Exhibit 4.1
                     CONSENT AND WAIVER TO CREDIT AGREEMENT
                     --------------------------------------

     This CONSENT AND WAIVER TO CREDIT  AGREEMENT (this  "Agreement") is entered
into and effective as of January 20, 2006, by and among ESCO TECHNOLOGIES  INC.,
a Missouri  corporation  ("Borrower"),  WELLS FARGO BANK,  NATIONAL  ASSOCIATION
("Wells Fargo"), as Administrative  Agent  ("Administrative  Agent"),  and Wells
Fargo  and  the  other  lenders  listed  on  the  signature  pages  hereto  (the
"Lenders").

                                    Recitals:
                                    ---------

A.   Borrower,  Administrative  Agent and Lenders have  heretofore  entered into
     that certain Credit  Agreement dated as of October 6, 2004, as amended from
     time to time (as amended,  the "Credit  Agreement;" all  capitalized  terms
     used and not otherwise  defined in this Agreement shall have the respective
     meanings  ascribed  to them in the  Credit  Agreement  as  amended  by this
     Agreement).

B.   Administrative  Agent,  Lenders and Borrower have agreed to the  provisions
     set forth herein on the terms and conditions contained herein.

                                    Agreement
                                    ---------

     Therefore,  in  consideration  of the  mutual  agreements  herein and other
sufficient consideration, the receipt of which is hereby acknowledged, Borrower,
Administrative Agent and Lenders hereby agree as follows:

1. Consent. Borrower has notified Administrative Agent and Lenders regarding the
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possible   acquisition  of  Hexagram,   Inc.,  an  Ohio  corporation,   by  ESCO
Technologies  Holding Inc. (the "Hexagram  Acquisition") for an initial purchase
price  not to  exceed  $70,000,000.00  plus  contingent  payments  not to exceed
$7,500,000.00 payable over five (5) years (collectively,  the "Purchase Price").
Section 5.14 of the Credit  Agreement  provides  that  Borrower will not, and it
will not cause or permit any Subsidiary to,  consummate any  Acquisitions  other
than  Permitted  Acquisitions.  The Purchase Price may exceed the limitation set
forth in subsection (e) of the definition of Permitted Acquisition. Furthermore,
the Purchase  Price,  when aggregated with the purchase price paid in connection
with the acquisition by ESCO Technologies Holding Inc. of Nexus Energy Software,
Inc.,   a   Massachusetts   corporation,   on  November  29,  2005  (the  "Nexus
Acquisition"),  exceeds  the  limitation  set  forth  in  subsection  (f) of the
definition of Permitted Acquisition. At the request of Borrower,  Administrative
Agent and Lenders hereby (a) waive the application of subsections (e) and (f) of
the definition of Permitted Acquisition to the Hexagram Acquisition, (b) consent
to the consummation of the Hexagram  Acquisition for consideration not exceeding
the Purchase Price,  subject to the  fulfillment of the remaining  conditions of
the definition of Permitted  Acquisition,  and (c) agree to exclude the Hexagram
Acquisition and Nexus  Acquisition  from the annual  Acquisition  limitation set
forth in subsection  (f) of the definition of Permitted  Acquisition;  provided,
however,  that (1)  contemporaneously  with  the  execution  of this  Agreement,
Borrower delivers, or causes to be delivered, to Administrative Agent, a Joinder
Agreement executed by Nexus Energy Software, Inc. in the form attached hereto as
Exhibit A (the "Nexus  Joinder") and (2) within ten (10) Business Days following
the closing date of the Hexagram  Acquisition,  Borrower  agrees to deliver,  or
cause to be delivered,  to Administrative Agent, a Joinder Agreement executed by
Hexagram,  Inc.  in the  form  attached  hereto  as  Exhibit  B  (the  "Hexagram
Joinder"). Borrower further agrees to deliver, or cause to be delivered, any and
all other  documentation,  including,  without  limitation,  certified corporate
documents,   resolutions  and  legal  opinions,   as  Administrative  Agent  may
reasonably  require  in  connection  with the  Nexus  Joinder  and the  Hexagram
Joinder, all of which must be in form and substance  reasonably  satisfactory to
Administrative  Agent.  Borrower further  acknowledges and agrees that Hexagram,
Inc. and Nexus Energy  Software,  Inc. will each become a "Domestic  Subsidiary"
and "Material Subsidiary" as such terms are defined in the Credit Agreement, and
it  will  cause  such  Subsidiaries  to  execute  any  and  all  such  documents
(including,  without limitation, the Joinder Agreements described above), as are
required  pursuant  to Section  5.17 of the Credit  Agreement.  The  consent and
waiver granted herein constitute the consent and waiver of Administrative  Agent
and Lenders only for the specific  purposes herein  described and upon the terms
and  conditions  set forth herein and shall not be deemed a consent to or waiver
of Section 5.14 or any other  provisions  of the Credit  Agreement for any other
Acquisition,  transaction  or  purpose  prohibited  by the  terms of the  Credit
Agreement or any other Loan Document.

