Smart Mixing Valve based HERMS?

I've had the (possibly) good fortune to aquire one of these:

Hass Manufacturing Company Online Search Results

from a retiring photographer who had a pretty advanced lab/darkroom. I am thinking about building a HERMS based on this unit, which is basically a highly precise hot/cold water mixing valve. It works by sensing temp with a thermocouple and moving 2 valves with stepper motors to achieve a given temp. I need to find a way to mod the thing to give a more appropriate temp range.

Heres basically what I'm thinking of doing:

MLT manifold>>Pump>>T fitting with one side feeding 50' 1/2" copper heat exchanger inside electric heated HLT for a 'HOT' side and the other bypassing it for a 'COLD' side of the mixing valve. Mixing Valve outlet>>MLT return manifold.

Sorry for the text description, I'll draw somthing up and post it when I get home from work later.

Potential problems with this system I can see:

Modding the Intellifaucet controls to allow greater temp range (I may need to simply use the valves/steppers and a different controller)

Possibly overheating the small amount of wort trapped in the heat exchanger once the MLT is up to the desired rest temp. Not sure on this one, obviously the HLT would be at sparge temperature, and if the wort in the heat exchanger was trapped there for a while, it would be heated to sparge temp as well. How problematic would this be? It could obviously denature the enzymes in that small volume of the wort, but would this be enough to notice?

This is a potential future project, I'm currently building the MLT and HLT for normal manual mashing/fly sparging and will brew a few batches to get the system tweaked and working manually before attmpting to step up to a HERMS, but I may build the heat exchanger into the HLT and just not use it yet.

Any advice would be appreciated.

Thanks,
Don

There is probably a calibration screw somewhere inside the unit that will allow you to tweak it a little bit. Depending on the type of sensor, there are different ways (involving solder and resistors) to fool the microprocessor into thinking the water is cooler than it really is.

There are some interesting mixing valves, some mechanical, some motorized, from Taco that are intended for hydronic heating applications which would also work quite well.

Yeah, the calibration screws appear to only adjust a few degrees one way or the other, so I'll probably have to trick the processor into thinking its much cooler than it actually is. I'm getting the valve for free, so it's worth a little work to make it function. It's all stainless, and is completely food grade, so it should be quite a nice piece once I figure out how to make it work for me.

The stock temp range is 65F to 115F I believe, so I need to offset the overall range upward. Anyone have experience fooling a thermocouple like this? I suppose I can pull the thermocouple and put it on a meter and see how it responds to various temps of water.

Worst case scenario I can build a basic stamp system to run the 2 stepper motors and use a normal temp probe to measure temp, but I really like the simplicity of the 'dial-a-temp' front panel control on this valve.

Any help is appreciated.

Thanks,
Don

Find out whether it's a two wire thermistor or a three wire sensor with a builtin amp. If it's just a thermistor you can put an ohmmeter across it and vary the temperature and see what you get. Then it's a simple matter to apply a series or shunt resistance to fool the uP. You can use either a fixed resistor or a pot.

There are some two-wire devices that are 4-20 mA current loop though not usually in packaged applications like that. If you get readings on the meter that don't make sense that might be what you have.

There are also some three wire devices that have an amplifier of some kind built in. You have to take these as they come and try to figure them out, though if you get a product number or part number you may be able to find a data sheet.

It is definitely a 2 wire device. The manufacturer calls it a thermocouple, which means it outputs a small voltage rather than varying in resistance. I'm going to stick a meter on it and see what kind of voltage it actually puts out. Thanks for the replies.

O/T: Anyone know a good free picture hosting site so I can upload my diagram for this, as well as a few pics I will take?

Thanks,
Don