Exhibit 10.7
                                                                    ------------


                 FOURTH AMENDMENT TO REVOLVING CREDIT, TERM LOAN
                             AND SECURITY AGREEMENT

     THIS FOURTH AMENDMENT TO REVOLVING CREDIT, TERM LOAN AND SECURITY AGREEMENT
(the  "Agreement") is entered into as of March 29, 2002 by and among  RHEOMETRIC
SCIENTIFIC,  INC.,  a  corporation  organized  under  the  laws of the  State of
Delaware ("RSI"),  RHEOMETRIC  SCIENTIFIC LIMITED, a company organized under the
laws of England and Wales ("RSL"),  RHEOMETRIC SCIENTIFIC FRANCE SARL, a limited
liability  company  organized  under  the  laws of  France  ("RSF"),  RHEOMETRIC
SCIENTIFIC  GmbH, a  corporation  organized  under the laws of Germany  ("RSG"),
RHEOMETRIC SCIENTIFIC F.E. LTD., a corporation organized under the laws of Japan
("RSLFE") , PROTEIN SOLUTIONS HOLDINGS,  INC., a corporation organized under the
laws of the State of Delaware ("PSI"),  PROTEIN  SOLUTIONS,  INC., a corporation
organized  under the laws of the  Commonwealth  of  Virginia  ("Protein  Inc."),
PROTEIN  SOLUTIONS  LIMITED,  a company  organized under the laws of England and
Wales ("Protein Ltd.") and AVIV INSTRUMENTS, INC., a corporation organized under
the laws of the State of Delaware  ("Aviv")  (collectively  the  "Borrowers" and
individually  each a "Borrower"),  the financial  institutions  which are now or
which  hereafter  become  a  party  hereto  (collectively,   the  "Lenders"  and
individually a "Lender") and PNC BANK,  NATIONAL  ASSOCIATION  ("PNC"), as agent
for Lenders (PNC, in such capacity, the "Agent").

                                    RECITALS

     Whereas,  the Borrowers and PNC entered into a Revolving Credit,  Term Loan
and  Security  Agreement  dated March 6, 2000,  as amended by a First  Amendment
thereto dated August 31, 2000 a Second  Amendment  thereto dated March 16, 2001,
and a Third Amendment thereto dated June 1, 2001 (as may be further amended, the
"Loan Agreement"); and

     Whereas,  Borrowers  and PNC have  agreed to  modify  the terms of the Loan
Agreement as set forth in this Agreement.

     Now, therefore, in consideration of PNC's continued extension of credit and
the agreements contained herein, the parties agree as follows:

                                    AGREEMENT

1)   ACKNOWLEDGMENT  OF  BALANCE.  Borrower  acknowledges  that the most  recent
     statement of account sent to Borrower  with respect to the  Obligations  is
     correct.

2)   MODIFICATIONS. The Loan Agreement be and hereby is modified as follows:


         A. The following  definition is hereby added to Section 1.2 of the Loan
Agreement to read as follows:


              "Net Worth" at a  particular  date,  shall mean (a) the  aggregate
              amount  of  all  assets  of  each  Borrower  as  may  properly  be
              classified as such in accordance  with GAAP  consistently  applied
              and such other assets as are properly  classified  as  "intangible
              assets", less (b) the aggregate amount of all Indebtedness of each
              Borrower.


         B.  Section 6.5 of the Loan  Agreement  be and hereby is  deleted,  and
replaced with a new Section 6.5, to read as follows:


                   6.5 Fixed  Charge  Coverage  Ratio.  Maintain a Fixed  Charge
              Coverage  Ratio of not  less  than  .20 to 1 for the  three  month
              period ending  03/31/02;  .65 to 1 for the six month period ending
              06/30/02, .85 to 1 for the nine month period ending 09/30/02, 1.10
              to 1 for the twelve month period  ending  12/31/02,  and maintain,
              thereafter,  on a quarterly basis at each quarter end on a rolling
              four-quarter basis, a Fixed Charge Coverage Ratio of not less than
              1.10 to 1.



<PAGE>

         C.  Section  6.9 is  hereby  added  to the  Loan  Agreement  to read as
follows:


                   6.9 Minimum Net Worth.  Maintain a Net Worth of not less than
              $2,800,000  for the period  ending  03/31/02;  $2,900,000  for the
              period ending 06/30/02, $3,300,000 for the period ending 09/30/02,
              $3,800,000 for the period ending 12/31/02 and thereafter, measured
              on a quarterly basis at each quarter end.

3)   WAIVER.  The Agent and the Lenders hereby waive the  Borrower's  failure to
     comply with Section 6.5,  the Fixed  Charge  Coverage  Ratio for the period
     ending 12/31/01.

4)   ACKNOWLEDGMENTS. Borrowers acknowledge and represent that:

     (A) the Loan Agreement and other Loan Documents,  as amended hereby, are in
     full force and effect without any defense,  claim,  counterclaim,  right or
     claim of set-off;

     (B) to the best of their  knowledge,  no default by the Agent or Lenders in
     the  performance of their duties under the Loan Agreement or the other Loan
     Documents has occurred;

     (C) all  representations  and warranties of the Borrowers  contained herein
     and in the  other  Loan  Documents  are true and  correct  in all  material
     respects as of this date,  except for any  representation  or warranty that
     specifically refers to an earlier date;

     (D) Borrowers  have taken all  necessary  action to authorize the execution
     and delivery of this Agreement; and

     (E) this Agreement is a modification of an existing obligation and is not a
     novation.