2. Representations and Warranties of  Borrower.  Borrower  hereby represents and
   -------------------------------------------
warrants to  Administrative  Agent and Lenders  that (i)  Borrower's  execution,
delivery and  performance  of this  Agreement  has been duly  authorized  by all
requisite  action of  Borrower,  (ii) no consents are  necessary  from any third
parties for Borrower's  execution,  delivery or  performance of this  Agreement,
(iii)  this  Agreement,  the  Credit  Agreement,  and  each  of the  other  Loan
Documents,  constitute  the legal,  valid and  binding  obligations  of Borrower
enforceable  against  Borrower in  accordance  with their  terms,  except to the
extent  that the  enforceability  thereof  against  Borrower  may be  limited by
bankruptcy,  insolvency or other laws affecting the  enforceability of creditors
rights generally or by equity principles of general application, (iv) all of the
representations  and warranties  contained in Article IV of the Credit Agreement
are true and correct  with the same force and effect as if made on and as of the
date of this Agreement,  (v) after giving effect to this Agreement,  there is no
Event of Default,  (vi) since the  Effective  Date,  there has been no change or
modification  to  the  organizational   documents  of  Borrower,  any  Borrowing
Subsidiary or any Guarantor  Subsidiary,  (vii) since the Effective Date,  there
has  been no  change  in the  financial  condition  or  business  operations  of
Borrower,  any  Borrowing  Subsidiary or any  Guarantor  Subsidiary  which could
reasonably be expected to result in a Material Adverse Effect,  (viii) there are
no  proceedings  of any  kind,  pending  or  threatened  against  Borrower,  any
Borrowing  Subsidiary or any  Guarantor  Subsidiary,  which could  reasonably be
expected  to result in a Material  Adverse  Effect,  and( ix) there are no Liens
with respect to Borrower or its Subsidiaries or any of their respective  assets,
except for those Liens permitted by Section 5.11 of the Credit Agreement.

3.  Reaffirmation.   Borrower  hereby  represents,  warrants,  acknowledges  and
    -------------
confirms that (i) the Credit  Agreement and the other Loan  Documents  remain in
full force and effect,  (ii) Borrower has no defenses to its  obligations  under
the Credit  Agreement and the other Loan  Documents,  and (iii)  Borrower has no
claim against  Administrative  Agent or any Lender arising from or in connection
with the  Credit  Agreement  or the other Loan  Documents  and any such claim is
hereby irrevocably waived and released and discharged forever.

4.  Definitions.  All references in the Credit Agreement to "this Agreement" and
    -----------
any  other  references  of  similar  import  shall  henceforth  mean the  Credit
Agreement as amended by this Agreement.

5.  Effectiveness of Agreement.  This Agreement shall become  effective,  unless
    --------------------------
otherwise  stated herein,  as of the date first written above,  but only if this
Agreement  has been executed by Borrower,  Administrative  Agent and Lenders and
acknowledged by each Guarantor.

6. Governing Law. This  Agreement  shall be governed by and construed  under the
   -------------
laws of the State of Missouri  without  giving  effect to choice or conflicts of
law principles thereunder.

7. Section Titles.  The section titles in this  Agreement are for convenience of
   --------------
reference only and shall not be construed so as to modify any provisions of this
Agreement.

