A system, comprising: a valve assembly (104,204) comprising a pneumatic actuator, a fluid coupling with valve elements coupled to the pneumatic actuator, and a valve positioner coupled to the pneumatic actuator; a first fluid supply providing a process fluid to the fluid coupling; a second fluid supply with a fluid supply line that provides an instrument gas to the valve positioner; and a measurement module (102, 202) coupled to the fluid supply line, the measurement module (102, 202) having a connector that couples electrically with the valve positioner, characterised by: the measurement module (102, 202) comprising: a housing enclosing a flow pathway with an inlet and an outlet, both being formed in the housing so that the outlet exhausts to environment, a chamber, separated from the flow pathway, the chamber being configured to receive the instrument gas, a sensor component disposed in the chamber, and a sample line coupling the chamber with the flow pathway; wherein the sensor component is configured to generate an output that reflects a characteristic of the instrument gas in the chamber; wherein the inlet of the flow pathway couples with the fluid supply line to direct a first sample of the instrument gas out of the fluid supply line to the inlet of the flow pathway with the flow pathway spaced apart from the fluid supply line; wherein the first sample originates from a location on the fluid supply line that is upstream of each component in the valve positioner that regulates a position of the actuator on the valve assembly (104,204): and wherein one or more valves regulate intake of samples of the instrument gas into the inlet. a valve assembly (104,204) comprising a pneumatic actuator, a fluid coupling with valve elements coupled to the pneumatic actuator, and a valve positioner coupled to the pneumatic actuator; a first fluid supply providing a process fluid to the fluid coupling; a second fluid supply with a fluid supply line that provides an instrument gas to the valve positioner; and a measurement module (102, 202) coupled to the fluid supply line, the measurement module (102, 202) having a connector that couples electrically with the valve positioner, characterised by: the measurement module (102, 202) comprising: a housing enclosing a flow pathway with an inlet and an outlet, both being formed in the housing so that the outlet exhausts to environment, a chamber, separated from the flow pathway, the chamber being configured to receive the instrument gas, a sensor component disposed in the chamber, and a sample line coupling the chamber with the flow pathway; wherein the sensor component is configured to generate an output that reflects a characteristic of the instrument gas in the chamber; wherein the inlet of the flow pathway couples with the fluid supply line to direct a first sample of the instrument gas out of the fluid supply line to the inlet of the flow pathway with the flow pathway spaced apart from the fluid supply line; wherein the first sample originates from a location on the fluid supply line that is upstream of each component in the valve positioner that regulates a position of the actuator on the valve assembly (104,204): and wherein one or more valves regulate intake of samples of the instrument gas into the inlet. the measurement module (102, 202) comprising: a housing enclosing a flow pathway with an inlet and an outlet, both being formed in the housing so that the outlet exhausts to environment, a chamber, separated from the flow pathway, the chamber being configured to receive the instrument gas, a sensor component disposed in the chamber, and a sample line coupling the chamber with the flow pathway; a housing enclosing a flow pathway with an inlet and an outlet, both being formed in the housing so that the outlet exhausts to environment, a chamber, separated from the flow pathway, the chamber being configured to receive the instrument gas, a sensor component disposed in the chamber, and a sample line coupling the chamber with the flow pathway; wherein the sensor component is configured to generate an output that reflects a characteristic of the instrument gas in the chamber; wherein the inlet of the flow pathway couples with the fluid supply line to direct a first sample of the instrument gas out of the fluid supply line to the inlet of the flow pathway with the flow pathway spaced apart from the fluid supply line; wherein the first sample originates from a location on the fluid supply line that is upstream of each component in the valve positioner that regulates a position of the actuator on the valve assembly (104,204): and wherein one or more valves regulate intake of samples of the instrument gas into the inlet.