Fixing a laboratory waterbath

[vrasp]

Re: Fixing a laboratory waterbath

Here is a website link where you can find temperature controllers

https://www.ebay.com/sch/i.html?_fro...oller&_sacat=0

I hope this helps

Thanks Sam!

Again it's still possible the voltages were not assigned properly at the transformer, however at this point with all the other damage on the board and their associated bodge fixes, I'm not sure where to start looking for problems...

Yes that makes sense. I m happy with the current way it works as it does everything it was built to do. Thank you for your help! I think this thread can finally go to sleep.

Thanks to everyone who helped.

[eccerr0r]

Re: Fixing a laboratory waterbath

Again it's still possible the voltages were not assigned properly at the transformer, however at this point with all the other damage on the board and their associated bodge fixes, I'm not sure where to start looking for problems...

[sam_sam_sam]

Re: Fixing a laboratory waterbath

Here is a website link where you can find temperature controllers

https://www.ebay.com/sch/i.html?_fro...oller&_sacat=0

I hope this helps

[vrasp]

Re: Fixing a laboratory waterbath

Hey guys,

Just an update.

I decided to reassemble everything. I couldn't figure out why the display is showing the wrong temperatures. I checked all the connections and it all seems to be good. Maybe the ICL needs to be replaced again, or the op amp. I don't fully understand how this works so it would take me a lot of time to investigate this issue. I ve decided it doesn't matter. The waterbath works, the temperature can be controlled, and I ve added a thermometer in there to get the actual temperature (2nd picture, bottom right).

Yesterday, I had to open the other waterbath we have (RM20, 1st picture). It's the same brand but a different model (the one I ve been working on is a RMS20). I looked at the TRIAC and noticed it doesn't have a mica but it has a washer. I remembered that when I first opened the RMS20 I did not see a mica. I put 2 and 2 together and realized the reason why I was not getting continuity with the heatsink was because it has an insulating layer on it (the 2 waterbaths have the same heatsink). So no mica needed. The TRIAC only has a washer because the screw makes contact with the exposed conductive part of the heatsink inside it. All these TRIACs I blew were likely due to the washer or screws (maybe size or not well assembled). Anyways, I had an "aha" moment when I saw there was no mica in the RM20.

I ve added a picture of the RM20 (1st picture). As you can see there is no display on this model, just a knob with temperatures. How complicated would it be to install a knob like this?

Thank you!

[eccerr0r]

Re: Fixing a laboratory waterbath

And aha! the manual says the RMS6-20 holds 14-20l of circulating fluid and the 1KW heater guess was spot on.

So 20 minutes to raise 20°C is extremely close to what it should be according to the math.

... and the manual also says that 60°C outside case, assuming the triac is one major contributor, is normal...

[vrasp]

Re: Fixing a laboratory waterbath

I checked in the manual. It says the capacity is 14 to 20L. It's close to being full so I d say 17L. That would mean ~24 min to raise to 20°C.

But again, I assumed it was around 20 min and I dont know the exact volume in the bath. I also dont know what mixture the technician put in there. He might have put all anti-freeze, or a 1:1 solution, or some other ratio. The lids are metal and absorb some of the heat, and other losses due to the system not being adiabatic. Lots of unknown variables here but like you said, it seems reasonable.

My new issue now is with the display. I get the correct temperature displayed when I turn on the waterbath. Then when I press the button to set the temp using the potentiometer, it shows random numbers constantly changing. It should show me the set temp and let me change it. When I release the button to go back to the current waterbath temp the values are not correct. Adjusting the potentiometer increases/decreases the number so that seems to work.

Turning the waterbath off and on goes back to giving me the correct temperature readings on the display.

I replaced the ICL a while ago. It wasn't easy installing it because it is so long and I had issues melting all the pads at the same time to insert it. As a result, most pins inserted through the holes as they should. a few pins did not go through and bent (like 3 pins I think). I put solder on those and made sure there was continuity between the pin and the pad on the other side of the board. I think it should be good but I ll double check.

I suspect the issue has to do with the long pins from the display board into the LED board. I had issues with the pins from the other boards and ended up removing them and putting wires instead. I dont think I have space to do that in this case. I m gonna try tinning the pins to increase their thickness and improve contact. I m open to any advice you guys may have.

Thank you!

