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Forum Index : Microcontroller and PC projects : DS18B20 Error
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| Phil23 Guru  Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
Seem to have one of my Spa sensors returning errors at the moment. Chicken or the egg scenario. Came back from a few days away to find 3 of the pipes in the solar panels literally melted, posted pics here. Temp must have been well in excess of 100°C to achieve that. But on replacing the pipes the output sensor is coincidently reading wrong. Few times I saw valid temps, but it spends a lot of time at either -0.1°C or 85.0°C. 85 is 0000 0101 0101 0000 from the sensor, not sure about the -0.1. Wondering if it's just a fluke or whether one lead to another. Presume the panels could have generated superheated steam in the right conditions with no flow & maybe that cooked the sensor, which is a fair pipe run away, or the sensor stopped the flow & let the panel overheat. Any thoughts? Do DS18B20's fail much or go out of spec? Phil. |
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| DaveC166 Regular Member  Joined: 13/09/2012 Location: New ZealandPosts: 44 |
Hi Phill, Ive just replaced a failed DS18B20 used to monitor the Duct temp of my Umite MK2 controlled ventilation system. The data pin had separated from the DS18B20 body.Most likely from the vibration of 5 years of air flow plus not actualy heat shrinking the heatshrink, plus the Hydrogen sulfide gas abundant in Rotorua NZ. With regard to your varying temp readings I found a quirk with Umites, in that a UMITE MK2(VER 4.7) gave wildly varying readings from a DS18B20 running at 10MHz clock. Rock solid readings at 20Mhz and above. Another problem ive found is that high temperatures affect the cable performance, resulting in either high or low temps being displayed. A 2.4k ohm or 2.7k ohm pullup resistor solved all problems |
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| robert.rozee Guru Joined: 31/12/2012 Location: New ZealandPosts: 2289 |
i believe there is an upper temperature limit for the DS18B20 in 2-wire (parasitic power) configuration. the datasheet i have here states 100 degrees C, but i am sure that i have read elsewhere of a somewhat lower limit, somewhere in the mid 80's. from the maxim datasheet: "The use of parasite power is not recommended for temperatures above +100°C since the DS18B20 may not be able to sustain communications due to the higher leakage currents that can exist at these temperatures. For applications in which such temperatures are likely, it is strongly recommended that the DS18B20 be powered by an external power supply." the abovementioned leakage is referring to normal leakage within the DS18B20 device itself, but my guess would be that the same issue may well apply to cables in a damp environment over extended periods of time. in any 'mission critical' application it is a far better bet to use the 3-wire configuration (pin 3 connected to the micromite's 3v3 supply), as well as having some sort of backup sensor on the system and doing sanity checks on all readings. cheers, rob :-) |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
Ah, Yeah.... There's a lot I need to polish up yet.... Still have a couple of manual interventions required, & code is far from complete. Need to move it all to one of the CGMicro MM+'s I have waiting; all IO pins in use on the 28 pinner, hence the 3 remaining relays that can not be controlled. Phil Phil |
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