The Chandra X-ray Observatory was successfully launched on July 23, 1999, and subsequently began an intensive calibration phase. We present preliminary results from in- flight calibration of the low energy response of the High Resolution Camera Spectroscopic readout (HRC-S) combined with the Low Energy Transmission Grating (LETG) aboard Chandra. These instruments comprise the Low Energy Transmission Grating Spectrometer (LETGS). For this calibration study, we employ a pure hydrogen non-LTE white dwarf emission model (Teff equals 25000 K and log g equals 9.0) for comparison with the Chandra observations of Sirius B. Pre-flight calibration of the LETGS effective area was conducted only at wavelengths shortward of 45 angstroms (E > 0.277 keV). Our Sirius B analysis shows that the HRC-S quantum efficiency (QE) model assumed for longer wavelengths overestimates the effective area on average by a factor of 1.6. We derive a correction to the low energy HRC-S QE model to match the predicted and observed Sirius B spectra over the wavelength range of 45 - 185 angstroms. We make an independent test of our results by comparing a Chandra LETGS observation of HZ 43 with pure hydrogen model atmosphere predictions and find good agreement.