To ensure the desired product yield and quality, tight pH control and monitoring are essential in many food production processes. Standard pH sensors are usually not used in-line for pH determination because they are made of glass, which always carries the risk of breaking. Instead, samples are typically taken, and their pH value is determined off-line in the laboratory or with a portable device.

This approach has several disadvantages: 

  • With off-line measurement, only a single-point result is available, which does not allow for real-time process control. 
  • Sampling and off-line measurement are labor-intensive and costly. 
  • Due to environmental influences; sampling, transport, and processing can cause the pH value to change. For example, the pH value changes with temperature. If a sample is measured in the laboratory at room temperature and the production process is at a higher or lower temperature, different results are obtained (contrary to popular belief, automatic temperature compensation does not consider this effect in pH measurement). 
M800 stainless steel transmitter for pH measurement
M800 stainless steel transmitter for pH measurement
Credit: Article translated and adapted from original written by Dr Klaus-Peter Mang

Alternative technologies to glass sensors, such as enamel electrodes or ion-selective field-effect transistor (ISFET) sensors, have not been able to establish themselves for in-line pH measurement, as enamel electrodes require about an hour to regenerate after each cleaning-in-place (CIP) cycle, and ISFET sensors are not clean-in-place (CIP) resistant. For these reasons, METTLER TOLEDO developed the InPro X1 HLS, which the company states is the world’s first food-safe, in-line pH sensor that withstands cleaning without sacrificing accuracy or sensor lifetime.

Latest technology for pH measurement

The most significant difference between traditional pH sensors and the InPro X1 is in place of a glass membrane, it has a pH-sensitive chip, the X-Chip, which is made of a composite of different materials. The X-Chip has exceptional mechanical resistance and uses the same potentiometric measurement principle as pH-sensitive glass normally used in pH sensors, the company states.

Tab 1: Comparison of different technologies for pH measurement
Tab 1: Comparison of different technologies for pH measurement

Also, unlike traditional glass pH sensors, the InPro X1 does not have an internal buffer. The reference system has a pre-pressurized liquid electrolyte that comes into contact with the measuring medium via a ceramic diaphragm. It withstands a process pressure of up to 4 bar(g). For many years, this design of reference system has proven itself thousands of times in other industries, such as the chemical and pharmaceutical industries, especially in biotechnology. METTLER TOLEDO notes.

The InPro X1 is fully food-compliant and meets all hygiene requirements and guidelines according to EC1935/2004, EHEDG, and 3A. However, its most important feature is that the InPro X1 is practically unbreakable. As well as the X-Chip being extremely robust, the sensor has a highly durable PEEK shaft. In fact, the InPro X1 has passed the US military’s MIL-STD-810H drop test. 

With the InPro X1, a pH sensor is available that can be used for continuous measurement in real-time, in-line, without the risk of glass breakage. Reliable control and monitoring of pH-sensitive process steps in the dairy industry are now possible.

High long-term stability

As mentioned, ISFET sensors do not tolerate cleaning-in-place. The hot caustic attacks the ISFET’s ion-selective layer, and the sensor will lose pH sensitivity after only a few CIP cycles. A currently available ISFET sensor drifts so far from zero offset after eight cleaning cycles that pH measurement is no longer possible. At the same time, the InPro X1 is practically inert to hot caustic (see Figure 1).

Figure 1: Comparison of CIP resistance of InPro X1 and ISFET sensors
Figure 1: Comparison of CIP resistance of InPro X1 and ISFET sensors

The InPro X1 was extensively tested under dairy production conditions. It was exposed to daily hot caustic CIP, and despite these demanding process conditions, service lives of up to one year were achieved, with calibration required only every three to six months, the company states. The sensor’s excellent long-term stability manifests in very low drift of the zero point and minimal decrease in electrode slope, significantly reducing the need for calibration and maintenance. This results in low operating costs, making in-line pH measurements superior to off-line sample measurements, METTLER TOLEDO adds. 

A prerequisite for correct pH measurement is a clean sensor. The fact that the InPro X1 can remain in the process during CIP ensures this requirement is met. And whereas ISFET sensors must be installed with a complex retractable fitting and removed from the process during CIP, installation of the InPro X1 with a stationary sensor housing is uncomplicated. 

Typical InPro X1 applications in dairy processes

Milk intake: The pH value of each milk delivery should be checked to prevent “spoiled” milk, easily recognizable by a low pH value, from being accepted and contaminating “good” milk in a storage tank. Spot measurement with a portable pH meter is error-prone and unreliable, especially if not performed by trained personnel. In-line measurement is accurate and reliable.

Post-acidification: In the fermentation of soy milk, the pH value has to be lowered using citric acid to achieve a sufficient shelf life of soy yogurt. Acid dosage is usually done by spot pH measurement and using the pH level to calculate the amount of citric acid to be added. In-line pH measurement that directly controls the dosage of citric acid is simpler and more reliable.

Fermentation and aging: Fermentation and aging processes are associated with a change in pH value. Progress and endpoint can be easily monitored or detected by utilizing in-line measurements.

Lactose production: The pH value during lactose crystallization determines product yield and quality. The abrasive behavior of the crystals can negatively affect the measuring performance of conventional electrodes.

Whey: To precipitate and separate whey proteins, the pH value must be adjusted by adding acid before heating to achieve the highest possible protein recovery.

Filling: pH measurement can reliably detect residues of acid or caustic from CIP processes.

Digital technology

The InPro X1 features METTLER TOLEDO’s Intelligent Sensor Management (ISM) technology. In ISM pH sensors, the primary electrochemical measurement signal is converted to a pH value directly in the sensor and is output as a digital signal. Digital transmission ensures an accurate measurement is received at the transmitter, regardless of cable length or interference from surrounding equipment.

Tab 2: Advantages of in-line measurement with the InPro X1
Tab 2: Advantages of in-line measurement with the InPro X1

ISM also provides diagnostics data that alert the user when sensor calibration or replacement will be necessary. When calibration is required, rather than performing the task at the measurement point, ISM sensors can be calibrated using a laptop and ISM Core software in any convenient location. When a calibrated sensor is connected to the transmitter at the measurement point, the transmitter uploads the calibration data and configures itself automatically. Further, for quality control, the complete life cycle of the sensor is recorded in ISM Core’s database.

Tab 3: Technical data of the InPro X1
Tab 3: Technical data of the InPro X1

The InPro X1 is compatible with most transmitters from METTLER TOLEDO’s range. These instruments display the measurement value and transmit it to higher-level systems (PLC, DCS). In addition to standard transmission as an analog 0/4...20 mA signal, digital interfaces such as HART, Profibus, Foundation Fieldbus, Ethernet/IP, and Profinet are also available. 


The practically unbreakable InPro X1 pH sensor enables in-line pH measurement in dairy processes without the risk of glass contaminating products. Now, unsatisfactory glass, enamel, or ISFET sensors can be easily and cost-effectively exchanged with reliable, durable sensor technology that also offers low operating costs and easy maintenance. Cumbersome sampling and error-prone off-line pH determination can be replaced by more accurate and secure in-line measurements to increase process control, avoid batch errors, and minimize operating costs.