<PAGE>
 
                                                                   Exhibit 10.14

                FIRST AMENDMENT TO AGREEMENT AND PLAN OF MERGER

          This First Amendment to Agreement and Plan of Merger (this
"AMENDMENT") is made and entered into as of August 5, 1998, by and among United
States Filter Corporation, a Delaware corporation ("PARENT"), Kinetics
Acquisition Corp., a California corporation and a wholly owned subsidiary of
Parent ("SUBCORP"), and Unit Instruments, Inc., a California corporation
("UNIT").

                             PRELIMINARY STATEMENTS

          A.  The parties entered into an Agreement and Plan of Merger dated as
of July 2, 1998 (the "MERGER AGREEMENT").

          B.  The parties have decided to amend the Merger Agreement by
amending, restating and supplementing certain provisions of the Merger Agreement
as set forth in this Amendment.

                                   AGREEMENT

          Now, therefore, in consideration of these premises and the mutual and
dependent promises hereinafter set forth, the parties hereto agree as follows:

          SECTION 1.  Section 2.2(a) of the Merger Agreement is amended and
          ---------                                                        
restated in its entirety as follows:

                      "(a) The "EXCHANGE RATIO" shall equal $11.03 divided by
     the Parent Share Value. The "PARENT SHARE VALUE" shall be either (i) the
     average of the closing price per share for shares of Parent Common Stock as
     reported on the New York Stock Exchange ("NYSE") Composite Tape ("NYSE
     COMPOSITE TAPE") on each of the twenty (20) consecutive trading days ending
     on the fifth (5th) trading day preceding the Closing Date (the "AVERAGE
     SHARE PRICE") if the Average Share Price is above $26.00, or (ii) $26.00 if
     the Average Share Price is equal to or less than $26.00. No certificates
     for fractional Shares of Parent Common Stock shall be issued as a result of
     the conversion provided for in Section 2.1(b)."

          SECTION 2.  In order to induce Subcorp and Parent to enter into this
          ---------                                                           
Amendment, Unit hereby represents and warrants to Parent and Subcorp as follows:

               (a) Unit has received the oral advice of Needham & Company, its
     financial advisors, to the effect that, as of the date of this Amendment,
     the Exchange Ratio is fair to the Unit Shareholders from a financial point
     of view. Such oral advice has not been withdrawn or revoked or modified in
     any material respect and is to be confirmed in a written fairness opinion
     of Needham & Company, which opinion will be dated as of the date of this
     Amendment. Unit will promptly provide copies of such opinion to Parent.
<PAGE>
 
               (b) The Board of Directors of Unit has authorized this Amendment
     and, by unanimous written consent, (i) determined that the Merger
     Agreement, as amended by this Amendment, and the transactions contemplated
     thereby and hereby, including the Merger, taken together, are fair to and
     in the best interests of the Unit Shareholders, and (ii) resolved to
     recommend that the Unit Shareholders approve the Merger Agreement, as
     amended by the Amendment, and the transactions contemplated therein and
     herein, including the Merger (which recommendation shall be deemed part of
     the Unit Board Recommendation).

          SECTION 3.  In addition to the conditions set forth in Section 6.3 of
          ---------                                                            
the Merger Agreement, the obligations of Parent and Subcorp to consummate the
Merger and the other transactions contemplated by the Merger Agreement, as
amended by this Amendment, shall be subject to the fulfillment of the following
conditions unless waived by Parent:

               (a) The representations and warranties of Unit set forth in this
     Amendment shall be true and correct in all material respects on and as of
     the Closing Date.

               (b) Needham & Company shall have waived, in form and substance
     reasonably acceptable to Parent and Subcorp, all of its registration rights
     under that certain Warrant to Purchase 100,000 Shares of Common Stock of
     Autoclave Engineers, Inc. dated as of October 17, 1994.

