How to Make the Best Choice of Moisture Sensor for your Natural Gas Process

When to choose a tunable diode laser spectroscopy-based analyzer or ceramic metal oxide moisture transmitter?

Tunable diode laser spectroscopy (TDLAS) and metal oxide moisture sensors are both popular and effective measurement techniques for controlling moisture in natural gas. They each offer benefits and advantages over the other so choosing the most effective option for your process depends on exactly how you are intending to employ the moisture analyzer and the outcomes you want. This guide examines and compares the different characteristics of each measurement technology and their relative strengths for specific applications within natural gas processing.

When to consider TDLAS:

Combustion is an exothermic reaction between a hydrocarbon fuel and oxygen
The Michell OptiPEAK TDL600 measures moisture reliably in changing background compositions
Processes involving sour gas or other chemically aggressive applications

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.

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 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 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.

You need the same level of accuracy across the complete measurement range

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 -70°C.

Minimal maintenance is important

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, especially if it is sited in remote, hard-to-reach locations.

When to consider metal-oxide moisture transmitters

Combustion is an exothermic reaction between a hydrocarbon fuel and oxygen
The Easidew PRO I.S. offers a low measurement range of -110 °C dew point
You need to measure down to very low moisture levels

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 can 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.

You need to measure water dew point as your 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 so dew point is a calculated parameter and there is a risk that errors may be introduced during this process.

You have a changing background gas composition

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 can automatically adjust for changes in background composition, they are still not as versatile as the metal-oxide impedance sensor.

Cost-effective installation and maintenance are important

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 to suit their operating protocols.

If there is no requirement to maintain a detailed calibration history for the specific sensor being used, Michell offers a national standards traceable calibration exchange program 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, including a new sensing element and all seals. Not only is this a fast and convenient option, it results in less waste in the environment.

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

Get expert advice

With all the options available for moisture measurement in natural gas processing and storage, getting good advice is always a good choice. At Process Sensing Technologies, we offer the widest range of moisture measurement techniques available from a single manufacturer. This means that we have both the expertise to provide good, impartial advice on the best measurement technique for your application. Contact us to find out more.

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Related Products

Moisture in Natural Gas Analyzer - Michell OptiPEAK TDL600
Transmitter for Moisture Analysis - Michell Easidew PRO I.S.

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