. ABOUT 6-7 GALLONS h2o
2. tEMP CONSTANCY very-very important to keep photo chems in the film cylinder at that level
3. I plan to use a single Hot water tank element 1500w/120vac and a single element thermostat
4. The plan is to circ the water using a fish tank pump
5. should be easily insulated
6. the rolling mech. is ,at this time, attached to the tub. The tub itself is a plastic tub bus boys use
the roller is to be turned by a 6-12 vdc motor attached to a wheel that will contact via friction the wheel casters on a stainless steel allthread rod
see the poor drawing

 

2. tEMP CONSTANCY very-very important to keep photo chems in the film cylinder at that level
3. I plan to use a single Hot water tank element 1500w/120vac and a single element thermostat

Do you believe the thermostat you have will give you the precision you need? The thermostat may have some built-in hysteresis. Let's please talk in numbers, for instance 100°±2°F. What exactly do you need?

 

OK
The water temp thermostat I have is meant for a home water heater.
In effect I do not know how accurate it will be.
I am open to any other suggestions.
My son is the photographer and tells me that a consistent temp is vital.
During my search on the net I found an astounding number of simile plans
To build a sous video machine.
One in particular is at YouTube Sansa ire $199 sous video machine.
This is very close to my needs, but without the roller parts

 

Well, your son really needs to quantify what is "constant temperature" (0.1°F, 1°F, 2°F?) so we can determine what would be adequate for the temperature control.

It would be difficult to maintain very close temperature control with such a large (1500W) heater element since you would tend to get a lot of localized hot spots and overshoot of the temperature. Better to use a heater of a few hundred watts or so, just enough to maintain the water temperature at about 110°F when continuously on.

 

These are all sound and valid questions.

How many gallons in the in the heated bath? Is it water or gasoline ;-)?


I had to ask, who would have a tray full of gasoline in a dark room?

 

Well, your son really needs to quantify what is "constant temperature" (0.1°F, 1°F, 2°F?) so we can determine what would be adequate for the temperature control.

It would be difficult to maintain very close temperature control with such a large (1500W) heater element since you would tend to get a lot of localized hot spots and overshoot of the temperature. Better to use a heater of a few hundred watts or so, just enough to maintain the water temperature at about 110°F when continuously on.

 

I appologize. I thought I had said earlier that the desired temp is 100 to 102 degrees F.
If you know a better solution I am all ears.

 

These are all sound and valid questions.

Also I am convinced that it is usually an I O problem......Idiot Operator.