Property, plant and equipment - Summary of elements of property, plant and equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Property, plant and equipment
Subtotal	$ 59,084	$ 57,405
Less: Accumulated depreciation	(39,709)	(36,837)
Total	19,375	20,568
Land
Property, plant and equipment
Subtotal	1,255	1,255
Land improvements
Property, plant and equipment
Subtotal	$ 319	319
Land improvements | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	6 years
Land improvements | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	10 years
Buildings
Property, plant and equipment
Subtotal	$ 1,446	1,446
Buildings | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	25 years
Buildings | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	39 years
Building improvements
Property, plant and equipment
Subtotal	$ 3,467	3,396
Building improvements | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	25 years
Building improvements | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	39 years
Cylinders
Property, plant and equipment
Subtotal	$ 13,220	13,315
Cylinders | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	15 years
Cylinders | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	30 years
Equipment
Property, plant and equipment
Subtotal	$ 29,397	27,258
Equipment | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	3 years
Equipment | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	10 years
Equipment under capital lease
Property, plant and equipment
Subtotal	$ 315	315
Equipment under capital lease | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	5 years
Equipment under capital lease | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	7 years
Vehicles
Property, plant and equipment
Subtotal	$ 1,790	1,773
Vehicles | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	3 years
Vehicles | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	5 years
Lab and computer equipment, software
Property, plant and equipment
Subtotal	$ 3,233	3,103
Lab and computer equipment, software | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	2 years
Lab and computer equipment, software | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	8 years
Furniture & fixtures
Property, plant and equipment
Subtotal	$ 933	840
Furniture & fixtures | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	5 years
Furniture & fixtures | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	10 years
Leasehold improvements
Property, plant and equipment
Subtotal	$ 865	852
Leasehold improvements | Minimum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	3 years
Leasehold improvements | Maximum
Property, plant and equipment
Property, plant and equipment, Estimated Lives	5 years
Construction-in-progress
Property, plant and equipment
Subtotal	$ 2,844	$ 3,533