Pt100 - esp8266 - max31865

Need Help With My Project
1

Friends, I describe the following problem. In a company I am wanting to optimize the temperature control of the processes. All controls are done with 3-wire PT100 sensors with 4-20ma RTD and PLC. My idea is to use a Nodemcu ESP8266 and a MAX31865 interface to read the PT100 values, since before I used a resistor interpolating values in the code but I did not have such exact measurements. I want to outline here everything I am doing and the values it gives me to help me since I think I am not making it work correctly.
In principle I have the board soldered in the following way:
2/3 WIRE SOLDERED
2 WIRE UNJOINED
24 3 JOINED FROM THE MIDDLE TO THE 3

Then I make the connections like the example code:

CS — D0
DI — D1
DO — D2
CLK — D3
VIN — VIN
GND — GND
RDY — NO CONNECT

Searching the forum and the internet I am trying with the example code of the library


#include <Adafruit_MAX31865.h>

// Use software SPI: CS, DI, DO, CLK
Adafruit_MAX31865 thermo = Adafruit_MAX31865(D0, D1, D2, D3);
// use hardware SPI, just pass in the CS pin
//Adafruit_MAX31865 thermo = Adafruit_MAX31865(10);

// The value of the Rref resistor. Use 430.0 for PT100 and 4300.0 for PT1000
#define RREF      430.0
// The 'nominal' 0-degrees-C resistance of the sensor
// 100.0 for PT100, 1000.0 for PT1000
#define RNOMINAL  100.0

void setup() {
  Serial.begin(115200);
  Serial.println("Adafruit MAX31865 PT100 Sensor Test!");

  thermo.begin(MAX31865_3WIRE);  // set to 2WIRE or 4WIRE as necessary
}


void loop() {
  uint16_t rtd = thermo.readRTD();

  Serial.print("RTD value: "); Serial.println(rtd);
  float ratio = rtd;
  ratio /= 32768;
  Serial.print("Ratio = "); Serial.println(ratio,8);
  Serial.print("Resistance = "); Serial.println(RREF*ratio,8);
  Serial.print("Temperature = "); Serial.println(thermo.temperature(RNOMINAL, RREF));

  // Check and print any faults
  uint8_t fault = thermo.readFault();
  if (fault) {
    Serial.print("Fault 0x"); Serial.println(fault, HEX);
    if (fault & MAX31865_FAULT_HIGHTHRESH) {
      Serial.println("RTD High Threshold"); 
    }
    if (fault & MAX31865_FAULT_LOWTHRESH) {
      Serial.println("RTD Low Threshold"); 
    }
    if (fault & MAX31865_FAULT_REFINLOW) {
      Serial.println("REFIN- > 0.85 x Bias"); 
    }
    if (fault & MAX31865_FAULT_REFINHIGH) {
      Serial.println("REFIN- < 0.85 x Bias - FORCE- open"); 
    }
    if (fault & MAX31865_FAULT_RTDINLOW) {
      Serial.println("RTDIN- < 0.85 x Bias - FORCE- open"); 
    }
    if (fault & MAX31865_FAULT_OVUV) {
      Serial.println("Under/Over voltage"); 
    }
    thermo.clearFault();
  }
  Serial.println();
  delay(1000);
}

Lastly and with the least knowledge but searching on the web I found that to connect a 3-wire RTD sensor, I must identify the two wires that are directly connected using a multimeter and connect them to the blocks labeled F+ and RTD+. The third wire, which is connected to the RTD, goes to the left, in the blocks marked F- or RTD-.
The values with this connection are:

RTD VALUE = 0
Ratio = 0.00000000
Resistence = 0.00000000
Temperature = -242.02

Now if I don’t connect the PT100 to the MAX31865 the values are the same.

Getting back to it… I just checked the soldering, the GND was poorly soldered on the MAX31865, now when I connect everything it gives me these values, but without connecting the PT100:
RTD VALUE = 14563
RATIO = 0.444442749
RESISTENCE = 191.10382080
TEMPERATURE = 241.77
Fault 0x18
REFIN- < 0.85 X BIAS – FORCE - OPEN
RTDIN- < 0.85 X BIAS – FORCE – OPEN
What am I doing wrong?
The Esp8266 doesn’t work well with the MAX31865? Should I use the ESP32?

I look forward to your suggestions, thanks.

2

I don’t understand that comment.

This seems like a strange choice of pins.

Pete.

3

PLC Programable Logic Controller
RTD Resistance Temperature Detector
See: RTD PLC Info

4

It’s the 4-20ma but that makes no sense to me.

Pete.

5

Perhaps the user means 4.20ma sensor:

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