8. Counterparts;  Facsimile Transmissions. This Agreement may be executed in one
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or more counterparts and on separate counterparts, each of which shall be deemed
an  original,  but all of  which  together  shall  constitute  one and the  same
instrument.  Signatures  to this  Agreement  may be given by  facsimile or other
electronic transmission, and such signatures shall be fully binding on the party
sending the same.

9. Incorporation By Reference. Administrative Agent, Lenders and Borrower hereby
   --------------------------
agree that all of the terms of the Loan Documents are incorporated in and made a
part of this Agreement by this reference.

10. Fees and Expenses.  Borrower shall promptly pay to Administrative  Agent all
    -----------------
fees, expenses and other amounts owing to Administrative  Agent under the Credit
Agreement and the other Loan Documents, including, without limitation, all fees,
costs and  expenses  incurred by  Administrative  Agent in  connection  with the
preparation, negotiation, execution, and delivery of this Agreement.

11. No Oral Agreements;  Entire Agreement.   This notice is provided pursuant to
    -------------------------------------
Section 432.047 R.S.Mo. As used herein,  "creditor" means  Administrative  Agent
and Lenders and "this writing" means this Agreement,  the Credit Agreement,  and
the other Loan Documents.  ORAL AGREEMENTS OR COMMITMENTS TO LOAN MONEY,  EXTEND
CREDIT OR TO FORBEAR FROM ENFORCING  REPAYMENT OF A DEBT  INCLUDING  PROMISES TO
EXTEND OR RENEW SUCH DEBT ARE NOT  ENFORCEABLE,  REGARDLESS  OF THE LEGAL THEORY
UPON WHICH IT IS BASED THAT IS IN ANY WAY RELATED TO THIS AGREEMENT.  TO PROTECT
YOU (BORROWER(S)) AND US (CREDITOR) FROM MISUNDERSTANDING OR DISAPPOINTMENT, ANY
AGREEMENTS WE REACH  COVERING SUCH MATTERS ARE CONTAINED IN THIS WRITING,  WHICH
IS THE COMPLETE AND EXCLUSIVE  STATEMENT OF THE AGREEMENT  BETWEEN US, EXCEPT AS
WE MAY LATER AGREE IN WRITING TO MODIFY IT.

12. Patriot Act Notice. Administrative Agent and each Lender hereby notifies the
    ------------------
Borrower  and each  Guarantor  that,  pursuant  to the  requirements  of the USA
Patriot Act, Title III of Pub. L. 107-56,  signed into law October 26, 2001 (the
"Act"), it is required to obtain,  verify and record information that identifies
the Borrower and each Guarantor, which information includes the name and address
of the  Borrower  and each  Guarantor  and other  information  that  will  allow
Administrative Agent or such Lender, as applicable, to identify the Borrower and
each Guarantor in accordance with the Act.

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<PAGE>

IN WITNESS  WHEREOF,  this Agreement has been duly executed as of the date first
above written.

ESCO TECHNOLOGIES, INC., a Missouri corporation, as Borrower

By:      /s/ V.L. Richey, Jr.
Name:
Title:


WELLS FARGO BANK, NATIONAL ASSOCIATION,
as Administrative Agent and a Lender


By:      /s/ Kevin L. Handley
Name:             Kevin L. Handley
Title:            Vice President


BANK OF AMERICA, N.A., Lender


By:      /s/ Jason R. Hickey
Name:             Jason R. Hickey
Title:            Senior Vice President


LASALLE BANK NATIONAL ASSOCIATION, Lender


By:      /s/ Michelle Dacey
Name:             Michelle Dacey
Title:            Commercial Banking Officer


JPMORGAN CHASE BANK, NATIONAL ASSOCIATION, Lender


By:      /s/ Christopher C. Cavaiani
Name:             Christopher Cavaiani
Title:            Vice President


COMMERCE BANK, N.A., Lender


By:      /s/ Mark Winker
Name:             Mark Winker
Title:            Vice President

<PAGE>

THE NORTHERN TRUST COMPANY, Lender


By:      /s/ David Sullivan
Name:             David Sullivan
Title:            Vice President


Acknowledged and Agreed to as of __________________, 2006.


