I use 18B20's on my solar panels. There have been a couple of occasions when the pump has failed and the panels and sensors have reached 127C and 3 of the 4 still work OK, the 4th works most of the time, don't know if it was the high temp that caused the problem with that, it certainly didn't happen at the same time.
One other consideration is the cable you use. Standard cable is only rated to about 90C. I bought some high temperature 3 core flex for mine, still only rated to about 110C but I though it was worth using as the sensors are up on the roof and I really don't want to have to get up there again. Cheers Mick On 02/10/14 06:38, Jerry Scharf wrote: > Hi, > > Thanks for all the input. I am guessing my experience with slow reads is > due to the long buses I have with my current setup. > > The bus will be very short because all the load resistors are on a rack > with water cooled heat sinks. I can make the main power and ground lines > any gauge I want, and the data line will be <20 feet. A couple amps is > easy to push around. The completed system will be sucking over 20kW of > power and dumping it into the load resistors. > > As for temperature change, when you dump 65W into a resistor that is has > a surface area of about 15mm square, the heat can change pretty quickly. > I was hoping to poll all the sensors at least every 30 seconds. I need > to measure both the resistors that exceed a threshold and the difference > between the coldest and hottest part. I expect the parts to be running > at 90-110C, so I will be able to use almost the full range of the chip. > There will be relays that can cut the power to any resistor that starts > to run away, so I just need to catch it in time. (carbon resistors > resistance drops with increased temperature, so as they start to get > hot, the resistance drops and the power through it goes up...) > > I am looking at using the Z version (SO) mounted to a board and the load > resistors bent over backwards. The board will have a hole that allows a > bolt to go through and bolt the board, 18b20 and TO247 load resistor to > the aluminum extrusion with two pipes of cooling water built in. This > will give a relatively accurate measure of the resistor temperature. > > Trying to handle over 300 thermocouples becomes a real chore. The only > risk I see is that we start having the load resistors getting well past > 125C and cooking the 18b20s. > > I promise to send pictures when we are done. > > jerry > > On 10/01/2014 03:52 PM, Roberto Spadim wrote: >> if it's not to control a process, i think could use without problems, >> like paul told, temperature don't change too fast, only if you are >> controlling something, in this case should check system update rate >> using modbus devices i got ~20ms to read 8 termocouples with rs485 >> (serial) 9600bauds >> i think it's a good number to think about, 60~50Hz update rate is very >> good to control systems using temperature termocouples >> >> 2014-10-01 19:01 GMT-03:00 Paul Alfille <paul.alfi...@gmail.com>: >>> I've done over 100 DS18S20's (passive) on passive, active, Link and DS9490 >>> successfully. It isn't fast, but then your temperature changes aren't that >>> fast. >>> >>> On Wed, Oct 1, 2014 at 4:53 PM, Roberto Spadim <robe...@spadim.com.br> >>> wrote: >>>> that what i stored here some years ago when testing ds1820 devices >>>> in other words 300*.01 = 3 seconds >>>> maybe less maybe more >>>> >>>> what update period you need? >>>> >>>> >>>> ------------------------------------------------------------------------------ >>>> Meet PCI DSS 3.0 Compliance Requirements with EventLog Analyzer >>>> Achieve PCI DSS 3.0 Compliant Status with Out-of-the-box PCI DSS Reports >>>> Are you Audit-Ready for PCI DSS 3.0 Compliance? 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