Property, plant and equipment - Summary of Elements of property, plant and equipment (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2022	Dec. 31, 2021
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 54,567	$ 53,854
Accumulated depreciation	35,380	33,761
Total	19,187	20,093
- Land
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	1,255	1,255
- Land improvements
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 319	319
- Land improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	6 years
- Land improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	10 years
- Buildings
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 1,446	1,446
- Buildings | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	25 years
- Buildings | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	39 years
- Building improvements
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 3,099	3,099
- Building improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	25 years
- Building improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	39 years
- Cylinders
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 13,269	13,272
- Cylinders | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	15 years
- Cylinders | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	30 years
- Equipment
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 26,505	26,653
- Equipment | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	3 years
- Equipment | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	10 years
- Equipment under capital lease
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 315	315
- Equipment under capital lease | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	5 years
- Equipment under capital lease | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	7 years
- Vehicles
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 1,773	1,773
- Vehicles | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	3 years
- Vehicles | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	5 years
- Lab and computer equipment, software
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 3,103	3,103
- Lab and computer equipment, software | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	2 years
- Lab and computer equipment, software | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	8 years
- Furniture & fixtures
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 840	837
- Furniture & fixtures | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	5 years
- Furniture & fixtures | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	10 years
- Leasehold improvements
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 852	852
- Leasehold improvements | Minimum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	3 years
- Leasehold improvements | Maximum
Property, Plant and Equipment [Line Items]
Property, plant and equipment, Estimated Lives	5 years
- Equipment under construction
Property, Plant and Equipment [Line Items]
Property plant and equipment gross	$ 1,791	$ 930