[eccerr0r]

Re: Fixing a laboratory waterbath

10L (assuming 100% water, which is not right if there's glycol but it makes the math easier) ~= 10kg * 4184J/kg°C = 41.84kJ/°C , * 20°C = 836.8kJ to raise 20°C

So if assuming a 1KW heater, it would take 836.8 seconds = 13 minutes to raise 20°C .

So yes your numbers are in the ballpark at least, though assumptions
- it should be faster as glycol holds less heat than water.
- 1KW heater is 1KW heater, not counting TRIAC and wire losses. 1KW=8⅓A @ 120VAC RMS. At 8⅓A, almost 10W (1%) is lost at the TRIAC.

So it sounds like you have less than 1KW heater, more water, or significant heat or wire loss to environment...

[vrasp]

Re: Fixing a laboratory waterbath

That's right. There is an LED light that is on when the heater is on. Once the water reaches the set temperature, the LED flashes, indicating the heater is turning on and off to keep that temperature constant. The TRIAC temperature would likely decrease once the set temp is reached.

I could put a fan on it but I dont want to put in the extra work. If the TRIAC dies from overheating I ll look into ways of reducing the temp. There is lots of surface area where the heatsink sits so I could also just replace it with a much bigger one.

This week I ll try to work on all the solder pads and cables I added so everything is nice, clean, and properly soldered so I can put everything back together. I wanted to keep the TRIAC fuses in there but they re too big (20 mm), especially with a fuse holder. In the end they aren't really necessary. It's more for peace of mind.

Thanks for the advice!

[eccerr0r]

Re: Fixing a laboratory waterbath

heatsink fins look like it's on the outside so i dont see why not....
however the duty cycle, despite the device being used for hours at a time, the heater will not be powered up the whole time as it needs to be shut off by the circuitry else the temperature will keep going up. One it has reached the setpoint temperature the heater and thus triac should be turned off for a while.

[sam_sam_sam]

Re: Fixing a laboratory waterbath

Is there a way to put a cooling fan on the heat sink safely

[vrasp]

Re: Fixing a laboratory waterbath

It's meant to be on for long periods of time. Sometimes 10 hours. We have another waterbath (same model but without a display) that I use all day long at 80 °C for protein gelation. I have checked the heatsink of that one and I d say it's more or less as hot.

The heating is probably as fast as before. Again comparing with the other waterbath which takes at least an hour to get to 80 °C. I didnt thoroughly compare but 20 °C in 20 min is reasonable. It holds I would say close to 10L of the antifreeze/water mixture which is a lot to heat up.

I dont know how to tell if it's fake but I trust the supplier. They re the go-to place for electronics. I m not too worried about it. If it stops working I ll get one from another store.

[eccerr0r]

Re: Fixing a laboratory waterbath

It's probably the way it is. 90°C is hot, but it could be hotter.
Also have to consider, is it hot when the heater/TRIAC is "off"? Should get some reprieve when the heater is off as it shouldn't be on 100% of the time.

Is the heater heating as fast as it was before? Perhaps the TRIAC you got is a fake?

[vrasp]

Re: Fixing a laboratory waterbath

I did do that. I can keep my thumb on the heatsink but it's definitely hot. I did a quick measurement but didn't write it down. I think it was around 60 °C. What I m worried about is reducing the TRIAC lifetime if it gets too hot. Do you think 90 °C is ok or should I try with some ZnO thermal paste? There isn't much else I can do. The TRIAC is tight on the heatsink.

[eccerr0r]

Re: Fixing a laboratory waterbath

No you don't want to peel mica, it's one of the characteristics that it can be split very thinly. And no there's no adhesive layer. Though thinness helps heat conduction, it also encourages shorts so you want to use full thickness. And peeling it, as you assumed, ruins it - so don't use the peeled version.

Question is how hot is the heatsink getting versus the TRIAC. At 90°C I'd think it's on the warm side though it's within limits. Rule of thumb is that if you can't keep your thumb on it, it's too hot. However don't do that as it's electrically live and you could get electrocuted. If the heatsink is also getting warm then you can probably leave it; if the heatsink stays cold then you might want to tighten the screw a bit more or look into heatsink paste.

[vrasp]

Re: Fixing a laboratory waterbath

I was going to mount the TRIAC with the new thicker mica I got. However, I can peel the layers off. I initially thought that there was a sticky part to stick to the TRIAC or something like that but below each peeled layer was another layer that is easily peeled. I added a photo below with a "full" mica and one peeled into it's layers. Anyways, not knowing if this was normal, I decided to use the other isolator I have which I believe is Silicone or Kapton.

