Detectors and Filaments - TCD's

Thermal Conductivity Detectors (TCD)

The TCD is one of the more widely used detectors for gas chromatography because it is simple in construction, rugged, versatile, sensitive, relatively linear over a wide range, nondestructive to the sample, and inexpensive. It is because of these qualities that it continues to be widely used.

The TCD consists of a block (usually metallic) containing a cavity through which the gas flows. A heated element (resistance wire or thermistor) is positioned in the flow path of the gas. For practical considerations a differential method is usually used that requires two cavities and two heated elements. Only carrier gas passes through one cavity and the column effluent through the other. For process analysis, a reference and sample gas are used.

Precise matching of filaments and thermistors over an extended current-resistance curve and under controlled heat dissipation conditions ensure superior detector performance
Rigid gas and electrical leakage tests and are run on specified zero gas
Micro-cells and GADEs are tested for signal output
Flow-through, semi-diffusion and diffusion geometries
Detector Sensitivity: depends on several factors. Most basic is the difference in thermal conductivity (or other characteristics) between the background (carrier, reference) gas and the sample.
Sensitivity of hot wire thermal conductivity detectors (TCD) depends on the internal geometry, type of detector elements, and the temperature differential (Dt) between filaments and cell block.
Thermistor detectors are at their best in sub-ambient and ambient operation.
Gas density detectors (GADE) vary in sensitivity directly with the difference in molecular weight, as well as with the number and type of sensor elements

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DC Power Supply & Bridge Control

Delivers a high quality direct current to a thermal conductivity detector (TCD) or a gas density balance (GADE)
Binary step-type attenuator control allows the user a wide range of sensitivity control for various applications
Polarity switch provides a positive up-scale reading on a recorder or galvanometer
LED display permits continuous monitoring of the detector bridge current
Three models to chose from:
- Model 40-200 (40-202, 220 V, 50 Hz) compatible with thermal conductivity cells or gas density balances containing GOW-MAC® filaments having a cold resistance from 12 ohms to 70 ohms. A zero control is provided to cover the broad resistance range of the detector filaments.
- Model 40-200A (40-202A, 22 V, 50 Hz) same as the Model 40-200 but includes a 10x signal amplifier for added sensitivity
- Model 40-400 (40-402, 220 V, 50 Hz) compatible with all thermistor thermal conductivity cells or thermistor gas density balances having a resistance from 7,000 to 10,000 ohms at 25 °C

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Temperature Regulated Thermal Conductivity Detectors

24-500 Series Temperature Regulated Detector Assemblies and Kits for gas chromatography and gas analysis

Complete with a proportional temperature controlled, high mass aluminum housing for detector stability.
Control module includes a power supply, temperature controller, 12-step attenuator and a temperature/bridge current readout.
x10 signal amplifier (optional)
Cells are made of stainless steel and can also be made of corrosion resistant monel, titanium, or gold-plated
Cells can be sealed with reference gas of H₂, He, N₂, Ar or other gas.

KIT: Customer to receive all parts necessary to construct the instrument. Substantial work is involved in assembling the parts. No case is provided for detector control module.

ASSEMBLY: Customer to receive most parts assembled into the proper components. The detector control module is complete and working

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Temperature Regulated Cell Assemblies for Agilent GCs

Models 24-580/590-2 and 24-580/590-3 Detector Assemblies are designed specifically for the Agilent 5880 and 5890 GCs. Also available are Models 24-6890 and 24-6890-1 for the Agilent 6890 GCs. The availability of these detector assemblies is important since the original manufacturer does not offer a corrosion resistant TC detector.

Available with capillary TC detectors that do not use fluidic valves which switch at 5 and 10 Hz.
The GOW-MAC capillary detector has an effective volume of 8 µL and a response time of 0.05 seconds at 10 mL/min. carrier flow.
A continuous sampling TC detector, with response times 2 to 4 times faster than a fluidic switching valve which switches every 0.1 or 0.2 seconds, assures that quantitative data is accurate, especially on fast eluting peaks.
Retrofit Agilent Detector Assemblies include power supply, bridge control, TCD installed in an insulated housing with heaters and RTD sensor (TCD assembly), and detector control unit containing a proportional temperature controller.

