Business Segment - Summary of Financial Information by Segment (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Revenue									$ 61,025	$ 58,023	$ 49,715
General and administrative	$ 5,446	$ 5,266	$ 4,927	$ 5,837	$ 4,985	$ 4,034	$ 3,927	$ 4,408	21,476	17,354	16,626
Sales and marketing	1,903	1,873	1,858	1,912	2,703	2,061	2,174	2,453	7,546	9,391	9,116
Research and development	5,220	4,270	3,605	3,917	4,819	3,038	3,077	2,509	17,012	13,443	10,136
Amortization of intangibles	156	158	158	158	158	157	158	158	630	631	631
Total operating expenses	12,725	11,567	10,548	11,824	12,665	9,290	9,336	9,528	46,664	40,819	36,509
Income from operations	1,420	5,650	4,240	(1,331)	4,340	3,550	1,006	353	9,978	9,249	3,426
Non-operating income									1,462	680	287
Income before income taxes									11,440	9,929	3,713
Operating Segments
Segment Reporting Information [Line Items]
General and administrative									3,849	2,966	2,081
Sales and marketing									7,047	8,015	8,061
Research and development									16,987	13,281	10,036
Amortization of intangibles									629	631	631
Total operating expenses									28,512	24,893	20,809
Income from operations									28,130	25,175	19,126
Operating Segments | Water
Segment Reporting Information [Line Items]
General and administrative									2,078	1,401	1,081
Sales and marketing									5,783	5,787	5,076
Research and development									1,711	1,064	1,331
Amortization of intangibles									629	631	631
Total operating expenses									10,201	8,883	8,119
Income from operations									33,100	29,386	23,073
Operating Segments | Oil & Gas
Segment Reporting Information [Line Items]
General and administrative									1,771	1,565	1,000
Sales and marketing									1,264	2,228	2,985
Research and development									15,276	12,217	8,705
Amortization of intangibles									0	0	0
Total operating expenses									18,311	16,010	12,690
Income from operations									(4,970)	(4,211)	(3,947)
Corporate
Segment Reporting Information [Line Items]
Total operating expenses									18,152	15,926	15,700
Product
Segment Reporting Information [Line Items]
Revenue	13,983	18,578	17,406	11,058	21,054	13,860	10,864	12,245	61,025	58,023	49,715
Product cost of revenue	3,561	5,022	5,976	3,314	6,660	4,217	3,572	4,612	17,873	19,061	17,849
Product gross profit	10,422	13,556	11,430	7,744	14,394	9,643	7,292	7,633	43,152	38,962	31,866
Product | Operating Segments
Segment Reporting Information [Line Items]
Revenue									61,025	58,023	49,715
Product cost of revenue									17,873	19,061	17,849
Product gross profit									43,152	38,962	31,866
Product | Operating Segments | Water
Segment Reporting Information [Line Items]
Revenue									60,512	54,301	47,545
Product cost of revenue									17,211	16,032	16,353
Product gross profit									43,301	38,269	31,192
Product | Operating Segments | Oil & Gas
Segment Reporting Information [Line Items]
Revenue									513	3,722	2,170
Product cost of revenue									662	3,029	1,496
Product gross profit									(149)	693	674
License and development revenue
Segment Reporting Information [Line Items]
Revenue	$ 3,723	$ 3,661	$ 3,358	$ 2,749	$ 2,611	$ 3,197	$ 3,050	$ 2,248	13,490	11,106	8,069
License and development revenue | Operating Segments
Segment Reporting Information [Line Items]
Revenue									13,490	11,106	8,069
License and development revenue | Operating Segments | Water
Segment Reporting Information [Line Items]
Revenue									0	0	0
License and development revenue | Operating Segments | Oil & Gas
Segment Reporting Information [Line Items]
Revenue									$ 13,490	$ 11,106	$ 8,069