Methods of calibrating a pH sensor fixed within an enclosed vessel are disclosed. The methods include introducing a buffer into the enclosed vessel, introducing a gas mixture comprising CO2 into the enclosed vessel, measuring a pH signal of the solution, measuring a CO2 concentration of a headspace gas of the solution, and calculating a pH value with a buffer calibration curve. The methods include calculating a calibration parameter with a sensor calibration curve and calibrating the pH sensor with the calibration parameter. Reactor systems are also disclosed. The systems include an enclosed reactor, a pH sensor, a CO2 sensor, a temperature control subsystem, and a controller. Methods of facilitating pH sensor calibration without sampling in a bioreactor system are also disclosed. The methods include providing a controller and providing instructions to operably connect the controller to the pH sensor and the CO2 sensor.