ph soil sensor for s7-1200

[xMilos]

Hi all, student project

I need suggestions for a pH sensor for testing the pH of soil. Using s7-1200 plc. It should be wired to the digital inputs, so i guess i need a sensor with digital output or can i use an analog one with A/D converter.

Thank you

 

[nonuke]

You need a sensor with an analog output connected to the analog input of the S7-1200 (0-10V). The PLC already has analog inputs.

 

[xMilos]

They are occupied, that is why it need to be connected to digital input. And i try not to use additional analog module.

 

[BryanG]

You could look at using a Signal Board (SB) rather than a Signal Module (SM). A single input SB costs a bit less than 1/3 of an SM. It fits on the front of the PLC, you have to remove an existing cover and plug the SB in instead.

edit: of course I could have been really helpful and have given the part number: 6ES7 231-4HA30-0XB0

http://media.automation24.com/Artikelbilder/Shop800px/101663.jpg

 

[scrambled]

I think you're getting a little confused about the meaning of a 'digital input' in the context of PLC's. A 'digital input' is a binary input, meaning it is either on or off. You want to measure pH, which is a numerical value. These are a completely different forms of measurement.

An A/D converter encodes a continuous voltage signal into discrete samples of finite resolution values. In fact, analog input modules should generally have an A/D converter already, as it needs to interprets the voltage level of an input signal as a 'number' for the PLC program to use in its logic. However, an A/D converter won't magically convert a numerical analog value to a binary on-or-off value.

If you want to measure soil pH, it will need to be fed into the PLC as an analog value one way or another.

 

[xMilos]

thank you all for replys,

can anyone suggest an equipment for getting the ph from soil. I found these but don't know if i can connect them.

http://www.atlas-scientific.com/product_pages/probes/ph_probe.html
http://www.aliexpress.com/item/Soil...ration-PLC-of-PH-meter-sensor/1709236786.html
http://www.pasco.com/prodCatalog/PS/PS-2102_pasport-ph-sensor/

 

[scrambled]

First and third choices are lab sensors, so they are probably not suitable for PLC applications. Second one employs a Modbus RS485 interface, which is usable with a PLC. However, I don't think S7 supports the Modbus protocol natively, so you may have to acquire a communications module.

If you want to read pH into a PLC input, you need a pH transmitter that can output either a 4-20mA signal or a 0-5V signal, as those standard signal ranges are what PLC's generally use.

EDIT: Something like this: http://www.omega.com/pptst/PHTX014.html

 

[boneless]

You can look for a PH sensor that will give you alarm outputs, low/high PH level?

You could make your own: https://www.sparkfun.com/products/10972

 

[xMilos]

Thanks again you were very helpful.

I decided to go with Sensorex PHMA pH Transmitter (2-Wire, Blind, Non-Isolated) + S8000 Modular pH sensor platform and additional analog module for the PLC.

 

[BryanG]

Non-Isolated

may well cause issues. One analogue channel may interfer with the readings of the others. If you use a Non-Isolated transmitter you may have to add a loop isolator.