Schedule of valuation and qualifying (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Schedule of valuation and qualifying
Balance at Beginning of Period	$ 547,000	$ 521,000	$ 561,000
Charged to Costs and Expenses	(48,000)	24,000	(44,000)
Other Increases (Decreases)	26,000	2,000	4,000
Balance at end of Period	473,000	547,000	521,000
Allowance for obsolete inventory
Schedule of valuation and qualifying
Balance at Beginning of Period	1,230,000	950,000	914,000
Charged to Costs and Expenses	349,000	408,000	187,000
Other Increases (Decreases)	831,000	128,000	151,000
Balance at end of Period	748,000	1,230,000	950,000
Allowance for deferred taxes
Schedule of valuation and qualifying
Balance at Beginning of Period	1,086,000	10,644,000	10,500,000
Charged to Costs and Expenses	216,000	(9,589,000)	538,000
Other Increases (Decreases)	304,000	31,000	394,000
Balance at end of Period	$ 1,606,000	$ 1,086,000	$ 10,644,000