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Working with OneFerm Single-Use pH Sensors

The OneFerm is an electrochemical pH sensor specifically designed for single-use (SU) applications. These products are designed for installation into the bag or bioreactor at your OEM supplier and gamma sterilized prior to usage on-site. OneFerm sensors function just like traditional glass membrane pH electrodes however, due to the unique installation requirements there are certain installation questions that should be considered prior to usage.


Why is my sensor discolored?

Gamma radiation will cause the OneFerm electrolyte to turn a light brown color as shown in the image below. This is completely normal and provides a good visual indicator that the sensor has undergone proper sterilization.

What is the small tag included with the OneFerm sensor?

Since OneFerm pH sensors are shipped pre-installed in the bag or bioreactor, normal buffer calibration is not practical. Each sensor comes with a calibration tag which includes sensitivity (slope) and zero-point (offset) values for the sensor. These values can be manually entered into the controller or transmitter. If using ArcAir, the small bar-code can be used to automate this process by scanning sensor information with a smart phone or tablet and loading into the Arc pH Module.

I lost my tag, how can I calibrate?

Calibration values can be calculated using information on the Declaration of Quality (DOQ) provided with each sensor. This document includes the millivolt test values found on 4 and 7pH buffers during production. An example of how this is done is shown below:

Information from DOQ: Reading in pH 4 Buffer: 184 mV Reading in pH 7 Buffer: 8 mV

Zero Point as mV(Offset) = 8 mV

Zero Point as pH (Offset) = ((7pH * 184mV) - (4pH * 8mV)) / (184mV - 8mV) = 6.68 pH

Sensitivity as mV (slope) = (8mV - 184mV) / 3 pH unit = -58.67 mV/pH unit

Sensitivity as percent (slope) = -58.67mV / -59.16mV = 99.16%

If the DOQ document is not available then it can be looked up via the Hamilton website at the link below (certifications). You will need to have the OneFerm ref number & serial number to access the corresponding document. Additionally, the DOQ document can be supplied by contacting Hamilton Technical Support.

DOQ Document Search

My sensor is dried out and some salt crystals appear near the tip

Glass membrane pH sensors normally require hydration to maintain their sensitivity. OneFerm sensors use a special membrane glass composition that allows them to be stored dry for up to 18 months.

During this time, it is common for a small amount of electrolyte to leach from the sensor and crystallize near the glass bulb. This out-flow is common and normal. The leaching is caused by the porous diaphragm present on the sensor. In traditional installations, the pH sensor is continuously wetted and the electrolyte may not be visible. Since single-use applications require dry storage, the electrolyte itself will show as harmless salt crystals. The electrolyte is non-cytotoxic and data is available upon request.

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