Temperature probe failure analysis - Page 1
Forum section: electronics / projects

author: dfmischler
author_rank: Frequent Contributor
timestamp: August 15, 2013, 01:25:58 pm
content: I have someDS18B20's potted into316 stainless steel tubes (closed end)with about three meters of outdoor grade category 5e cable. These are measuring seedling root zone temperatures in a greenhouse. One of them started reporting "Parasite Power" which implies that the sensor is no longer seeing power on the VDD line. I disconnected the sensor and its properly working neighbor and hooked them back up, and saw no change in their status. So I replaced the apparently failing sensor and now I would like to know if the cable failed (quite possible: solid conductors suck for something like this), or if the solder joints or DS18B20 failed in some way.The new sensor is working fine, implying that the cable leading to the final terminal strip and the screw terminals themselves are OK. I hooked the old sensor up to a 1-wire bus just for testing purposes and it works as expected; when I ground the VDD wire the sensor reports "Parasite Power" and when it is tied to 5V it works normally (draws about .76 mA during conversion). I have tried flexing the cable about every inch (2.5 cm) while testing this and haven't seen a problem. I also tried a tone generator and probe on the cable while flexing and couldn't find an intermittent break that way.I tried heating the probe to about 121F (50C) with a heat gun to see if that would have an impact but I didn't see anything odd. Leaving the sensors exposed to sunlight out in the greenhouse got them to about 115F (46C) yesterday so I couldn't see going any higher.Any further thoughts about this kind of thing? Do I attribute all this to a bit of dirt and assume that the sensor assembly could be reused? Or do I toss it assuming it has some cryptic problem? Any ideas about isolating the problem further? My goal in isolating the problem is to improve the probe construction if needed.

author: JackOfVA
author_rank: Unknown
timestamp: August 15, 2013, 08:23:59 pm
content: Maybe moisture related? What happens when you submerge the probe end with the sensor in a bucket of water?

author: dfmischler
author_rank: Frequent Contributor
timestamp: August 15, 2013, 09:41:17 pm
content: Sometime within the last two hours the sensor assembly failed to the point that it no longer reports "Parasite Power" if I ground the VDD line. When connected to 5V no current registers on the VDD line on the uA meter during conversion. So I assume the wire no longer makes it all the way to the chip in the DS18B20. It is still reading temperatures and it is unclear why it isn't reporting that it is on parasite power (I guess the DS18B20 isn't sure unless the VDD line is tied to ground).I threw the probe and a few inches of cable in the water anyway and it is still reading temperatures.

author: dfmischler
author_rank: Frequent Contributor
timestamp: August 15, 2013, 09:57:16 pm
content: I pulled it out of the water and cut off all but around 3 inches (7.6 cm) of the cable. The cut off piece of cable rings out OK with my DMM. I removed all of the cable jacket except where the epoxy is holding it, and it appears OK outside the SS tube, and I get the same failure where it just doesn't see the VDD line. So I'm assuming the problem is inside the tube. I know I can get rid of the epoxy with heat, but I suspect the solder will melt before the stuff holding the DS18B20 in the probe tip (rated for 500F or 260C).

author: JackOfVA
author_rank: Unknown
timestamp: August 16, 2013, 01:19:13 am
content: It's been a few years since I tinkered with the DS18B chips (interfaced to a PIC microcontroller) and at that time my one-wire bus was reasonably short - a few feet. Can't recall any generic problems I ran into with it, once the read and write commands were debugged.Sort of sounds like a high resistance connection in the wire to sensor chip, enough current can make it through to perform some functions but not enough to power the chip to the conversion routine that, if I recall correctly, consumes more current than other functions.To remove the epoxy without heat you are left with using a fairly nasty solvent or mechanically removing it. File away the tubing (assuming it's metal) and then cut or scrape the epoxy off to expose the wire or sensor. Or if the budget permits, have it X-rayed.There might be some off-the-wall failure mechanism if you have water inside a long tube such that the CAT-V cable is in a waterlogged environment. For one thing, it will increase the wire to wire capacitance and second, I'm not sure if the wire insulation is impermeable to standing water. Some CAT-V cable is made with a waterproof jacket, of course, but I don't know if the individual pair insulation will withstand long term immersion.One thing I've run into with some of my duplex powered products (DC power to the remove amplifier is carried over coaxial cable as well as the returned RF signal) is that it only takes a tiny bit of water to cause electrolytic corrosion. The DC power will rather quickly convert copper into a brownish green paste that is partially conducting. I've even seen where the wire insulation appears intact, as the water enters through a pinhole and moves via capillary action. The electrolytic action can happen very quickly if sufficient current is available and the sludge is a partial conductor. In the case of the 1-wire parasitic interface you are limited to a mA or so through the pull-up resistor so it won't happen as fast as what I've seen with several hundred mA available at 20V DC.

