<PAGE>   1
                                                                    EXHIBIT 10.9


                            SECURED PROMISSORY NOTE



     $285,601.00                  Houston, Texas             April 9, 1998



         FOR VALUE RECEIVED, B. Tom Carter, Jr. ("Maker") hereby promises to
pay to the order of HydroChem Industrial Services, Inc., a Delaware corporation
("Payee"), the principal amount of Two Hundred Eighty Five Thousand Six Hundred
One and 00/100 Dollars ($285,601.00) plus interest thereon at the rate of Five
and 70/100 percent (5.7%) per annum, payable as provided herein in lawful money
of the United States of America, at Houston, Texas, or at such other place as
Payee of this Note may from time to time designate by written notice to Maker.

         The principal amount of this Note shall be due and payable upon the
earlier of April 8, 2004 or any termination of Maker's employment, but not
earlier than two years from the date hereof.  Accrued interest payments in the
amount of Sixteen Thousand Two Hundred Seventy Nine and 25/100 Dollars
($16,279.25) each shall be due and payable on April 8, 1999 and each April 8
thereafter through and including April 8, 2004.

         Pursuant to a Pledge Agreement of even date herewith between Maker and
Payee, and as more specifically described therein, this Note is secured by the
pledge of certain common stock of HydroChem Holding, Inc. which stock is owned
beneficially and of record by Maker.

         This Note may be prepaid in whole or in part without premium or
penalty.

         If this Note is placed in the hands of an attorney for collection
after default, or if all or any part of the indebtedness represented hereby is
proved, established or collected in any court or in any bankruptcy,
receivership, debtor relief, probate or other court proceedings, then Maker
agrees to pay reasonable attorneys' fees and collection costs to Payee in
addition to the principal and interest payable hereunder.

         If default is made in the payment of the principal or interest under
this Note, or if a default occurs under any other instrument evidencing or
securing payment hereof, then in any one or more such events the entire
principal balance and accrued interest owing hereon shall at once become due
and payable, at the option of Payee.  Failure to exercise this option shall not
constitute a waiver of the right to exercise the same in the event of any
subsequent default.

         Maker, signers, sureties, and endorsers of this Note severally waive
notice of acceleration of maturity if such shall occur, demand, presentment,
notice of dishonor, diligence in collecting, grace, notice and protest, and
agree to one or more extensions for any period or periods of time and partial
payments, before or after maturity, without prejudice to Payee.


                                 Page 1 of 2
<PAGE>   2
                                                                   EXHIBIT 10.10



         THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE
LAWS OF THE STATE OF TEXAS.


                                        MAKER:
                                      
                                      
                                        /s/ B. TOM CARTER, JR.
                                        -------------------------------------
                                        B. Tom Carter, Jr.
                                      


                                 Page 2 of 2