In the analysis of dissolved gases in transformer oil, TCD (Thermal Conductivity Detector) is the core component for detecting inorganic gases such as hydrogen (H ₂), carbon monoxide (CO), and carbon dioxide (CO ₂). Unlike FID, TCD is a universal detector that responds to both organic and inorganic substances, and cannot be replaced in DGA whole component analysis.

The working principle of TCD

There are four symmetrical chambers inside TCD, each containing a tungsten wire thermistor, forming a Wheatstone bridge circuit. Two chambers are connected to a reference carrier gas (pure carrier gas only), while the other two chambers are connected to a measurement gas (carrier gas+sample components).

When using pure carrier gas, the four arms have the same thermal conductivity, the bridge is balanced, and the baseline signal is output. After the sample enters the measuring cell, the thermal conductivity of different components is different from that of pure carrier gas, resulting in temperature changes in the tungsten wire, loss of balance in the electric bridge, and output of differential signals. The TCD sensitivity of HZGC-1212A is S ≥ 10000mV · ml/mg, baseline noise ≤ 20 µ V, and linear range ≥ 10 ⁵.

The iron rule of TCD operation: first ventilate, then heat up, and then add bridge flow

This is the most important operation sequence in TCD use, any violation may result in tungsten wire burning:

Step 1: Open the carrier gas and confirm that it flows steadily through TCD; Step 2: Start up and heat up until the TCD temperature reaches the set value and stabilizes; Step 3: Finally, set the bridge current.

Never set up a bridge current without a carrier gas - when there is no carrier gas, the tungsten wire will quickly oxidize and burn in the air, causing irreparable damage. The HZGC-1212A is specifically designed with a bridge current setting value that is not saved when power is turned off (automatically reset to zero when turned on), in order to prevent forgetting to ventilate before connecting the bridge current during the next power on.

The shutdown sequence is equally crucial

The shutdown must follow the following steps: first turn off the bridge current → cool down → wait for the TCD temperature to drop to around room temperature → then turn off the carrier gas. During the cooling process, it is necessary to maintain the passage of carrier gas to protect TCD from oxidation.

Principles for setting up bridge flow

Bridge current should not be set too high. Although high bridge current can improve sensitivity, it will accelerate tungsten wire oxidation and significantly shorten the detector life. It is recommended to choose the lowest possible bridge current value (usually 100-150mA) while meeting the analysis sensitivity requirements.

Special requirements for carrier gas purity

TCD is extremely sensitive to oxygen in the carrier gas, and the presence of oxygen in the carrier gas accelerates the oxidation of tungsten wires. The instruction manual clearly requires that TCD carrier gas must undergo thorough deoxygenation treatment to achieve a purity of 99.999%.