hello everyone /

I want to transfer the reading of a pt100 into voltage so i can read it from an analog input , I used a transmitter this one

https://www.amazon.com/Uxcell-4-20mA-Temperature-Sensor-Transmitter/dp/B00GN6X25U/ref=sr_1_fkmr0_1?s=industrial&ie=UTF8&qid=1486983398&sr=8-1-fkmr0&keywords=PT100+Temperature+Transmitter+Sensor+-50+~++150C+OUT+4-20mA+Power+24VDC


but there is no change in amps or voltage in the output of this transmitter ?

what should i do in this case , or is there a better way to change the reading of the resistance of pt100 into voltage and then into temperature ?



thank you in advance

 

How do you have it connected?

 

How do you have it connected?

 

the analog input i use reads voltage

 

The earth end of the 100Ω resistor should connect to both the negative of the 24V supply and the ground of whatever you are using to read the voltage across the resistor. You could check the voltage across the resistor with a multimeter without connecting your proposed measuring system as a check.

 

The earth end of the 100Ω resistor should connect to both the negative of the 24V supply and the ground of whatever you are using to read the voltage across the resistor. You could check the voltage across the resistor with a multimeter without connecting your proposed measuring system as a check.

we tried it , we did not get the right reading its still stable when we put it in cold or hot water? what do you think is wrong ?

 

What reading did you get?

 

What reading did you get?

 

3.39V across 100Ω indicates a loop current of 33.9mA which is out of range for a 4-20mA loop. This might indicate an open circuit or not connected PT100.

 

3.39V across 100Ω indicates a loop current of 33.9mA which is out of range for a 4-20mA loop. This might indicate an open circuit or not connected PT100.

If i test the reading of the resistance the pt100 itself i get a reading of the right amount of resistance , so do you think there is something wrong with the transmitter maybe its broken ?

 

Hello,

You will need an extra piece of wire:



Bertus

 

Hello,

You will need an extra piece of wire:

View attachment 120907

Bertus

the sensor i use is 2 wire sensor , so what should i connect in these ports?

 

Hello,

The third connection is to sense the wire with a 3 wire sensor.
When a two wire sensor is used, just short the left connector to the middle one with a piece of wire, as indicated on the ports.

Bertus

 

Hello,

The third connection is to sense the wire with a 3 wire sensor.
When a two wire sensor is used, just short the left connector to the middle one with a piece of wire, as indicated on the ports.

Bertus

yes it finally worked i see difference when in the resistor of 100ohm , thanks a lot . one more question do you think this code is correct because i don't get right reading when i transfer the voltage into temperature ?


void setup() {
// put your setup code here, to run once:
pinMode(A0,INPUT);
Serial.begin(9600);
}

void loop() {
// put your main code here, to run repeatedly:
float v=analogRead(A0);
float Rt=(100*(1024-v))/v;
Serial.print(Rt);
Serial.print(" - ");
float R0=100;
float A=3.9083E-3;
float B=-5.775E-7;
float t = (-R0*A +sqrt(R0*R0*A*A - 4*R0*B*(R0 - Rt)))/(2*R0*B);
Serial.println(t);
delay(1000);
}

 

I know that

at 0 degree = 100 ohm

for every 10 degree we add 3.8ohm

 

But your microcontroller is not reading ohms, it is reading volts, that are caused by the current flowing from the converter making a voltage drop on your sense resistor.
That converter seems to convert -50C to 4ma and +150C to 20mA, but that is not certain. You should calibrate it and find out the real constants.

 

Hello,

Linearisation is likely done in the transmitter.
There are two calibration scews (potentiometers) on the transmitter.
The following picture shows wich screw has the function of offset (o) and gain:



You will need two known temperatures to calibrate.
Usualy melting ice (0 °C) and boiling water (100 °C) are used.

Bertus

 

Hello,

Linearisation is likely done in the transmitter.
There are two calibration scews (potentiometers) on the transmitter.
The following picture shows wich screw has the function of offset (o) and gain:

View attachment 120912

You will need two known temperatures to calibrate.
Usualy melting ice (0 °C) and boiling water (100 °C) are used.

Bertus

nice i did not know that

 

I use the same basic devices for doing temperature to voltage conversion for my boiler contorl system. The 4 - 20 mA units are crap for making temperature to voltage converters due to their inherent lower limit 4 mA offset current.

Even with using diodes in series with the output to null the offset voltage the T/V ratio across the load resistor will not be exactly proportionally linear for the rated I/O span of the device so you can never set it to have more than one single point in the whole measurement span that's accurate no matter how you set the gain and span ranges with the two potentiometers on the device.

The correct units for what you want to do are the 0- 5 or 0- 10 volt output type.

These are the units I switched to after giving up on the 4 - 20 ma transmitter based design.
https://www.newegg.com/Product/Product.aspx?Item=9SIA3XT29S3859&ignorebbr=1&nm_mc=KNC-GoogleMKP-PC&cm_mmc=KNC-GoogleMKP-PC-_-pla-_-HI+-+Test+&+Measurement+Tools-_-9SIA3XT29S3859&gclid=CjwKEAiAxKrFBRDm25f60OegtwwSJABgEC-Z5T6Mw4nJl5PYBWncYLSamhiWZvpA2iOadB8lbQroCBoCl6_w_wcB&gclsrc=aw.ds

 

Hello,

You are using a 100 resistor to convert the current to voltage.
This will give you an output range of 0.4 to 2 Volts.
The 0.4 Volts will be at - 50 °C.
The 2 Volts will be at 150 °C.
If you want to calibrate with the melting ice and boiling water, the points to check are:
0 °C at 0.8 Volts and 100 °C at 1.6 Volts.

Bertus