Topics - Time constant
In chromatography a time constant is introduced into the electronic circuit of a detector in the form of capacitor resistance network. The time constant is made sufficiently large, so that the high frequency noise (from the electronics or other sources) is filtered out and the low frequency chromatographic signal (the eluted peak) remains unmodified. The actual time constant of the circuit is the time taken for the charge on the capacitor to be reduced to 36.8% of its fully charged value, by shorting it through the resistor to ground. Alternatively, it is the time taken for the condenser to reach 63.2% of its maximum charge when charged through the resistor. The time constant in seconds can be calculated by multiplying the capacity in farads by the resistance in ohms. For example, if the resistance was one megohm and the capacitor one microfarad then the time constant would be one second.