author: dfmischler
author_rank: Frequent Contributor
timestamp: August 16, 2013, 02:51:28 pm
content: I filed a window in the side of the probe tube and here is the image. It still works to the same level at this point, i.e. 1-wire protocol and temperature conversions work but the VDD line isn't working and the DS18B20 isn't reporting that it is on parasite power. It was not fun filing the 316 stainless tube because it is pretty hard, but I was afraid that I would do too much heat and mechanical damage with an angle grinder.blue & orange1-wire datawhite/blue & white/orange1-wire groundgreenVDD (5V)The black material is the body of the DS18B20. The gray stuff is JB Weld and the almost clear stuff is Devcon 5 minute epoxy. I am surprised how far from the tip of the probe the sensor wound up, and the odd voids. Maybe I should be thinning the JB Weld so it runs during assembly. I believe the exposed copper on the orange wire happened while filing the probe tube. I see no obvious signs of water damage.I'm going to see what I have for solvents.

author: Niklas
author_rank: Frequent Contributor
timestamp: August 16, 2013, 06:01:34 pm
content: Have you used this product?http://www.jbweld.com/product/j-b-weld/If so, the datasheet:http://www.jbweld.com/wp-content/uploads/2012/08/J-B-Weld-MSDS.pdfmight give some hint in the listing of ingredients. Iron...At my former work we potted a similar sensor device inside a stainless steel tube using 3M Scotch-Weld DP190 epoxy.http://multimedia.3m.com/mws/mediawebserver?66666UuZjcFSLXTtnxfclXs6EVuQEcuZgVs6EVs6E666666--

author: nctnico
author_rank: Super Contributor
timestamp: August 16, 2013, 06:20:44 pm
content: Using epoxy as a potting compound is not a good idea. Epoxy shrinks so it can cause mechanical damage. PU based resins don't shrink.

author: dfmischler
author_rank: Frequent Contributor
timestamp: August 16, 2013, 06:40:09 pm
content: I had a paint stripper with 40% methylene chloride plus acetone, methanol and toluene and gooped it on there. After two hours it softened less than 1/2 mm of the 5 minute epoxy. It seemed to have no effect on the JB Weld. So I tried heating the stuff with an old 45 watt soldering iron. 5 minutes of that just got me frustrated. So I tried to gently heat it with a propane torch. The result isn't pretty, but all of the wires were still attached to the chip leads, one of which is no longer attached to the chip (but that's not the VDD lead that had failed). And the DMM says that the VDD lead is continuous to the remains of the cable. So at this point I'm assuming the failure was inside the DS18B20 itself.However, I'm really rethinking the use of the 5 minute epoxy to insulate the solder connections before potting the sensor into the tube with the JB Weld. The JB Weld is made to fasten steel, and has a similar coefficient of thermal expansion due to the iron powder suspended in it (don't worry it is still an insulator, though I haven't tried to measure its capacitance or inductance). So I think it should cause less mechanical stress than the 5 minute epoxy.I also should probably put something for TVS closer to these particular sensors. I think the nearest transient suppression diodes are over 70 feet (21 meters) away in cable distance and the inside of a greenhouse is practically outside for lightning issues, etc.Quote from: Niklas on August 16, 2013, 06:01:34 pmYeah, I know the 3M products will do exactly what 3M claims they will. But the properties of JB Weld should be good enough for my application and the low quantities I need to make. And I can get it at the local hardware store. The sensor assembly that failed was in service for about 20 months before failing, and it was apparently a failure of the actual sensor IC rather than my assembly.Thanks for the help, or any additional comments.

author: dfmischler
author_rank: Frequent Contributor
timestamp: August 16, 2013, 06:50:34 pm
content: Quote from: nctnico on August 16, 2013, 06:20:44 pmGood to know. Thanks.