8/3/2009 11:05 AM | |
Posts: 173 Rating: (95) |
Hi n3quik .. I think you might get your fingers burnt! 1) A Pt100 sensor is actually a resistor which is 100.0 ohms at 0.01°C. The resistance of the platinum filament (very fine wire spiral) inside the sensor changes resistance quite linearly over the usable temperature range -50...+200°C (I do not know what the min & max resistance values will be, but I am sure it can be looked up on the internet ) 2) Because the AM2 PT100 from Siemens tried to 'balance' a bridge circuit using this Pt100 as one of the 'legs' of the bridge, any changes in resistance have a big effect on the value obtained. 3) That is why the PT100 module offers the option of connecting as a 2 wire (not so good) or a 3 wire connection (better) so that the effect of the resistance generated in the wires going to the sesning element (the platinum filament) canbe determined and subtracted from the resistance value used in balancing the bridge circuit. It does this by passing a known current down the 2 parallel legs and determining the resistance - or somehting like that. It was way inthe past that I last explained this sort of thing, so I am a bit cloudy 4) If you use the Pt100 sensor as you described, directly connected to a 0..10 V analog input on a LOGO! 0BA5, together with a voltage divider with other resistors, then I think that you have to be VERY careful with the size of the resistors chosen, thier TOLERANCE (temperature stability) and the power that they disipate (in Watts). 5) Ordinary 1/2 or 1/4 W carbon resistors typically have a tolerance of 5-10%, meaning that their value can drift by 5-10% depending on thier temperature (which rises as current flows through them!) So a 560 ohm resistor could 'drift' by up to ±56 ohms in either direction. This would wipe-out or drown-out the resistance change in the Pt100 that is sensing the process temperature that you want. Better to find HIGH quality resistors (see attached) where you can select the best value and know that it will be stable. Typically available from most good quality catalog sellers like RS-Components 6) You can always correct for the range of values that you want by doing the necessary scaling in the LOGO! program - a great idea is to use the excel spreadsheet generated by Betel to calculate the necessary gain and offset values and even how to cascade analog amplifiers to get the exact range you want. 7) A good reference point to test your sensor would be to make up an ice-slush which will be pretty close to 0.01°C. put the Pt100 in there, keep the other resistors constant and check the value you get in LOGO! Soft Comfort. This will be your 'zero' point and you can calibrate better from this known value. Hope this helps! Cheers Aaaaarrggh! Attachment500ohmVeryHighPrecision.pdf (329 Downloads) |
8/26/2014 7:52 AM | |
Joined: 9/1/2005 Last visit: 10/4/2024 Posts: 4105 Rating: (195) |
new question published by rajinderis split to a seperate thread with the subjectPT100 question - connect it with logo 0ba7 12/24 dc direct. Best regards CS Moderator |
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