SECOND AMENDMENT TO REVOLVING CREDIT AGREEMENT

This  Second  Amendment  to  Revolving  Credit  Agreement  made this 10th day of
October  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, as amended by the First Amendment dated 20th
day of February, 1998 (collectively 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.   The  introductory  paragraph  of  Section  3.3(b)  is  amended  in its
          entirety to read as follows:

          (b)  Selection  of Interest  Periods.  The  Borrower  shall  select an
               interest period (each, an "Interest  Period") to be applicable to
               each  Eurodollar  Loan,  pursuant  to the  applicable  notice  of
               Borrowing or Notice of  Conversion/Continuation,  as the case may
               be, which Interest Period shall be, at the Borrower's  option,  a
               30-, 60-, 90-, 180- or 360-day period;  provided, that all of the
               following shall be satisfied with respect thereto:

     2.   Section 8.6(f) is amended in its entirety to read as follows:

          (f)  Indebtedness   related  to  Capitalized   Leases   providing  for
               aggregate outstanding liabilities of not more than $5,000,000;

     3.   Section 8.7(e) is amended in its entirety to read as follows:

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

     5.   This Amendment may be executed in counterparts.

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