Warranty costs (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Changes in product warranty accrual [Roll Forward]
Warranty accrual, beginning of the period	$ 892,920	$ 580,451	$ 785,412	$ 529,374
Charges to cost and expenses relating to new sales	486,509	193,950	884,690	472,575
Costs of product warranty claims/change of estimate	(354,392)	(258,697)	(645,065)	(486,245)
Warranty accrual, end of period	$ 1,025,037	$ 515,704	$ 1,025,037	$ 515,704