Trace moisture measurement plays an important role in ensuring the quality of products and the efficiency of hundreds of thousands of plants and factories around the world. Ensuring reliable and accurate measurements requires the correct sampling techniques, and a basic understanding of how water vapour behaves under different conditions of temperature and pressure.
All materials are permeable to water vapour since water molecules are extremely small compared to the structure of solids, even including the crystalline structure of metals. The graph below demonstrates this effect by showing the increase in dew point temperature seen when passing very dry gas through tubing of different materials, , where the exterior of the tubing is in the ambient environment.
What this demonstrates is the dramatic effect that different tubing materials have on the humidity levels of a gas passed through them. Many materials contain moisture as part of their structure and when these are used as tubing for a dry gas the gas will absorb some of the moisture. Always avoid using organic materials (e.g. rubber), materials containing salts and anything which has small pores which can easily trap moisture (e.g. nylon).
As well as trapping moisture, porous sampling materials will also allow moisture vapour to ingress into the sample line from outside. This effect is called diffusion and occurs when the partial water vapour pressure exerted on the outside of an airline is higher than on the inside. Remember that water molecules are very small so in this case the term ‘porous’ applies to materials that would be considered impermeable in an everyday sense – such as polyethylene or PTFE. Stainless steel and other metals can be considered as practically impermeable and it is surface finish of pipework that becomes the dominant factor. Electropolished stainless steel gives the best results over the shortest time period.
Top sampling tip: Choosing the right sample tubing materials avoids inconsistent or incorrect moisture measurements. Firstly, take into consideration the gas you are measuring, and then choose materials appropriate to the results you need. Very dry gases are affected more by diffusion or moisture trapped in materials than those with a high humidity. If you are measuring moisture in parts per billion, just a few more water molecules present will have a greater impact on your results than if you are measuring relative humidity at 50 to 60 %rh.
Selecting the right materials is only part of the story. Humidity is also affected by temperature and pressure, so maintaining a consistent environment is vital to ensure reliable, repeatable measurements.
As the temperature or pressure of the environment fluctuates, water molecules are adsorbed and desorbed from the internal surfaces of the sample tubing, causing small fluctuations in the measured dew point. Ensuring the temperature of the location where sampling takes place is kept at consistent levels is important, and not always easy. If the measurements are made on an industrial process, this consistency may need to be maintained over a number of years, allowing for seasonal changes. In some cases, the sampling systems are placed outside – for example on gas pipelines – and integral heating and cooling systems are built into the sample conditioning system to maintain this consistent environment no matter the outside temperatures.
|Adsorption is the adhesion of atoms, ions, or molecules from a gas, liquid, or dissolved solid to the surface of a material, creating a film. The rate of adsorption is increased at higher pressures and lower temperatures.
Desorption is the release of a substance from or through the surface of a material. In constant environmental conditions, an adsorbed substance will remain on a surface almost indefinitely. However, as the temperature rises, so does the likelihood of desorption occurring.
If temperatures drop below ambient dew point, water may condense in sample tubing and affect the accuracy of measurements.
Maintaining the temperature of the sample system tubing above the dew point of the sample is vital to prevent condensation. Any condensation invalidates the sampling process as it reduces the water vapour content of the gas being measured. Condensed liquid can also alter the humidity elsewhere by dripping or running to other locations where it may re-evaporate.
Although ambient pressure does not change drastically in a single location, the gas sample pressure does need to be kept constant to avoid inconsistencies introduced by adsorption or desorption. The integrity of all connections is also an important consideration, especially when sampling low dew points at an elevated pressure. If a small leak occurs in a high-pressure line, gas will leak out, however, vortices at the leak point and a negative vapour pressure differential will also allow water vapour to contaminate the flow.
Theoretically flow rate has no direct effect on the measured moisture content, but in practice it can have unanticipated effects on response speed and accuracy. An inadequate flow rate may:
An excessively high flow rate can:
Top sampling tip: relative humidity and dew point are parameters that are both affected by temperature and pressure. Reliable humidity measurements require both a stable ambient environment and a consistent sample gas temperature, pressure and flow rate. The optimal flow rate varies depending on the measurement technology and can always be found in the instrument or sensor manual.
The more complicated the sample system, the more areas there are for trapped moisture to hide. The key pitfalls to look out for here are the length of the sample tubing and dead volumes.
The sample point should always be as close as possible to the critical measurement point to obtain a truly representative measurement. The length of the sample line to the sensor or instrument should be as short as possible. Interconnection points and valves trap moisture, so using the simplest sampling arrangement possible will reduce the time it takes for the sample system to dry out when purged with dry gas.
Over a long tubing run, water will inevitably migrate into any line, and the effects of adsorption and desorption will become more apparent.
Dead volumes (areas which are not in a direct flow path) in sample lines, hold onto water molecules which are slowly released into the passing gas. This results in increased purge and response times, and wetter than expected readings. Hygroscopic materials in filters, valves (e.g. rubber from pressure regulators) or any other parts of the system can also trap moisture.
Top sampling tip: plan your sampling system to ensure that the sample point and the measurement point are as close as possible to avoid long runs of tubing and dead volumes.
All trace moisture measurement instruments and sensors are by their nature sensitive devices. Many processes contain dust, dirt or liquid droplets. Particulate filters are used for removing dirt, rust, scale and any other solids that may be in a sample stream. For protection against liquids, a coalescing or membrane filter should be used. The membrane provides protection from liquid droplets and can even stop flow to the analyser completely when a large slug of liquid is encountered, saving the sensor from potentially irreparable damage
Top sampling tip: Protecting your moisture sensor by using the correct filtration not only ensures accurate measurements, it may also prolong the life of the sensor and save money on maintenance.
The measurement of humidity is a complex subject, but in practice it does not need to be difficult. Keeping consistent environmental conditions and using appropriate sample handing materials will help to ensure reliable and accurate results. There is also no substitute for experience, so seeking advice from an expert in sampling system design or purchasing a system from reputable manufacturer is advisable. They will be sure to have encountered applications like yours before and will be able to help you avoid common pitfalls.