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Thermal Conductivity Detector Filaments

Hotwires
Made of gold-sheathed tungsten, tungsten, rhenium-tungsten, nickel and nickel alloy. Each provides specific performance characteristics such as corrosion resistance, oxidation resistance, catalytic reactions and sensitivity for use under different application conditions.

Thermistors

GOW-MAC thermistor detector elements utilize the same mounts as hotwires.
A thermistor is a thermally sensitive resistor having a high negative co-efficient of resistance.
Thermistors have relatively low operating temperatures, and should not be used in hydrogen atmospheres.
They have high sensitivity at low temperatures and can be used at sub-ambient temperatures.

Selection Chart
Chemical Resistance Behavior of Various GOW-MAC Filaments

Operating Conditions
Maximum Operating Currents for Various Cell Temperatures Chart

It is important to remember that the detector bridge should be operated at the lowest temperature (sic current) consistent with the sensitivity required for analysis. The lower the temperature the longer the life. Lower temperatures also reduce noise and increase stability.

The filament chart indicates maximum bridge currents. Recommended values at any temperature are 60 to 70% of those shown. The thermistor chart indicates recommended values for maximum sensitivity. The sensitivity of thermistor cells tends to have a peak value. Exceeding this value can result in lowered sensitivity. Since the cold resistance varies between pairs, it is recommended that the exact value be determined by experimentation, using the chart as a guide.

Order Information for GOW-MAC Filaments and Thermistors

The basic filaments and thermistors manufactured by GOW-MAC have special lead lengths and spade lugs for easy installation in GOW-MAC GCs. For details of part numbers for GOW-MAC GCs, please contact us or send for our Repair/Replacement List. GOW-MAC also manufacturers replacement detectors and filaments for most GCs made. Refer to the section "Replacement TCDs and Filaments for GCs" for a listing of those GCs.

All shipments are made FOB Bethlehem, PA USA. Terms of sale are Net 30 Days. It is our policy to maintain all filaments and thermistors offered, in stock, for immediate shipment.

All GOW-MAC filaments are tested before shipment. Most filaments are tested and matched in helium but can be matched in other gases on special request.

Pairs vs. Quads

GOW-MAC filaments are sold in matched pairs of quads. A matched pair consists of two elements matched in gas under varying current conditions. While the filaments in a pair are matched, one pair is not matched to another pair. When ordering, we suggest purchasing 3 or 4 pairs to make a well balanced bridge. A quad consists of four elements matched as in a cell. All four filaments exhibit characteristics which tend to compensate for changes in current, temperature and flow. Quads are supplied with 18" electrical leads (longer or shorter leads are available on request) and new tube nuts and are ready for installation in the detector block.

GOW-MAC also offers special filaments for other manufacturers instruments, e.g., Beckman, Perkin Elmer, Agilent (Hewlett Packard), Finnigan, Varian and many more. Contact us for a listing of those manufacturers.

Repair Service

GOW-MAC offers complete cell repair service at our Bethlehem, PA USA facility. We clean, repair and refilament to original equipment specifications. Pricing information is available. Contact us at repairs@gow-mac.com

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Replacement TCDs and Filaments for Gas Chromatographs

GOW-MAC® manufactures filaments and new replacement thermal conductivity detectors for all of GOW-MAC's instruments and most of the major gas chromatographs on the market today, including many older instruments. If the list below does not contain your GCs manufacturer, call us at (610) 954-9000 with the manufacturer and GC model number and we'll determine whether a replacement detector is available or if one can be manufactured by GOW-MAC. Fittings are not supplied with replacement detectors.