COMTRAK TECHNOLOGIES, L.L.C., a Missouri limited liability company


By:      /s/ V.L. Richey, Jr.
Name:
Title:


DISTRIBUTION CONTROL SYSTEMS, INC., a Missouri corporation


By:      /s/ V.L. Richey, Jr.
Name:
Title:


ETS-LINDGREN L.P., a Texas limited partnership

By:  RANTEC COMMERCIAL, INC.


By:      /s/ V.L. Richey, Jr.
Name:
Title:


ESCO TECHNOLOGIES HOLDING INC., a Delaware corporation


By:      /s/ V.L. Richey, Jr.
Name:
Title:


<PAGE>


FILTERTEK INC., a Delaware corporation


By:      /s/ V.L. Richey, Jr.
Name:
Title:


LINDGREN, INC., a Delaware corporation


By:      /s/ V.L. Richey, Jr.
Name:
Title:


LINDGREN R.F. ENCLOSURES, INC., an Illinois corporation


By:      /s/ V.L. Richey, Jr.
Name:
Title:


PTI TECHNOLOGIES INC., a Delaware corporation


By:      /s/ V.L. Richey, Jr.
Name:
Title:


RANTEC HOLDINGS, INC., a Missouri corporation


By:      /s/ V.L. Richey, Jr.
Name:
Title:


RANTEC COMMERCIAL, INC., a California corporation


By:      /s/ V.L. Richey, Jr.
Name:
Title:


VACCO INDUSTRIES, a California corporation


By:      /s/ V.L. Richey, Jr.
Name:
Title:
<PAGE>

Exhibit A


                                JOINDER AGREEMENT
                                -----------------


     THIS JOINDER AGREEMENT (the "Agreement"),  dated as of January 20, 2006, is
by and between NEXUS ENERGY  SOFTWARE,  INC., a Massachusetts  corporation  (the
"Subsidiary"),  and WELLS FARGO BANK, NATIONAL  ASSOCIATION,  in its capacity as
Administrative  Agent under that certain Credit Agreement dated as of October 6,
2004, among ESCO  Technologies,  Inc., a Missouri  corporation (the "Borrower"),
the Lenders identified therein and Wells Fargo Bank,  National  Association,  as
Administrative  Agent (as the same may from time to time be  amended,  modified,
extended, renewed or restated, the "Credit Agreement"). All of the defined terms
in the Credit Agreement are incorporated herein by reference.

     Reference is further made to that certain  Continuing  Guaranty dated as of
October 6, 2004 by and among Comtrak Technologies,  L.L.C., Distribution Control
Systems,  Inc.,  ETS-Lindgren  L.P., ESCO Technologies  Holding Inc.,  Filtertek
Inc.,  Lindgren,  Inc., Lindgren R.F.  Enclosures,  Inc., PTI Technologies Inc.,
Rantec  Holdings,  Inc.,  Rantec  Commercial,  Inc.,  and Vacco  Industries,  as
Guarantors,  in favor of Administrative  Agent and Lenders (as the same may from
time  to  time  be  amended,  modified,   extended,  renewed  or  restated,  the
"Guaranty").

     The Borrower is required by Section  5.17 of the Credit  Agreement to cause
the  Subsidiary  to  guaranty  the  payment  and   performance  of  all  of  the
Obligations.

     Accordingly,  the Subsidiary hereby agrees with the  Administrative  Agent,
for the benefit of the Lenders as follows:

