Note 3 - Revenue 1 (Details Textual) Pure in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2022 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2021 USD ($)
Contract with Customer, Liability, Current	$ 522		$ 390
Contract with Customer, Liability, Revenue Recognized	$ 230	$ 108
Contract with Customer, Contracts in Progress, Number of Contracts Identified as Loss Contracts	0		0
Revenue, Remaining Performance Obligation, Amount	$ 9,552
Air Pollution Control [Member]
Contract with Customer, Asset, after Allowance for Credit Loss, Total	$ 2,222		$ 1,277