Backflow Test Kits: How Do They Work?

Backflow test kits have been on the market for over 50 years. Some of the original design technologies have stood the test of time and continue to provide reliable, trouble-free operation. There are also evolving technologies and new styles of test kits becoming available. To answer the above question, we will focus on the most common test kit — elastomer diaphragm/magnetic coupling — and an evolving technology: the differential pressure transducer.

Regardless of the specific design, all backflow test kits share common elements and meet the same requirements.

COMMON ELEMENTS

1. Pressure-containing element — The components or housing that contain the system pressure inside of the test kit. This is similar to the body of a backflow prevention assembly.
2. Sensing element — A mechanical element that deflects in a linear motion as pressure or differential pressure is applied. This is similar to the components of backflow prevention assemblies that open or close as pressure changes inside the assemblies.
3. Indicator — The mechanism that translates the deflection of the sensing element into a pressure value that can be read and recorded.
4. Valves/Manifolds — Used to control the pressure across the backflow prevention assembly components so the test kit can indicate and record the pressure values to determine if the assembly is functioning properly or requires repair.
5. Hoses — The components that connect the test kit to the backflow prevention assembly.

COMMON REQUIREMENTS

1. 0-15 PSID (0-100 kPa, 0-1.0 bar) full-scale range.
2. Over-range protection to full rated working pressure, high side over low side, and low side over high side.
3. Two valves between the high side and low side of the test kit.
4. Valves configured in a way to allow bleeding of air from the inside of the test kit.

MECHANICAL ANALOG TEST KITS

These were the first designs to use differential pressure gauges that were plumbed up with valves or manifolds to become the first backflow prevention test kits. The elastomer diaphragm/magnetic coupling style test kit is the most common type of test kit in use today.

ELASTOMER DIAPHRAGM/MAGNETIC COUPLING

This design has become the most prevalent test kit currently on the market, with varying models available from Apollo, Aquestia A.R.I., Mid-West. Instrument, Watts, Zurn, and others in America, Australia, Canada, and Europe.

The sensing element is an elastomer diaphragm assembly clamped between a high-side pressure housing and a low-side pressure housing. A range spring is located on the low side of the sensing element, which allows the gauge to be calibrated to a full-scale range of 15 PSID. With the test kit exposed to atmospheric pressure, the range spring pushes the sensing element to zero position. When the high-side pressure exceeds the low-side pressure plus the spring force, the sensing element deflects away from the zero position until the force on both sides of the diaphragm is equal. When the pressure differential decreases, the range spring deflects the sensing element back towards the zero position until the force on both sides of the diaphragm is equal.

The indicator is a magnetic coupling that translates the linear deflection of the diaphragm to a rotary deflection of a pointer that indicates the pressure differential on a graduated dial scale. A driver magnet is part of the elastomer diaphragm assembly, which deflects the same linear distance as the elastomer diaphragm. A rotary follower magnet assembly on the outside of the pressure housing tracks the linear deflection of the driver magnet. As the north pole of the driver magnet passes the follower magnet, the south pole of the follower magnet rotates. The follower magnet assembly consists of a round shaft that extends through a graduated dial. A pointer is affixed to the end of the shaft. The combination of the driver and follower magnet spacing, the range spring, range spring preload and the graduated dial layout allows the test kit to be calibrated.

The elastomer diaphragm is a thin flexible component roughly the thickness of three sheets of paper. This is protected from damage when full system pressure is applied to either the high side or the low side. The elastomer diaphragm assembly has support plates on the high side and low side of the diaphragm. The high-side and low-side gauge bodies have a contour that matches the shape of the elastomer diaphragm. When the diaphragm assembly is deflected all the way to zero or all the way to full scale, the combination of the support plates and the gauge body contour fully supports the thin elastomer diaphragm, thus protecting it from rupture or damage. Imagine a piece of paper held between two hands. It would be easy for someone to poke a hole in the paper. Place the piece of paper flat on a table. You cannot poke a hole in the paper unless you can poke a hole through the table. This is the “over-range” protection for the diaphragm assembly.

The support plate also acts as a full-scale travel limiter, so the range spring is never compressed solid, ensuring the range springs does not fail from repeated full-scale deflections.

