Design and Application of a Virtual Ion Meter Based on Graphical Programming Software

The ion meter is one of the most widely used analytical instruments in scientific research and routine laboratory practice in the biomedical, chemical, and environmental fields.

In general, the data from a traditional ion meter are obtained manually or displayed on a recorder. The analysis and processing of testing data obtained using a traditional ion meter can only be done manually, making it difficult to avoid errors.

Virtual instruments (VIs) expand upon and improve the functions of traditional systems. VIs offer real-time data acquisition, data storage, data display, data processing, and remote monitoring and control of test control sites for the Internet, in addition to allowing the use of a single computer for multitasking.1,2 LabVIEW graphical programming software (National Instruments, Austin, TX), which is widely used for instrument control, automated testing, and data processing, is an effective platform for VI development. It offers a user-friendly interface; an easily visible virtual instrument panel; and a flow programming mode, which significantly enhances the efficiency of VI development. The software is particularly well-suited for instrument measurement and control.3,4

Investigations focusing on ion meters connected to computers have been reported in recent years.5 However, these systems are rare, and the software was developed mainly in text-based programming language. Text-based language has the disadvantage of long development periods and difficulty in maintaining and improving upon the systems. Performance data on a virtual ion meter based on LabVIEW have not yet been reported. The virtual ion meter developed by the authors is based on LabVIEW 8.0 graphical programming language, which provides automatic slope, temperature, and positioning calibrations, in addition to automatic data acquisition, data storage, data processing, and results display, among other functions. Compared to ion meters connected to computers, it not only has the advantages of simple design, easy expandability, friendly user interface, and ease of operation, but also achieves remote, real-time monitoring and control of measurements, and uses a single computer for multitasking.

Structure of testing system

Figure 1 - System diagram.

A diagram of the test system is shown in Figure 1. The system hardware handles such functions as the conversion of nonelectrical energy for ion concentrations in solution, filtering and acquisition of the analog signal, A/D conversion, and results display. As the central part of VI, the software drives the DAQ board, displays data in real time, controls the design of the VI panel, provides analysis and processing of data, and generates the results report.

Electrode system

The ion selective electrode and the reference electrode are immersed in the test solution. The potential generated by the electrode is fed to the computer via the DAQ board after impedance transformation and filtering via the high-input impedance conditioning circuit.

Conditioning circuit

The laboratory-built conditioning circuit is made up of one high-input impedance voltage follower with CA3140 (Radio Corp. of America, New York, NY) and second-order active low-pass filter with low-drift OP07 (Analog Devices, Norwood, MA). Specifically, the CA3140 is used to achieve impedance transformation of the electrode system, and the low-drift OP07 provides the output signal filtering.

DAQ board

The PCI2005 DAQ board (Beijing Art Science and Technology Development Corp. Ltd., Beijing, China) is a 16-bit A/D converter based on a PCI bus. It acquires data through 16 single channels or eight double channels. It has an input resistance of 100 MΩ, sampling frequency of 200 kHz, and analog input of ±10 V. By inserting the DAQ board into the built-in PCI plug of the computer and activating the corresponding driving program, real-time data acquisition is obtained via LabVIEW programming. For the other type of DAQ board, such as the PCI 2008, data acquisition is done the same way using a corresponding driving program.

Digital thermometer

The SWC-IID precise digital thermometer (Nanjing Sangli Electronic Equipment Factory, Nanjing, China) provides output of the direct digital signal. It connects to the computer via an RS232 serial port and inputs the temperature signal into the computer to perform automatic temperature compensation. Digital thermometers that offer direct signal output can be used as well.

Computer

The computer has an available PCI plug and at least 256 MB memory, 220 MB hard disk, 486/DX CPU, and 600 × 800 resolution.

LabVIEW 8.0 program

In accordance with the practical application of the ion meter and configuration of the VI developed by the authors, the VI tracks and acquires the potential signal from the electrode system. Activating the driving program to perform the A/D conversion and changing the voltage signal to a digital pX value, the realtime value can be expressed by programming LabVIEW. For different applications, the real-time display and the method of data processing vary. Thus, LabVIEW VI includes a common program comprised of a DAQ board driver, a real-time data display, and LabVIEW data processing and analysis program that performs the corresponding testing.

DAQ board driver

Because the testing system uses a DAQ board that is not a National Instruments product, LabVIEW cannot initiate the driving program automatically, but rather needs to activate the Function>Advanced>>CLF (call library function) node6 to perform data acquisition. The driving program is based on a Stacked Sequence Structure consisting of seven subprograms. The subprograms independently perform DAQ board parameter assignment, equipment initialization, A/D equipment activation, results of A/D conversion, A/D equipment termination, A/D release, and turning off of the equipment. Details of the driving program are not provided here.