Available TCDs for Various Manufacturers GCS

AGILENT (HEWLETT PACKARD): F&M 185, 300, 400, 500, 700, 720, 810 / HP 5710, 5710A, 5712, 5730A, 5732, 5734, 5722A, 5720A, 5890

BECKMAN: GC2, GC2A, GC4, GC45, GC5, GC55, GC65, GC72-5, GC-M, 6700

BENDIX:2110, 2200, 2300, 2500

CARLE:100, 5660, 5752, 5754B, 5830, 5790A, 5840, 7620, 7645, 7620

CARLO ERBA: ATC405, ATCF, G1-450, G1-252, 1101, 1102, 1104, 2305, 2400, 2500, 2200, 2351, 2450

CHROMPACK (PACKARD): 406, 409, 417, 419, 420, 421, 428, 886, 7300, 7400, 1455, 428, 886

FISHER SCIENTIFIC (VICTOREEN): 29, 1200, 2400, 4400

NUCLEAR CHICAGO (BARBER COLMAN): 5000, 5340

PERKIN ELMER: 154, 800, 810, 900, 910, 990, 3920, 9677, F11, F17, F22, F30, Sigma 1, Sigma 2, Sigma 3, Sigma 4, PE Autosystem

PYE-UNICAM: Call

THERMO FINNIGAN (TREMETRICS , TRACOR , MIKRO-TEK): 220, 2000, DS161, DS171, 540, 550, 563, 565, 9000

VARIAN CHROMATOGRAPHY (AEROGRAPH): A90P, 200, 350B, 700, 920, 1520, 90P3, 1400, 1500, 1700, 1720, 1800, 2700, 2800, 3400, 3600, 3700, 3800

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Gas Density Balance Detector (GADE)

The GOW-MAC® Gas Density Detector is the only detector in use today that measures a single additive physical property of the chromatographic gas, i.e., a property that depends upon only the number and kind of atoms in the molecule.

Particularly useful in monitoring the most corrosive samples
Ideal for the detection of highly corrosive or reactive samples

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Additional Detector Notes

More tidbits of information

Zero Stability

Drift and noise are affected by numerous variables. GOW-MAC quality control procedures ensure superior detector performance, by precise matching of filaments and thermistors over an extended current-resistance curve and under controlled heat dissipation conditions. Compatible filament pairs are installed symmetrically in a cell block. Under conditions of constant cell current, cell block temperature, carrier gas flow and pressure, negligible drift and noise will be noted on a 1 mV potentiometric recorder. The signal can be amplified many times under such precisely controlled conditions.

Speed of Response

Flow-through, semi-diffusion and diffusion geometries are offered to meet detector needs for each application whether gas chromatographic or continuous process.

Oxidation and Corrosion Resistance

Thermal conductivity detectors (TCD) can be supplied with gold-sheathed tungsten, rhenium-tungsten, nickel, or other special filaments installed in cell blocks of Monel, etc.

Gas Density Detectors (GADE) with elements in the inert reference stream are not exposed to the sample and are capable of long service life with highly corrosive samples.

GOW-MAC Quality Control

Includes rigid gas and electrical leakage tests and are run on specified zero gas. Micro-cells and GADEs are tested for signal output.

Calibration of TCDs and FIDs is made empirically by the customer on samples of known analysis, after reaching steady state on reference (carrier) gas.

Detector Sensitivity

This depends on several factors. Most basic is the difference in thermal conductivity (or other characteristics) between the background (carrier, reference) gas and the sample.

Sensitivity of hot wire thermal conductivity detectors (TCD) depends on the internal geometry, type of detector elements, and the temperature differential (Dt) between filaments and cell block. Thermistor detectors are at their best in sub-ambient and ambient operation. Experiments with filament and 8K thermistor detectors of identical geometry demonstrate that greater signal is obtained from the hot wires at cell temperatures of 75 °C and above.

Gas density detectors (GADE) vary in sensitivity directly with the difference in molecular weight, as well as with the number and type of sensor elements.

Flame ionization (FID) sensitivity varies with the applied polarization voltage (until a plateau is reached), also with the temperature of the flame, which is a function of the air/oxygen fuel mixture, as distinguished from temperature of the detector assembly.

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