Moisture Measurements for Natural Gas Quality

The moisture content is a key parameter for natural gas quality. There are many technologies on the market, and a lot of conflicting information about which is best for the job. Here we take an unbiased look at the pros and cons of two of the main contenders: Tunable Diode Laser Spectroscopy (TDLAS) and aluminium oxide, also known as metal oxide or ceramic impedance, sensors.

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Why choose TDLAS?

Tuneable Diode Laser Absorption Spectroscopy is a non-contact form of measurement

Because TDLAS analyzers measure moisture content based on the effects of the laser energy on water molecules, there is no contact between the sensing element and the sample gas. This means that it is highly resistant to chemically aggressive gases such as sour natural gas.

Metal-oxide impedance sensors must be in contact with the sample gas to make the measurement, which makes them more prone to contamination or damage from the sample.

Tuneable Diode Laser Absorption Spectroscopy directly measures moisture content

With a TDLAS analyzer, the sensor tells you immediately what the moisture content of the sample is. Moisture content is not dependent on sample temperature or pressure, unlike the dew-point temperature which changes according to the pressure of the sample.

In applications where moisture content is the required parameter – such as refinery and petrochemical – TDLAS analyzers have the advantage over metal-oxide moisture sensors as they directly measure moisture content. Although a metal-oxide sensor can provide readings of ppmV, these need to be calculated from measurements of dew point, plus a pressure input calculated from measurements of dew point and also an additional pressure measurement, which could come from a pressure transducer, or from a fixed pressure value. A fixed pressure input can easily introduce calculation error if the process pressure fluctuates.

Tuneable Diode Laser Absorption Spectroscopy is consistently accurate

With a TDLAS analyzer you can be sure that the accuracy of the measurement is consistent at both high and low-end measurements. Metal-oxide impedance sensors, while able to measure to lower moisture levels, reduce in accuracy once the dew point drops below -60°C.

Tuneable Diode Laser Absorption Spectroscopy Analyzers need minimal maintenance

Partly thanks to its non-contact measurement technique, TDLAS analyzers need a major service only every five years with a simple annual field-check in between. In comparison, metal-oxide impedance sensors need a yearly recalibration of the sensor.

An analyzer that will operate reliable for a long time before needing a full service is an attractive cost-saving.

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Why choose a metal oxide impedance sensor?

Metal-oxide impedance moisture sensors offer the widest measurement range of any other technology

Impedance is the only moisture measurement technology to offer user the ability to measure down to parts per billion, as well as up to 30,000 ppmV. This versatility is a great advantage to systems integrators who are able to stock just a few sensor types, which meet the needs of a wide variety of application types.

While TDLAS analyzers are highly accurate through the full measurement range, for ultra-dry applications where measurements need to be in parts per billion, a multi-pass optical measurement cell is needed to increase sensitivity. Unfortunately the technology is not advanced enough to create one of these which would be robust enough to survive in an industrial setting.

Metal-oxide impedance sensors measure water dew point as a primary parameter

Water dew-point at pipeline pressure is a key parameter specified in contractual agreements for natural gas suppliers. Metal-oxide impedance sensors’ primary measurement is of dew point, and they are also able to make measurements at line pressure – up to 350 barg. For natural gas suppliers, this is an important feature of the technology as the ability to measure dew point directly at line pressure increases their confidence in the measurements.

Other analyzers, such as TDLAS, measure moisture content and dew point is a calculated parameter and there is a risk that errors may be introduced during this process.

Metal-oxide impedance sensors do not need to compensate for the composition of the sample gas

A ceramic sensor calibration is independent of the carrier gas, so they will reliably measure moisture in a sample, regardless of the type of gas or its composition. This makes them a robust and reliable technology, which is suitable for a wide range of situations. Ease of installation and maintenance helps to keep costs down for the end user.

Although recent developments featured in the newer models of TDLAS analyzer – such as the OptiPEAK TDL600 – mean they are able to automatically adjust for changes in background composition, they are still not as versatile as the metal-oxide impedance sensor.

Metal-oxide impedance sensors are cost-effective to install and maintain

The Metal-oxide impedance moisture sensor technology is simple and well-established. This makes it both cost-effective to buy and maintain as well as easy to install.

When calibration or service is due, the sensor can be replaced in less than five minutes. This process needs no specialist training or knowledge, unlike the maintenance of more complex technologies. Then users have a choice, depending on whether their process requires calibration traceability or not.

If no traceable calibration is needed, then the sensor can be simply replaced with a freshly calibrated one. Michell offer a calibration exchange programme to make this process even simpler, which effectively works like a life-time guarantee. When calibration is due, order a replacement sensor and when it arrives, fit in place of the old sensor and send this one back to Michell where it is checked and refurbished. Not only is this a fast and convenient option, it results in less waste in the environment.

Where a traceable calibration in needed, the sensors is removed and sent back to Michell for calibration and service. Michell has service centres located around the world which means that no matter where the sensor has been installed, there is local recalibration option.

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