FIRST AMENDMENT TO REVOLVING CREDIT AGREEMENT

This  First  Amendment  to  Revolving  Credit  Agreement  made  this 20th day of
February  1998,  among HACH  COMPANY,  a Delaware  corporation,  and HACH EUROPE
S.A./N.V.,  A Belgian limited liability  company  (collectively the "Borrower"),
and U.S. BANK NATIONAL ASSOCIATION f/k/a COLORADO NATIONAL BANK ("Lender").

Whereas,  the parties have heretofore entered into that certain Revolving Credit
Agreement dated as of July 7, 1997, (the "Agreement"); and

Whereas, the parties desire to amend the Agreement as hereinafter provided.

Now, therefore, for good and valuable consideration, the receipt and sufficiency
of which is  hereby  acknowledged,  the  parties  hereby  covenant  and agree as
follows:

     1.   Effective January 5, 1998, the following definition is amended in its
          entirety to read as follows:

          "Adjusted Eurodollar Rate" means, for any Interest Rate Determination
          Date, the rate per annum obtained by dividing (i) the London Interbank
          Offered Rate  (expressed as a rate per annum and rounded upward to the
          nearest  1/16 of one percent)  appearing  on the  Telerate  system (as
          quoted  by the  Bank's  standard  administrative  procedures)  on such
          Interest Rate  Determination  Date for U.S. dollar deposits of amounts
          in same day funds comparable to the principal amount of the Eurodollar
          Loan for which the Adjusted  Eurodollar Rate is then being  determined
          with  maturities  comparable  to the  Interest  Period  for which such
          Adjusted Eurodollar Rate will apply by (ii) a percentage equal to 100%
          minus the actual rate of all reserve requirements (including,  without
          limitation, any marginal,  emergency,  supplemental,  special or other
          reserves)   actually  imposed  on  the  Bank  on  such  Interest  Rate
          Determination Date.

     2.   The following is added as Section 8.7(g):

          (g)  Liens of an entity  acquired by the  Borrower  which exist at the
               time of acquisition, provided that:

               (i)  such entity is maintained  as a separate  corporate or other
                    limited liability subsidiary; and

               (ii) such lien is nonrecourse to Borrower or its assets.

     3.   Unless otherwise  provided herein,  the defined terms shall have their
          ascribed meanings as provided in the Agreement.

     4.   This Amendment may be executed in counterparts.

     5.   The Agreement,  as amended hereby, and the Note is hereby ratified and
          confirmed between the parties.

In  witness  whereof,   the  parties  have  executed  this  First  Amendment  to
Warehousing  Credit and Security  Agreement  as of the day and year  first-above
written.

COMPANY:

HACH EUROPE S.A./N.V.,
a Belgian limited liability company

By: /s/ Bruce J. Hach
    ----------------------------------------------------
    Bruce J. Hach, President and Chief Executive Officer

HACH COMPANY

By: /s/ Bruce J. Hach
    ----------------------------------------------------
    Bruce J. Hach, President and Chief Executive Officer


LENDER:

U.S. BANK NATIONAL ASSOCIATION
f/n/a COLORADO NATIONAL BANK

By: /s/ Brian T. McKinney
    ---------------------------------
    Brian T. McKinney, Vice President