November 2004 

Split Compartment Test for Vehicle Tank Meters 

By G. Diane Lee 

As fall and winter approach more vehicle tank meters (VTMs) will be in use across the 

U.S. delivering products such as home heating fuels. To ensure that the metering system 

and its associated equipment are operating accurately, the testing of a VTM must 

include, in addition to basic accuracy tests, a special test called the split compartment 

test. This article provides information on how to perform the split compartment test on 

VTMs. 

Because most designs of meters used on VTMs cannot differentiate between product and 

air or vapor passing through the measuring chamber, it is crucial that a VTM be equipped 

with an effective means for eliminating air and vapor from the system (as required by 

NIST Handbook 44 VTM Code, Paragraph S.2.1.). Most VTMs are equipped with a 

device called an air eliminator, which vents air and vapor away from the meter so that the 

meter registers fuel and not air or vapor. This helps to ensure that the customer is not 

paying for air and vapor. While conventional air eliminators are not effective at 

eliminating air that is entrained in the product, they can effectively eliminate air that 

enters the VTM system through the tank, a common source (though not the only source) 

of air in the system. 

The split compartment test examines the capability of the system to prevent air and vapor 

from entering the meter by testing the device under the most demanding circumstance 

when this might occur; this is, when the fuel supply is exhausted and air floods the intake 

lines. Such a condition would occur in commercial use when a tank compartment is 

emptied while a delivery is in progress. 

Multi-Compartment Tanks 

1. Start the test with an amount of product equal to less than one-half the nominal 

capacity of the prover in the largest compartment. 

2. Begin delivery to the prover, drawing product from the largest compartment. Because 

of the extended drain times from the largest compartment, drawing from the largest 

compartment provides a more rigorous test of the system and its capability to prevent the 

registration of air or vapor. 

3. Dispense the product from the tank into the prover at the system’s full flow rate. 

4. When the supply of product is exhausted, the air eliminator should close the intake to 

the meter to prevent registration of air, and the register should stop and hold its 

indications. 

5. If the register does not stop and hold the indications, some air or vapor is leaking 

through the shut-off valve into the meter. 

6. Next, the pump is left engaged while the manifold valve to the empty compartment is 

closed and the valve to a compartment containing a sufficient amount of the same product 
to complete the draft is opened. (Note that the mechanism for switching compartments 

may vary according to the design of the system.) 

7. To complete the test, the delivery is continued until the register indicates the nominal 

capacity of the prover, and the error is recorded.

Single-Compartment Tanks 

1. Start the test with an amount of product equal to less than one-half the nominal 

capacity of the prover in the tank. 

2. Follow steps 3 – 5 above. 

3. Next, additional product must be added to the tank. This additional product may be 

supplied from another truck or from a terminal. 

4. If no source of additional product supply is available at the operator’s location, you 

must arrange to perform this test (at least) at a distribution site or elsewhere. 

5. Sufficient product must be added with the pump disengaged if the system is power 

operated. This will interrupt the delivery, but this is what will happen in commercial use. 

In commercial use, the delivery would be discontinued and the single compartment truck 

would be driven back to the terminal to be refilled. 

6. When the tank is refilled, allow sufficient time for the product to settle (particularly if 

it is susceptible to foaming), but keep in mind the possible effects of temperature and 

evaporation of the product already in the prover. Keep the rest period to a minimum. 

8. To complete the test, the delivery is resumed without resetting the meter and continued 

until the register indicates the nominal capacity of the prover, and the error is recorded.

Tolerances to Apply 

Even a system that is properly designed, installed and maintained may permit a small 

amount of air and or vapor to pass through the meter; therefore, the more liberal special 

tolerance applies to the results of the split compartment test. NIST Handbook Section 

3.31, Paragraph T.2. Table 1 permits a special test tolerance of ± 0.45 % for meters 

delivering petroleum products. 

Note that the NCWM Specifications and Tolerances Committee is presently considering 

changes to the requirements for conducting a split compartment test. A "restructuring" of 

the tolerances applied to the results of the split compartment test is being considered. 

As a general rule, the greater the difference between the results obtained for the normal 

test and the split compartment test of a vehicle tank metering system, the stronger the 

indication that the air eliminator is not functioning properly and is in need of repair. 

It is important that the split compartment test be included as part of a complete test of the 

VTM system to verify that the air eliminator is functioning properly and to ensure that 

customers are not being charged for air and vapor.