5)   PRECONDITIONS.  As a  precondition  to  the  effectiveness  of  any  of the
     modifications,  consents,  or waivers contained herein, the Borrower agrees
     to:

     (A) provide the Bank with a resolution, in form and substance acceptable to
     the Bank, which approves the modification contemplated hereby.

     (B) pay to the Bank a waiver fee in the amount of $50,000.

     (C) pay all other fees and costs incurred by the Bank in entering into this
     Agreement.

6)   POST-CONDITION.  Within one hundred  twenty  (120) days of the date hereof,
     Borrower  shall have  received a cash  infusion  in an amount not less than
     $1,000,000   in  the  form  of  equity.   Failure   to  comply   with  this
     post-condition  shall  be  deemed  an  Event  of  Default  under  the  Loan
     Agreement.

7)   MISCELLANEOUS.  This  Agreement  shall be construed in accordance  with and
     governed by the laws of the State of New Jersey,  without reference to that
     state's  conflicts of law  principles.  This  Agreement  and the other Loan
     Documents  constitute the sole agreement of the parties with respect to the
     subject  matter  thereof  and  supersede  all oral  negotiations  and prior
     writings with respect to the subject matter  thereof.  No amendment of this
     Agreement,  and no waiver of any one or more of the provisions hereof shall
     be effective  unless set forth in writing and signed by the parties hereto.
     The illegality,  unenforceability or inconsistency of any provision of this
     Agreement   shall  not  in  any  way   affect  or  impair   the   legality,
     enforceability or consistency of the remaining provisions of this Agreement
     or the other Loan  Documents.  This  Agreement and the other Loan Documents
     are intended to be consistent. However, in the event of any inconsistencies
     among  this  Agreement  and any of the Loan  Documents,  the  terms of this
     Agreement,  then the Loan Agreement,  shall control.  This Agreement may be
     executed  in any number of  counterparts  and by the  different  parties on
     separate  counterparts.  Each such counterpart shall be deemed an original,
     but all  such  counterparts  shall  together  constitute  one and the  same
     agreement.

8)   DEFINITIONS.  The terms used herein and not  otherwise  defined or modified
     herein shall have the meanings ascribed to them in the Loan Agreement.  The
     terms used herein and not otherwise  defined or modified  herein or defined
     in the Loan  Agreement  shall  have the  meanings  ascribed  to them by the
     Uniform Commercial Code as enacted in New Jersey.



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<PAGE>

IN WITNESS  WHEREOF,  the undersigned  have signed and sealed this Agreement the
day and year first above written.


ATTEST:                                      RHEOMETRIC SCIENTIFIC, INC.



BY: /s/ Joseph Musanti                       By: /s/ Mark F. Callaghan
    --------------------------                  --------------------------------
       Name:  JOSEPH MUSANTI                    Name:  MARK F. CALLAGHAN
       Title:  Asst. Secretary                  Title:  Executive Vice President


                                             RHEOMETRIC SCIENTIFIC LIMITED



                                             By: /s/ Joseph Musanti
                                                --------------------------------
                                                Name: JOSEPH MUSANTI
                                                Title: Director


                                             RHEOMETRIC SCIENTIFIC FRANCE SARL



                                             By:/s/ Joseph Musanti
                                                --------------------------------
                                                Name:  JOSEPH MUSANTI
                                                Title:  Manager


                                             RHEOMETRIC SCIENTIFIC GmbH



                                             By:/s/ Joseph Musanti
                                                --------------------------------
                                                Name:  JOSEPH MUSANTI
                                                Title:  Under Power of Attorney
                                                from Manfred Herzog,
                                                Managing Director


                                             RHEOMETRIC SCIENTIFIC F.E. LTD.



                                             By:/s/ Joseph Musanti
                                                --------------------------------
                                                Name:  JOSEPH MUSANTI
                                                Title:  Chairman and
                                                Representative Director



ATTEST:                                      PROTEIN SOLUTIONS HOLDINGS, INC.



By: /s/ Joseph Musanti                       By: /s/ Mark F. Callaghan
    --------------------                        --------------------------------
      JOSEPH MUSANTI                            Name:   MARK F. CALLAGHAN
      Asst. Secretary                           Title:  Vice President



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<PAGE>

ATTEST:                                      PROTEIN SOLUTIONS, INC.



By: /s/ Joseph Musanti                       By: /s/ Mark F. Callaghan
    --------------------                        --------------------------------
      JOSEPH MUSANTI                            Name:  MARK F. CALLAGHAN
      Asst. Secretary                           Title:   Vice President


                                             PROTEIN SOLUTIONS LIMITED



                                             By: /s/ Mark F. Callaghan
                                                --------------------------------
                                                Name:  MARK F. CALLAGHAN
                                                Title:   Director


ATTEST:                                      AVIV INSTRUMENTS, INC.



By: /s/ Joseph Musanti                       By: /s/ Mark F. Callaghan
    --------------------                        --------------------------------
      JOSEPH MUSANTI                            Name:  MARK F. CALLALGHAN
       Asst. Secretary                          Title:  Vice President


                                             PNC BANK, NATIONAL ASSOCIATION
                                             Lender and as Agent



                                             By: /s/ Michelle Stanley-Nurse
                                                --------------------------------
                                                Name:  MICHELLE STANLEY-NURSE
                                                Title: Vice President






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