I wonder how much the elevated temperature might be effecting the load cell. It is a bit away from the heated area and has quite a bit of mass between it and the heat. Can comfortably touch the coupling to it, but it’s definitely not ambient.
How bad is it??
- Thread starter no704
- Start date
The load cell spec/cal sheet should have a range listed.
If you can touch it it's not that hot.
Are you doing this experiment/test professionally?
8x115w heaters in a 2”x3.375 block of Al with a 28mm bore thru. I’m really pleased with its performance, had planned on having to insulate it and wait longer. Testing laser welded joints on t2 titanium capsules. The ER16 collets holding the samples are still working but not happy about it!
If by professionally you mean not just as a hobby and actually being paid then yes. We tend to do things that really haven’t been done before and sometimes have to come up with ways to test for the most extreme conditions we are possibly going to see. These capsules will be inserted into a Nucler reactor core so we like to have our Is crossed and Ts dotted. Data collected from this test will and is being scrutinized by a team of engineers.
Sorry, but it is concerning that such haphazard work is allowed when the company is working with radioisotopes.
You should know how to read a cal sheet, what the effects of temperature may be or know how to wire a test device safely.
You should know how to read a cal sheet, what the effects of temperature may be or know how to wire a test device safely.
walta
Well-known member
The way I see it the first thing the black wire of the line cord connects to must be a fuse or circuit breaker sized at 125% of your peak amp draw for over current protection. Hell, no you cannot count on the breaker in the panel to provide this protection.
Second your attitude about grounding is cavalier it is a mater of life and death. The last thing you want to do is build the machine that kills someone and it may well be you. Put a green wire on each one of the heater blocks and the frame of that machine.
This should work for the thermal fuse but keep the current thru the fuse under 7 amp. One fuse for every 2 heaters?
LR4-900F LITTELFUSE - Fuse: PTC polymer | 9A; RF1534-000 | TME - Electronic components India
LITTELFUSE LR4-900F | Fuse: PTC polymer; 9A - This product is available in Transfer Multisort Elektronik. Check out our wide range of products.
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Walta
rlitman
Well-known member
Mostly agree, but I'm not sure about why you suggested a PTC fuse. Not saying it's wrong, I'm just not understanding your thought process.The way I see it the first thing the black wire of the line cord connects to must be a fuse or circuit breaker sized at 125% of your peak amp draw for over current protection. Hell, no you cannot count on the breaker in the panel to provide this protection.
Second your attitude about grounding is cavalier it is a mater of life and death. The last thing you want to do is build the machine that kills someone and it may well be you. Put a green wire on each one of the heater blocks and the frame of that machine.
This should work for the thermal fuse but keep the current thru the fuse under 7 amp. One fuse for every 2 heaters?
LR4-900F LITTELFUSE - Fuse: PTC polymer | 9A; RF1534-000 | TME - Electronic components India
LITTELFUSE LR4-900F | Fuse: PTC polymer; 9A - This product is available in Transfer Multisort Elektronik. Check out our wide range of products.
Walta
Here are the concerns I have:
1) Grounding. Plugging this into a GFCI would go a long way to making it safer, but so would grounding each of those two blocks. That's not particularly easy, because of how hot the blocks will get. I would bolt on a length of TGGT (high temperature, teflon/fiberglass jacketed "appliance wire") crimped in nickel-plated steel high-temperature ring terminals (NOT copper or brass) to ground them (the TGGT needs to be long enough to get your wire out of the heat, and you can use more normal wire from there). The ground wire should be sized no smaller than the largest conductor (16AWG?).
2) Isolation. Those heaters should be completely isolated from ground. They are supposed to have a siliconed fiberglass sleeve that covers the otherwise bare nichrome wire where it connects to the pigtail. If that insulating sleeve gets pulled up, or if there was an error in the manufacturing, you can short to the case. There was nothing wrong with burying the heaters in the thermal block, but clearly one of the heaters had either been made wrong, was damaged, or failed (any one of these is possible). If you had tested for resistance between each lead and the case, you could have identified the bad one before it burned up, but keep in mind that this is still a potential future failure mode for the rest of them, which is why grounding is SO important.
3) OCPD. I count eight 1A heaters, so this could be fused at 10A. Without this, the grounding doesn't help enough (though it would help when a GFCI is present). Your enclosure has plenty of room for a pop-up breaker like this:
Though the push button pop up breakers are generally thermal only, and a thermal-magnetic should open faster on a dead short. I think you could find space for a DIN rail breaker in there, but then you'd need to remove the covers when it trips.
4) Thermal runaway. I hear that this will only be used under supervision, but you still only have one sensor, so when it starts reporting lower temperatures than reality, the block will get hotter and hotter until the heaters are on full power continuously. The question I have is, will something be damaged by that? What thermal equilibrium will those blocks reach when you're continuously pumping 1250 watts of heat into them, and will it be hot enough to damage something?
5) Secondary monitoring. I hope you're not using an infrared thermometer or thermal imager to monitor the external temperature of those heat blocks, because the emissivity of aluminum is very low. If you do want to use that as a backup source of temperature, put on a piece of kapton tape. That's got an emissivity of 0.95.
Well I fixed a couple of outlets and an extension cord today. Got my cord strain reliefs and a 15A breaker today, so I was able to complete the enclosure. Entire unit was pulling 11A according to my clamp meter. Heater blocks currently have a ground on one of the clamp feet, screwed into the block. I’ll add another to the other side. I think the silicone boots on the heater leads might have been getting pulled up when they were recessed into the Al block, pulled the heaters out of the block a little. There is a little notch on the side of the blocks that allows a secondary thermocouple to pass thru that is hooked onto the test part that is independently monitored.
I have not done a runaway test, while not a bad idea, I fear that would most likely end up frying the heaters. Might try that with one heater in a test block.
I have not done a runaway test, while not a bad idea, I fear that would most likely end up frying the heaters. Might try that with one heater in a test block.
Thank you for your concern. I can garentee that safety is all ways our highest concern. That being said please understand that this is still a test that is under development, and what you have seen on this thread was/is still very much a proof of concept and not a finished product.