Manometric (also called Sieverts or volumetric) gas sorption measurements rely on precise control and reading of the pressure and temperature within the analyzer and calibrated dosing and reactor volumes.
The method begins by loading a known quantity of sample into the system, evacuating it, and degassing/activating the sample. A defined quantity of gas is dosed into the sample cell and the resulting pressure change is used to calculate the amount of gas adsorbed or desorbed. The process is then repeated to build up a series of equilibrium points, usually plotted as a sorption isotherm or PCT (pressure-composition-temperature) curve.
Pressure Measurement Accuracy
- The measurement relies on accurate pressure readings.
- Any changes in pressure due to sensor drift, calibration errors, or even leaks will give inaccurate sorption values.
- Regular recalibration and checks across the full sensor range will improve the accuracy of the measurements.
Temperature Measurement and Accuracy
- Stable uniform temperatures are needed for accurate gas moles calculations.
- All temperature sensors (cabinet and manifolds, dosing volume and sample volume) must be accurate and stable at all times.
- Regular recalibration and checks across the temperature range will ensure accurate measurements.
Leaks in the System
- Any leak will cause a pressure change that is not due to sorption.
- A pressure drop due to an outboard leak will appear as adsorption and a pressure increase due to an inboard leak will appear as desorption.
- Routine vacuum hold and pressure hold tests should be conducted to ensure the system is leak tight.
Accurate System Volumes
- For accurate measurements, the dosing volume and sample cell volume must be calibrated precisely.
- The volume calibrations depend on:
- Accurate pressure readings
- Accurate temperature readings
- A leak tight system
- Any errors in the volumes will cause errors in every adsorption and desorption point.
Manometric measurements calculate the uptake based on the assumption that all the observed pressure changes are due solely to sorption or desorption. Any other changes to the temperature or pressure readings will introduce error into the uptake values.
Here at Hiden Isochema, every IMI analyzer is rigorously tested during each stage of the commissioning procedure and is verified and validated using industry standards. This comprehensive approach guarantees that you will receive an analyzer capable of delivering accurate and reproducible measurements, giving you confidence in the reliability of your data.
