A turbulator for promoting mixing of fluids passing through a body lumen includes an elongate flow perturbing component and an elongate shape controlling component freely slidable within the flow perturbing component. The method includes receiving the customer data at a point of sale system and, during a check out process: transmitting the customer data to a server; searching a database for the customer data; and based upon results of the searching, performing a credit worthiness check and providing said customer with an invitation to open a charge account. For helical magnetic recording systems, magnetic layers with a coercivity of at least 2,000 Oe with underlayers having a coercivity between 25 and 800 Oe are used. Upon receiving the frequency difference signal, the control device, preferably operating within a controlled-feedback loop, will begin to adjust the underlying clock frequency to approximate the incoming data frequency.