Building a water-bath for Science Olympiad

[adrenalynn]

I just thought I'd share this project with the LightObject community - and maybe it'll show-up in a Google search for other SciOly coordinators looking for a solution.

I'm the Physics / Physical Sciences coordinator for our Regional (and sometimes State) Science Olympiad. This year, I needed to construct a water bath that could maintain a constant 65C or better (key is "constant") water temperature throughout the day.

I purchased the JLD612DC (the AC was out of stock), a 25A SSR, and PT100 "premium" probe.

As I write this - I've only just fired-up the low voltage side and confirmed no fuses blow, lights glow, and things otherwise look happy. I'm still terrified to light up the line-voltage side. I'll update once I do.

The project is built in a cheap $10 Harbor Freight toolbox with appropriate holes cut in it. Once functionality is confirmed, everything will get a good dose of RTV. I'm looking for splash resistance. The AC output plug is a 15A GFCI outlet housed in an in-use wet-environment exterior box. The input is fused with a 15A ceramic fuse.

The input cord is a cannibalized 10AWG/10ft extension cord. All internal AC wiring is 12AWG.

The DC input is a barrel plug in the opposite side, and is fused with a 2A Buss fuse.

Switches are lighted Radioshack, 12v automotive and 120vAC as appropriate.

The bath will be a 12Qt Allclad sauce pan, and the heater is a 1200wt GE electric hot plate, decades old from my college dorm days.

The event is this weekend, and I'm totally out of time. I tossed this together this morning, and plan to have it hopefully working and tested tonight. That considered: comments welcome!

More updates as testing continues.

[richiem]

I'm a little confused -- you're using a DC controller but have AC wiring? How are you converting AC to DC? Or are you using a DC output power supply to run the controller?

[adrenalynn]

I'm a little confused -- you're using a DC controller but have AC wiring? How are you converting AC to DC? Or are you using a DC output power supply to run the controller?

The Controller (JLD612DC) is powered by DC. I'm using the DC Input/AC Switched SSR. http://www.lightobject.com/25A-Solid-St ... t-P61.aspx

The Controller is powered by an independent 12vDC wall-wort, although if I had more time, I'd probably build a 12v switching supply right into the box, but that's another story...

The Controller outputs the 12vDC on a switch condition to the input pins of the SSR. The SSR has the hot wire from the AC (behind the fuse and switch) into the AC side of the SSR, then that goes out to the plug powering the heater.

Does that make sense?

Here it is running! WooHoo! Seems to be solid so far! The shortest fin on the heatsink is 9C above ambient. The longest fin is 4C above ambient. I'm still going to put a small fan inside the toolbox for all-day running - since I have a 12vDC supply handy anyway, no reason not to!

[richiem]

Yep, makes sense. Good job all around. Be careful to not let the probe touch the bottom of the pan. I also don't think you need the fan, given the very low temperature rise of the heat sink, but as you say, it can't hurt.

[adrenalynn]

Thanks!

The probe is supported by [cough] a super-high-tech - ok, I fib - super-low-tech length of 12AWG copper wire that has been stripped of insulation and wrapped twice around the upper body to prevent it from flopping around. The wire is then bent over the edges of the vat. By changing the bend of the supporting wire, the thermometer can be raised or lowered so that at any water level the tip is at the mid-depth level of the water.

It seems reasonably precise. When the JLD is reading 60C, a precision lab-grade digital thermometer is reading 58.92. At 65C :: 64.41C. A couple percent in a device like this has got to be "reasonable error".

I did add a high-temp circulation pump supported by a copper stand-off on the bottom, just to stir the water up a bit and make sure it's consistent. Given the relatively small volume, it's probably overkill - but I'm trying to avoid writing a thermodynamics exam until the last possible instant.

[adrenalynn]

As my daddy always said:

"Might as well: Can't dance, and it's too wet to stack BB's". I never had the opportunity to rate his dancing ability, and not sure the desirability of BB-stacking - but here's the result:



It's on the switched 12vDC supply and only took about 20mins to add.

[Architeuthis]

Ok, so this is exactly the type of setup I want to make, I want to use the PID controller to control a temperature bath. I don't have any experience with DIY electronics but I am a science guy (albeit without practical experience in this area, I work in health care currently) and I have soldering equipment and voltmeter. I want to construct a machine like the one above but don't understand all the circuitry. I drew a diagram found here:

https://docs.google.com/drawings/d/1eDq ... LD6qk/edit

Can anyone tell me what my next steps are? Like I said this is my first DIY electronics project so don't skip the details like if I have to use the voltmeter for something or if I need to use special wire I won't know.

[richiem]

Use at least a 60 Amp SSR -- I would use a 90. SSRs have a very steep derating curve with both internal heat and ambient temperature rise.

Use the AC powered version of the JLD612, just for simplicity.

Use 12 gauge 12/3 household wire for the AC line to SSR to AC output socket connections. AC line black goes to an SSR AC terminal and from the other SSR terminal to the hot side of the socket.

The white neutral side of the AC line goes directly to the neutral side of the socket.

The green ground from the AC line also goes directly to the socket ground.

The AC power connections to the JLD612 can be made with 18 gauge lamp cord.

The connections from the 612's SSR output to the SSR control input can be made with 22 gauge two-conductor "bell wire" which is color-coded for polarity, usually red and white. Make sure that the + connection of the 612's SSR output goes to the + connection of the SSR's control input, and - to -.

You may not need the circulation pump for stable temperature in the bath unless it is a big bath. If you don't, then you don't need the DC power supply, and you save wiring and money.

[Architeuthis]

Thanks for your reply richiem!
I'm still a bit confused (still learning as I go).

How should I hook up the AC input, i.e. the connection from the wall to the box.
And that 16A 125V black thing is just a rocker switch, so that should turn it on/off? The original poster said he had two switches but I didn't understand what they were for.

Thanks a lot for helping me, modifying the drawing really helps

Also, should all connections be soldered? The temperature probe and everything? Where the wires are in a 'Y' should I just solder two wires together?
Sorry if some of these questions are newbie, but I am a newbie. Thanks again for your help

[richiem]

For AC power input, I would use a regular 16/3 orange power extension cord and cut the female connector end off, and strip the insulation back to reveal the wires. These cords are cheap and effective. Use a strain relief or bushing where the cord enters the box.

I use crimp connectors for my AC connections -- they come in various terminal types -- spade, ring, fast-on, butt splice etc -- to suit the other components. I also use spade types for the screw connections on the JLD612. The store you buy them from will also have an inexpensive crimping tool for attaching them. You just strip insulation to expose about 1/4" of bare wire, slip the connector on and crimp. No need to solder.

A switch for power needs to be rated for the current being delivered, which will typically be 15 or 16 amps for a hot plate.

[TinCook]

Use at least a 60 Amp SSR -- I would use a 90. SSRs have a very steep derating curve with both internal heat and ambient temperature rise.

How come the massive overspeccing? I've ordered a 25 amp w/ the recomended heatsink. I figured that'd be enough since I'm just running off of wall current. Just how hot is this thing going to get?

[richiem]

The typical SSR spec has 100% current rating at a base plate temp of 25 deg C -- as the base plate temp goes up, the current rating heads for the sewer. Most SSRs will lose over 1/2 of their rating as the base plate temp goes up 20 deg C. So a really good heat sink AND over-rating the current capability will yield big reliability dividends for a small cost increase. Especially if the location it's in can have a big heat increase in ambient temp. Eskimos probably don't have to worry if the location is outdoors....