I cleaned off the surface of the TRIAC and heatsink and used the new washers I got. Checked for resistance, everything was good (max resistance between the metal tab and heatsink/scew). Turned everything on and it worked!

I then proceeded to resolder the heating element instead of the lamp. Turned on the waterbath and the water heated as it should. It took about 20 min to go from 20 °C to 40 °C. I then proceeded to measure the TRIAC temperature with an infrared thermometer. After 20 min of heating, the TRIAC temperature was approx. 80 °C to 90 °C depending on where I point the laser. The datasheet mentions temperatures from -40 °C to +125 °C so it should be good. I assume that the TRIAC reaches it's operating temperature quickly and that this temperature stays more of less constant over time (assuming constant room temp) because the current going through is always the same. In other words, leaving the waterbath on for 20 min or 24 hours should not have a significant effect on the temperatures I measured. Can someone confirm that?

There are two other minor issues that I believe are due to bad soldering. The first one is that the display is missing some LED bars that show the numbers and also the dot for decimals (it's still readable so no big deal if I cant fix that). I had changed the ICL a while ago and have to check each pin is properly soldered on its pad. The other minor issue is that I sometimes have to turn the waterbath on - off then on again for the display to show the temperature. So the first time I turn it on the display turns on but doesnt show numbers (just a some LED bars are on) then if I turn it off and on again it shows the proper temperature and it's good to go. Again, it probably has to do with bad soldering. I ll have to double check all the connections.

Thanks for your help guys.

[vrasp]

Re: Fixing a laboratory waterbath

You probably could just stick your multimeter probes into the heatsink compound and see if there's any conductivity.

No need to scrape or scrub the heat sink, just need to make sure the surfaces are flat and rid of any shrapnel that might be large or sharp enough to damage the insulator. Again don't bother with thermal compound for now. And the main reason for not torquing down the screw more than a foot pound or so is to prevent warping, along with not damaging the insulator.

Sounds good. I ll do that this coming week.

[eccerr0r]

Re: Fixing a laboratory waterbath

You probably could just stick your multimeter probes into the heatsink compound and see if there's any conductivity.

No need to scrape or scrub the heat sink, just need to make sure the surfaces are flat and rid of any shrapnel that might be large or sharp enough to damage the insulator. Again don't bother with thermal compound for now. And the main reason for not torquing down the screw more than a foot pound or so is to prevent warping, along with not damaging the insulator.

[vrasp]

Re: Fixing a laboratory waterbath

That thermal compound had no label so I have no idea if it's conductive.

I just got back from the store with some NTE ZnO thermal compound, a few mica (they re thicker than the one provided with the TRIAC), and while I was there I also got some more washers. I love this store - they have everything!

The plastic washer I have works fine, it slides into the screw like you said. The screw didn't touch the TRIAC at all. I think everything was correctly assembled so it has to be something with the thermal paste or a damaged mica.

With all of this, I should be good to go. I'll clean the heatsink of the old thermal compound, scrub it with the scotch-brite pad, and assemble it all.

[eccerr0r]

Re: Fixing a laboratory waterbath

TBH the only thing that makes sense ASSUMING you assembled the TRIAC properly is that your heatsink compound is conductive or you somehow have shrapnel on the screw.

Actually I'd have to say: since you're using an insulator sheet (mica, kapton, or silicone), don't use any heatsink compound at all, especially if you don't know if it's conductive or not. These materials are soft enough to expel air bubbles, just do not overtighten the screw (5-10 inch-lbs is sufficient). Make sure it's cleaned off of all old compound. Clean off any metal debris you can see.

Mica is clear, brittle, and cleaves little shards. Kapton is orange and flexible. Silicone is gray and rubbery.

Again use the sharp points of the multimeter in the highest resistance range to check the dielectric. Make sure your meter is actually working too... Scrape the top side of the metal tab of the TRIAC and check against the screw.

[eccerr0r]

Re: Fixing a laboratory waterbath

new hole is unlikely going to fix the problem.
BTW that plastic shoulder washer... if you're screwing the screw in, the screw is too big. The screw should slide into the plastic.

The screw screws into the heatsink so it will be grounded. The heatsink should have an small air gap or the insulator material between it and the metal tab.