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DIGITAL TEST KITS WITH A DIFFERENTIAL PRESSURE TRANSDUCER

Recently, affordable wet/wet differential pressure transducers with over-range protection became available, allowing water pressure to be present on both the high side and low side of the transducer. This type of test kit is available from Arbiter and Mid-West. Instrument. Similar to the construction of the mechanical analog test kits, the differential pressure transducer sensing element is clamped between a high-side pressure housing and low-side pressure housing. The sensing element has a high-side and low-side isolation diaphragm, which is exposed to water pressure. Because the isolation diaphragms are metal, they have a spring rate that allows the differential pressure transducer to be selected for the proper full-scale range of 15 PSID. The inside of the sensing element has a sensing diaphragm, “electronics,” and silicone fill fluid. The isolation diaphragms deflect under differential pressure, transmitting pressure through the silicone fill fluid, deflecting the sensing diaphragm. The “electronics” convert the deflection of the sensing diaphragm to a change in voltage, resistance, or capacitance.

The indicator is a circuit board and a digital display. The electrical output from the differential pressure transducer is converted to a calibrated digital pressure value via a microprocessor on the circuit board for viewing on an LCD display or wirelessly transmitted to a smartphone.

“Over-range” protection is achieved by the thin metal isolation diaphragms being fully supported at either side of full-scale travel. When over-ranged high over low, the high-side thin metal diaphragm is fully supported and no longer transmits pressure through the silicone fill fluid, so the sensing diaphragm and low-side isolation diaphragm are protected. When over-ranged low over high, the low-side thin metal diaphragm is fully supported and no longer transmits pressure through the silicone fill fluid, so the sensing diaphragm and high-side isolation diaphragm are protected.

VALVES/MANIFOLDS

The mandatory requirements for the plumbing of backflow test kits is two control valves between the high side and low side, and the low-side control valve shall be a needle valve. Backflow test kits are currently available in five-valve, three-valve, and two-valve configurations. The plumbing can be individual valves plumbed with tubing or manifold assemblies. Valve styles include ball valves, plug valves or needle valves. Needle valves may be hard seated or soft seated.

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FIVE-VALVE TEST KITS

Five-valve test kits have three control valves located on the bottom of the test kit and a high-side and low-side bleed valve located on the top of the test kit. Water enters the bottom of the test kit and fills the gauge bodies. Air is expelled from the test kit through the high-side and low-side bleed valves located on the top of the test kit. The control valves located on the bottom of the test kit control the flow of pressure between the high-side and low-side hoses of the test kit or the highside and bypass hoses of the test kit. Testing the relief valve opening point on an RP requires opening the high-side control valve and the low-side control valve (no more than a quarter turn). Backpressure testing of the No. 2 check valve on an RP requires closing the low-side control valve and opening the high-side control valve and bypass control valve.

THREE-VALVE TEST KITS

Three-valve test kits have three valves located on the top of the test kit. These valves act as both bleed valves and control valves. Water enters the bottom of the test kit and fills the gauge bodies. Air is expelled from the high side of the test kit by opening the high-side and bypass control valves. Air is expelled from the low side by opening the low-side and bypass control valves. Testing the relief valve opening point on an RP requires opening the high-side control valve and the low-side control valve (no more than a quarter turn). Backpressure testing of the No. 2 check valve on an RP requires closing the low-side control valve and opening the high-side control valve and bypass control valve.

TWO-VALVE TEST KITS

Two-valve test kits have two valves located on the top of the test kit. These valves act as both bleed valves and control valves. Water enters the bottom of the test kit and fills the gauge bodies. Air is expelled from the test kit through the high-side and low-side control valves located on the top of the test kit. Testing the relief valve opening point on an RP requires connecting a hose between the control valves and opening the high-side control valve and the low-side control valve (no more than a quarter turn). Backpressure testing of the No. 2 check valve on an RP requires closing both control valves, disconnecting the hose from the low control valve, connecting the hose between the high-side control valve and the No. 4 test cock, and opening the high-side control valve.

HOSES

Backflow test kits are supplied with three hoses that are generally color-coded red for high-side hose, blue for low-side hose and yellow for bypass hose. The most common hose length is 5 feet, while some test kits use 3-foot hoses. Most manufacturers supply hoses with serviceable filter assemblies to prevent unwanted debris in the water from entering the test kit assembly.

The filter assemblies are not centered in the hoses and should always be positioned closest to the backflow prevention assembly being tested.

Any test kit construction described in this article is capable of testing any model of RP, DC, PVB or SVB. Today’s backflow test kits are available in a wide variety of options, styles, shapes, and sizes. It is the tester’s preference as to the construction and valve configuration that they prefer to use.