Significant accounting policies Long lived assets (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Jan. 31, 2015	Jan. 31, 2014
Long lived assets [Abstract]
Depreciation and amortization expense	$ 5,897	$ 5,785
Land, buildings and improvements	36,493us-gaap_RealEstateGrossAtCarryingValue	36,535us-gaap_RealEstateGrossAtCarryingValue
Machinery and equipment	54,277us-gaap_MachineryAndEquipmentGross	50,793us-gaap_MachineryAndEquipmentGross
Furniture, office equipment and computer systems	10,426us-gaap_FurnitureAndFixturesGross	9,723us-gaap_FurnitureAndFixturesGross
Transportation equipment	192us-gaap_PropertyPlantAndEquipmentOther	206us-gaap_PropertyPlantAndEquipmentOther
Subtotal	101,388us-gaap_PropertyPlantAndEquipmentGross	97,257us-gaap_PropertyPlantAndEquipmentGross
Less accumulated depreciation and amortization	59,368	54,716
Property, plant and equipment, net of accumulated depreciation	42,020us-gaap_PropertyPlantAndEquipmentNet	42,541us-gaap_PropertyPlantAndEquipmentNet
Filtration Products [Member]
Long lived assets [Abstract]
Depreciation and amortization expense	$ 1,710	$ 1,729

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Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

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Amount before accumulated depreciation of equipment commonly used in offices and stores that have no permanent connection to the structure of a building or utilities. Examples include, but are not limited to, desks, chairs, tables, and bookcases.

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Amount before accumulated depreciation of tangible personal property used to produce goods and services, including, but is not limited to, tools, dies and molds, computer and office equipment.

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Amount before accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Amount after accumulated depreciation, depletion and amortization of physical assets used in the normal conduct of business to produce goods and services and not intended for resale. Examples include, but are not limited to, land, buildings, machinery and equipment, office equipment, and furniture and fixtures.

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Amount before accumulated depreciation, depletion and amortization of other physical assets used in the normal conduct of business to produce goods and services and not intended for resale.

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Amount, before accumulated depreciation, of real estate investments for entities with a substantial portion of business acquiring and holding investment real estate.

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