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Using an ADS1115 with PT100
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Using an ADS1115 with PT100

by scotts on Wed Sep 11, 2013 8:11 am

Hi, I'm trying to use a PT100 temperature sensor with my Raspberry Pi along with a ADS1115 ADC because I need a highly accurate temperature reading with good resolution for my nanobrewery. I have a software, not hardware, background so I might be in a little over my head… bear with me.

1. WIRING

The PT100 has three wires, and I assume from trial and error this is how it's wired.

white wire: 3.3v rail, with some resistors in front.
red wire #1: A0 on the ADS1115
red wire #2: GND

I then connect the ADS1115 to the corresponding GPIO pins on the RPI. I've successfully used the Adafruit_ADS1x15 python library with a 10k pot, as well as the PT100 in single ended mode using the above wiring configuration. Or is there a way to use it in differential mode with this PT100, since I've heard this is more accurate?

2. CURRENT

I will need to limit the current going to the PT100 to 1mA according to spec. Using 3.3v, I believe I need to use 3300 ohms of resistance, correct? (10 330-ohm resistors in sequence?)

3. BOOSTING THE SIGNAL

After I limit the PT100 input to 1mA, I'll need to read it on the ADS1115, but since a 1*C temperature change will result in a 0.384 ohm change, I believe I'll need to use an op amp to boost the signal -- the ADS1115 has up to a 16x gain, effectively 0.256v max, so I'll need more. How do I calculate how much more?

Would one such as the TS922IN work?
http://www.adafruit.com/products/808

And how would I connect it to provide the gain I need?


There have been a lot of questions on this forum and others throughout the internet about PT100s, but commenters always recommends switching to thermocouples since they're easy to work with. PT100s are used throughout industry and are commonly available in thermowells and various piping systems, so it seems like there should be at least one layman's guide to hooking them up to a RPI/Adruino. At any rate, I'd like to persevere with the PT100s if anyone can offer some help...

Thanks,
Scott
scotts
 
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Re: Using an ADS1115 with PT100

by adafruit_support_bill on Wed Sep 11, 2013 8:38 am

RTDs can be tricky to work with. For a nice summary of the different RTD configurations and how they are wired, see this link: http://www.thermometricscorp.com/rtd.html (click on the images for more details)

Using differential mode on the ADS1115 will give you better noise immunity. http://learn.adafruit.com/adafruit-4-ch ... onnections

adafruit_support_bill
 
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Re: Using an ADS1115 with PT100

by scotts on Wed Sep 11, 2013 9:13 am

In differential mode, do I leave the PT100 connected as I've described, and then connect an additional wire from the end of the resistor sequence to A1 on the ADS1115, to see the difference between A0 and A1?
scotts
 
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Re: Using an ADS1115 with PT100

by scotts on Wed Sep 11, 2013 9:28 am

Correction, it looks like from this link, I connect the white wire to the power source (with resistors) and the two red wires to A0 and A1.

http://www.thermometricscorp.com/3-wire-rtd.html

The ADS1115 tutorial seems to indicate as much, too. However, the signal is now much too small notice in the readings, which brings me to the next issue: boosting the signal.
scotts
 
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Re: Using an ADS1115 with PT100

by adafruit_support_bill on Wed Sep 11, 2013 9:53 am

If you hunt around a bit, most of the major chip manufacturers have app notes on this sort of thing. Here's one from Microchip.
http://ww1.microchip.com/downloads/en/A ... 01154B.pdf

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Re: Using an ADS1115 with PT100

by GregRR on Thu Feb 05, 2015 6:52 pm

I realize this is an old post, but I'm trying to do the same thing using a BeagleBone Black and am wondering if you got this all working successfully.

GregRR
 
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Re: Using an ADS1115 with PT100

by dhouha on Thu May 14, 2015 12:19 pm

Hi
I'm trying to do the same thing using a Raspberrypi B+ and am wondering if you got this all working successfully.

Thank you

dhouha
 
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