Many key factors are in place to ensure your pH sensor is functioning properly and providing consistent, accurate readings. The first step in ensuring accurate measurements is understanding the calibration process and its impact on your pH measurement range. When calibrating sensors by establishing two points on the pH scale, you expect a process to operate within the standard calibration methods. This method is known as 2-point calibration and acts as the industry standard for calibrating pH sensors. Single-point and 3-point calibrations are less popular, but each method has a role in measuring pH. This article will examine the following: 

• The role of calibration in producing accurate measurements 
• Characteristics of 2-point and 3-point calibration 
• When to utilize each method of calibration

Calibration Basics 

Calibrating your sensor is critical to producing accurate measurements. You and your operators and technicians can perform calibrations in numerous ways. Depending on the configuration and the process involved, some methods may be better utilized than others. 

 Before diving into the calibration process, it’s important first to understand the slope and zero-point of a pH sensor. These values are specific to each sensor. The sensor’s slope is the linear correlation between the raw voltage(mV) reading and pH value. This means that the mV value at pH 7 is zero in a perfect world. It should then increase or decrease by 59.16 mV/per unit of pH as the value increases or decreases from pH 7. Unfortunately, each sensor is unique, so the perfect world does not exist. The purpose of calibrating each sensor to the slope and zero-point relative to the theoretical values is so the sensor can adjust for the changes it has experienced due to process exposure and aging. 

Using 2-Point Calibration

2-point calibration is the standard and recommended method of calibration. It is a common misconception in various pH measuring applications that the more calibration buffers used, the more accurate a measured value will be. This is only sometimes true. As industry technology has evolved, users have found that controlled 2-point calibrations can provide highly precise and reliable readings in most applications. Adjusting your electrode with two specific buffers on the pH scale will allow the sensor to accurately interpret a solution’s pH. We often recommend utilizing 2-point calibrations with pH buffers 4 & 7 followed by a third-point validation. 

Validating your 2-point calibration with a third point allows for those accustomed to 3 or multi-point calibration to receive that extra assurance that the sensor is reading accurately. Using 3-point calibration, A technician can bring a Portavo portable meter into the field when completing routine maintenance or addressing a specific performance issue. By taking a sample of the process and then measuring and data logging with the Portavo, they now have a third source of data to compare to the process reading and lab measurement. You now have a rapid and reliable validation of the process measurement without permanently including a third, potentially unnecessary variable. 

Using 3-Point Calibration

We can utilize 3-point calibration when expecting to cover an even wider range of the pH scale. As previously explained, exceeding two buffer points does not automatically mean a reading will produce more accuracy. Again, most 2-point calibrations utilize the recommended buffers of 4 and 7 on the pH scale.

3-point calibration often requires users to set buffers higher along the pH scale. Higher buffers in alkaline ranges are historically prone to cause degradation. This is especially true when exposed to the harsh environments the sensors operate in. Over time, this exposure can cause inaccurate readings and eventually lead to a misalignment of data. In many of these environments, for example, the Food & Beverage and Pharmaceutical CIP (clean-in-place) industries and fertilizer production, such pH data interruption could be detrimental to costs, time, and production. However, in some applications, especially in processes where measurements are frequently found exceeding pH 10, a 3-point Calibration may be necessary to cover that broader range.

2-point and 3-point Calibration Summary

Remember, each sensor calibration is unique to the process you are analyzing. 2-point and 3-point calibration processes both have their benefits. But understanding which is better for your application is key to confirming accuracy. Ensuring your buffers are applicable to this process is the key to determining which method best suits your analysis. Check out our instructive videos and helpful guides for more information on calibrations and buffers.