Q1 2009 / Fast and Efficient: The Thermal Conductivity Detector

A thermal conductivity detector (TCD) is a concentration-based detector that senses the change in the thermal conductivity of a gas stream. The two most common uses of the TCD are in binary gas analyzers and gas chromatography applications. These detectors can respond to any compound that has a thermal conductivity different than that of a reference gas. In binary analyzer applications the TCD offers both practicality and ease of operation. 

The TCD is a good general purpose detector because it responds to all compounds. It is often used for initial investigations with unknown samples. Since all compounds, organic and inorganic, have a thermal conductivity different from helium, all compounds can be detected by this detector provided that they are gases or liquids that can be maintained in a vapor state by temperature. TCDs are often used in the analysis of argon, oxygen, nitrogen, carbon dioxide, xenon, krypton and neon.

What’s Inside a TCD?

The detector is composed of a metallic block with both reference and sample stream inlets. Internally, these inlets lead to the sample and reference cavities that house its sensors which are typically either wire filaments or thermistors.

Thermistors, (a word formed by combining thermal with resistor), are devices in which electrical resistance is controlled by temperature. (Figure 1)

The “block” is held at a constant temperature, and the filaments are heated by passing a constant current through them. The detector block acts as a heat sink for the filaments. The filaments themselves, however, operate at an elevated temperature due to the current passing through them. 

TCDs in Action

Both helium and hydrogen possess a high thermal conductivity value. They allow the filament heat to dissipate to the TCD block at a constant rate. When an impurity gas, such as nitrogen, having a lower thermal conductivity (TC) value, enters the sample stream, it lowers the TC value and causes heat loss from the filament to the block to decrease. This in turn raises the temperature of the filament and its resistance. The increase in resistance is attributable to the filament having a positive coefficient of resistance, the hotter the filament the higher the resistance.

Employing a reference and zero gas that has a low TC value – such as argon or CO₂ – or the introduction of a gas with a higher TC value (such as N₂ or air) will cause the sample filaments to cool slightly thereby lowering the resistance of the filaments.

Electrically speaking, the filaments are arranged in a Wheatstone bridge circuit. When the resistance of the sample side filaments increases this produces a measureable voltage that is proportional to the difference. The bridge output may then be sent to a meter, recorder or software program. The converse is similar with a low TC value reference gas, i.e., CO₂, and a higher TC value sample gas, such as N₂. 

In this case the filament temperature will actually decrease resulting in a drop in filament resistance. This in turn also results in the Wheatstone circuit showing a voltage change.

Types in Operation

Depending on the application, there are generally two types of TCD geometries in the field today. The first is the diffusion geometry used for high flow rates, flows that experience irregularities, and process control. The second is a flow-through design employed in chromatographic applications. Being a concentration based detector, the smaller the internal volume of the detector, the greater the sensitivity and the faster the response. Detector sensitivity depends on several factors. Most basic is the difference in thermal conductivity between the background (carrier, reference) gas and the sample. Also, sensitivity of hot wire thermal conductivity detectors (TCD) depends on the internal geometry, type of detector elements, and the temperature differential (?t) between filaments and detector block.

Analyze This: Binary Gases

Due to their ease of operation, fast response, sensitivity and robustness TCDs are ideal for binary gas analysis. Typical applications are listed in Table 1 (See next page). TC detector blocks are available with monel, nickel, tantalum, or other corrosion resistant metals; and filaments may be selected from nickel, gold, tungsten, rhenium tungsten or other metals and alloys. Besides the more common aforementioned applications, binary analyzers may be employed in the on-line analysis of PH₃/H₂ and corrosive gas streams. 

Five Easy Steps

The ease of operation for a TCD binary gas analyzer is a five-step procedure:

• Establish zero/reference flows by adjusting inlet pressure and flow rate to manufacturer’s recommendation and purge the analyzer to ensure ambient air has been removed from the flow system.

• Set the detector current.

• Zero the analyzer by adjusting zero controls until the readout indicates 0.

• Turn off zero gas and introduce calibration/span gas; adjust the calibration controls until the readout agrees with the calibration gas.

• Test the sample stream/cylinders.

• The total time involved for these steps for a trained operator can be as little as 5 minutes. In the case of testing a batch of cylinders, analysis time is on the order of one to two minutes per cylinder.  

Typical Applications

Thermal conductivity binary gas analyzers such as the GOW-MAC Series 20 or Series 50 instruments offer a fast, cost efficient, and reliable means of providing accurate on-line analysis for binary gas mixtures.

The applications are wide ranging. For example, the GOW-MAC 20 Series Binary Gas Analyzer is being used to determine the helium level in lighter than air ships (dirigibles).  Before departure, the crew needs to verify the helium concentration in the gas envelope.

In another application the Series 20 Series analyzer helped one farmer maintain the CO₂ level in his corn silos. Carbon dioxide is used in the fumigation of corn and rice and the CO₂ level had to be held at about 55% to be effective. Using the 20 Series Binary Gas Analyzer with alarms, the farmer was able to control silo’s atmosphere by turning the CO₂ on and off at precise levels. 

These analyzers are employed in submarines for atmosphere testing, in lamp bulb manufacturing facilities to monitor the fill gas, in annealing furnace applications to monitor the blanket/inerting gas, and gas reclamation systems. 

So from the depths of the oceans to the sky above, thermal conductivity binary gas analyzers are in operation throughout our world. They offer a practical alternative to the chromatographic analysis of binary gas mixes. Their combination of rugged design, stable detector, and user-friendly controls gives the gas industry an analytical tool whose life is measured in decades of service. SGR

Kenneth B. Fincke

Kenneth B. Fincke is Vice President of Sales & Marketing for GOW-MAC Instrument Co., Bethlehem, PA., a leading manufacturer of trace gas analyzers, gas chromatographs, and gas handling equipment. Ken, an alumnus of Rutgers University, has over 40 years of analytical, engineering, and business experience.

He can be reached at kbfincke@gow-mac.com or by phone at 610-954-9000 ext. 239