     1. The  Subsidiary  hereby  acknowledges,  agrees and confirms that, by its
execution of this Agreement,  the Subsidiary will be deemed to be a party to the
Guaranty and a "Guarantor" for all purposes of the Guaranty,  and shall have all
of the obligations of a Guarantor thereunder as if it had executed the Guaranty.
The Subsidiary  hereby ratifies,  as of the date hereof,  and agrees to be bound
by, all of the terms,  provisions  and  conditions  applicable to the Guarantors
contained in the  Guaranty.  Without  limiting the  generality  of the foregoing
terms,  the  Subsidiary  hereby  absolutely  and  unconditionally   jointly  and
severally  guarantees  to each  Lender,  the Swing  Line  Lender,  the  Offshore
Currency  Fronting  Lender,  the Letter of Credit Issuer and the  Administrative
Agent  the  prompt  and  complete  payment  when due in  accordance  with  their
respective  terms  (whether  by  reason of  demand,  maturity,  acceleration  or
otherwise)  of any  and  all  of the  Obligations  (whether  heretofore,  now or
hereafter  made,  incurred or created,  whether  voluntary  or  involuntary  and
however arising, absolute or contingent, liquidated or unliquidated,  determined
or undetermined,  whether the Borrower or any Borrowing Subsidiary may be liable
individually or jointly with others and whether  recovery upon such  Obligations
may be or hereafter  becomes  unenforceable)  in  accordance  with the terms and
conditions  hereof.  In addition,  the Subsidiary shall and agrees to be jointly
and  severally  liable to each  Lender,  the Swing  Line  Lender,  the  Offshore
Currency  Fronting  Lender,  the Letter of Credit Issuer and the  Administrative
Agent  for all costs and  expenses  incurred  by such  Person in  attempting  or
effecting  collection  under the Guaranty  (whether or not  litigation  shall be
commenced in aid thereof) and in connection with  representation  of such Person
in connection with bankruptcy or insolvency proceedings relating to or affecting
the Guaranty,  including,  without  limitation,  reasonable  attorneys' fees and
expenses.

     2. This  Agreement  may be  executed in two or more  counterparts,  each of
which shall  constitute an original but all of which when taken  together  shall
constitute one contract.

     3. This  Agreement  shall be governed by and construed and  interpreted  in
accordance  with the laws of the State of  Missouri.

     IN WITNESS WHEREOF,  the Subsidiary has caused this Joinder Agreement to be
duly executed by its authorized officers,  and the Administrative Agent, for the
benefit of the  Lenders,  has caused the same to be accepted  by its  authorized
officer, as of the day and year first above written.


                                            NEXUS ENERGY SOFTWARE, INC.


                                            By:      s/ V. L. Richey
                                            Name:    V. L. Richey
                                            Title:   Chairman and CEO


                                            Acknowledged and accepted:


                                            WELLS FARGO BANK,
                                            NATIONAL ASSOCIATION
                                            as Administrative Agent


                                            By:      s/Kevin L. Handley
                                            Name:    Kevin L. Handley
                                            Title:   Vice President

<PAGE>


Exhibit B

                             SECRETARY'S CERTIFICATE
                                       OF
                           NEXUS ENERGY SOFTWARE, INC.

     I am Secretary of Nexus Energy Software, Inc., a Massachusetts  corporation
(the "Company"), and do hereby certify further that:

     1. Attached hereto as Exhibit A is a true and complete copy of the Articles
of Organization of the Company certified by the Secretary of the Commonwealth of
Massachusetts and in effect on this date.

     2. Attached  hereto as Exhibit B is a true and complete copy of the By-Laws
of the Company as in effect on this date.

     3. Each persons named in Exhibit C hereto is a duly elected,  qualified and
acting officer or authorized signatory of the Company, holding the office in the
Company  set  forth  opposite  his or her name on such  Exhibit,  and set  forth
opposite his or her name on such Exhibit is the genuine signature of such person
or a true facsimile of such genuine signature.

     4. No further amendment to the Articles of Incorporation of the Company has
been approved by its Board of Directors or  stockholders  or has been filed with
the Secretary of the Commonwealth of Massachusetts.

     5. The Company is in good standing as a  corporation  under the laws of the
Commonwealth of  Massachusetts,  no proceeding has been commenced or to the best
of my  knowledge is  contemplated  for the  dissolution  or  liquidation  of the
Company.

     6. Attached  hereto as Exhibit D is a true and complete copy of resolutions
duly  adopted by  unanimous  written  consent of the Board of  Directors  of the
Company  authorizing the transactions  contemplated  therein,  which resolutions
have not been amended and are in full force and effect on this date.

     IN  WITNESS  WHEREOF,  the  undersigned  has  caused  this  Certificate  of
Secretary to be executed on January 20, 2006.


(Seal)                                   s/Alyson S. Barclay
                                         Alyson S. Barclay, Secretary

     I am Vice  President of the Company.  I hereby  certify that on the date of
the Certificate set forth above Alyson S. Barclay is the duly elected, qualified
and acting  Secretary of the Company and that the  signature  set forth above is
his genuine signature.

                                         s/V. L. Richey
                                         V. L. Richey, ____________