Lumped capacitance models have been introduced to study transient thermal response of data centers. Chilled water interruption of a Computer Room Air Handling (CRAH) unit is one of several failure scenarios of data center cooling infrastructures. In such a scenario, predicting the transient thermal response of the CRAH unit depends requires the determination of the CRAH lumped capacitance model parameters: the thermal capacitance (thermal mass) and the time constant. In this paper, we propose an experimental methodology to extract sufficient information for the lumped capacitance modeling of CRAH units. The method requires measurements of inlet and exit air temperature, air flow rate and CRAH fan power. If the chilled water supply to a CRAH unit is intentionally interrupted in a data center with multiple redundant CRAH units, sufficient information to estimate the CRAH lumped capacitance parameters can be obtained without disturbing the data center operation.