Learn how to perform a 2 point calibration on Memosens pH sensors using the MemoRail Modbus transmitter.

Modbus RTU Protocol

Using Modbus RTU protocol with our MemoRail Modbus transmitter not only allows us to pull the measured values and Memosens diagnostics data, but it also allows us to write data to our sensor. This then allows us to perform a calibration on our connected sensor directly through the control system, HMI, or the device type manager, which we’ll be using today.

Establishing Communication

We’ll start by ensuring that at least one sensor is connected to our MemoRail. Our MemoRail should connect to a 24 volt DC power supply and our Modbus RTU terminals should connect to our PC via an RS-485 to USB adapter. We’ll now connect to our MemoRail by establishing communication to the device through our DTM framework. We’re using PACTware in this example. Once we’ve established communication and it brought up our MemoRail DTM, we’ll begin reading our measured value registers and ensure there are no Modbus channel errors. If you’re utilizing a system other than a PC with the DTM you can find these read registers in the Modbus Command Specification file.

Calibration Process

It’s now time to calibrate our pH sensor. First ensure that you rinse your sensor with distilled water and then dab it dry. For this 2 point calibration we will use pH buffer 7 and 4. Let’s begin by placing our pH sensor in buffer 7 and stirring lightly. As always, we want to ensure that the entire pH bulb as well as the sensor’s junction are completely immersed.

We can see on our DTM screen that our measured value is changing. Please note that, we want to compensate for the temperature value of our buffer solution. This should be done by monitoring the measured temperature value and cross-checking it with the table provided with your buffer solution. We’ll now enter the temperature compensated value of the first buffer solution in register 2528 and hit enter to continue. Once we have entered the proper value of our first buffer solution, we will write the value by using action code 2004 in register 800 and pressing enter.

We’ll now rinse our sensor again, dab it dry, and place it in our second buffer solution. Stir lightly and let your measured value stabilize. Now write the temperature compensated value of the second buffer in register 2540 and press Enter.

Final Steps

We will finalize our calibration by writing action code 2005 in register 800 and monitor our measured values. We can now click read under the sensor adjustment tab in the DTM to view our calibration status. This concludes our 2 point calibration of our Memosens pH sensor using MemoRail Modbus. Note that you can always conduct calibrations offline using the Knick Portavo series portable meter. We’re now ready to reinstall our sensor and go online with our measurement.

[vc_row][vc_column width=”1/1″][vc_widget_sidebar sidebar_id=”ups-sidebar-1704″][/vc_column][/vc_row]