          SECTION 4.  In order to induce Unit to enter into this Amendment,
          ---------                                                        
Parent and Subcorp agree with Unit that, except as set forth in or contemplated
by Section 3 above, and notwithstanding the delivery to Unit of the letter from
Parent's counsel dated July 31, 1998 pursuant to Section 6.3(f) of the Merger
Agreement (the "INVESTIGATION NOTICE"):

               (a) The condition to Parent's and Subcorp's obligations set forth
     in Section 6.3(f) of the Merger Agreement shall be deemed to be satisfied;

               (b) The matters set forth in the Investigation Notice, to the
     extent of information pertaining thereto that has been specifically
     disclosed by Unit to Parent in the draft of Unit's annual report on Form 
     10-K delivered by Unit to Parent on July 22, 1998, in Unit's press release
     dated July 17, 1998 announcing certain workforce reductions (the "PRESS
     RELEASE"), or in other written material delivered by Unit to Parent and
     Subcorp prior to Parent's execution of this Amendment (collectively, the
     "PREVIOUSLY DISCLOSED INFORMATION"), shall be deemed to amend and
     supplement the Unit Disclosure Schedules for all purposes under Article IV
     of the Merger Agreement;

               (c) Parent and Subcorp hereby waive any breach of Unit's
     covenants under Section 5.1(c) or 5.3(c) of the Merger Agreement with
     respect to the issuance by Unit of the Press Release and the taking by Unit
     of actions specified in and consistent with the Press Release;

                                       2

<PAGE>
 
               (d) Unit shall be deemed to have complied with its notice
     obligations under Section 5.3(h) of the Merger Agreement with respect to
     the Previously Disclosed Information; and

               (e) The parties further agree that (i) for purposes of the
     condition set forth in Section 6.3(d) of the Merger Agreement, the
     Previously Disclosed Information shall be deemed to have been disclosed by
     Unit to Parent not less than five (5) business days prior to the
     Investigation Date and (ii) any deterioration in Unit's prospects or
     operating results after July 22, 1998, to the extent such deterioration is
     attributable to a decline after such date in the market demand for
     semiconductor manufacturing equipment, shall not be deemed a Material
     Adverse Change for the purposes of Section 4.7 or Section 8.3(b) of the
     Merger Agreement.

          SECTION 5--MISCELLANEOUS.
          ------------------------ 

               (a) EFFECT OF AMENDMENT.  The provisions of this Amendment are 
                   -------------------    
     hereby incorporated into and made part of the Merger Agreement. Except as
     amended by this Amendment, all of the provisions of the Merger Agreement
     shall continue in full force and effect.
 
               (b) DEFINITIONS.  Unless otherwise defined in this Amendment, 
                   -----------   
     capitalized terms have the meanings given in the Merger Agreement.

               (c) ENTIRE AGREEMENT.  The Merger Agreement (including the 
                   ----------------   
     documents and the instruments referred to therein), the Confidentiality
     Agreement, and this Amendment constitute the entire agreement among the
     parties and supersede all prior agreements, understandings and
     representations by or among the parties, written and oral, with respect to
     the subject matter hereof and thereof.

               (d) COUNTERPARTS.  This Amendment may be executed in 
                   ------------   
     counterparts, which shall constitute one and the same instrument. The
     parties may execute more than one copy of this Amendment, each of which
     shall constitute an original.
 
                                       3
<PAGE>
 
          IN WITNESS WHEREOF, Parent, Subcorp and Unit have signed this
Amendment as of the date first written above.

                                    "PARENT"

                                    UNITED STATES FILTER CORPORATION, a 
                                    Delaware corporation

                               

                                           By: ______________________________
                                           Name: ____________________________
                                           Title: ___________________________

                                    "SUBCORP"

                                    KINETICS ACQUISITION CORP.,
                                    a California corporation



                                           By: ______________________________
                                           Name: ____________________________
                                           Title: ___________________________

                                    "UNIT"

                                    UNIT INSTRUMENTS, INC., a California 
                                    corporation



                                           By: ______________________________
                                           Name: ____________________________
                                           Title: ___________________________

                                       4