FAQ - Backflow Test Kits

Print this Article

Backflow Prevention Assembly Test Kits

A backflow prevention assembly test kit is a tool. A tool that when properly used, can diagnose how a backflow prevention assembly is operating. A test kit on its own does nothing. Only when this tool is placed in the hands of a skilled operator following proper test procedures, does this tool do what it was designed for, which is to generate data. A knowledgeable tester must interpret the data that is generated. The test kit does not tell you if the assembly is working, it only generates data that a knowledgeable certified tester can interpret. A test kit's purpose is to perform a diagnostic analysis of a backflow preventer at one point in space and time. To be sure this test kit can do this properly, we must understand what this tool can and cannot do. To understand how to use this tool properly, we must evaluate how it is built and how it works.


We first need to clarify a few terms. A test kit consists of a differential gauge, needle valves, hoses, test cock fittings and carrying case. Many times, the terms "gauge" and "kit" are used interchangeably. These words, however, mean two different things. A gauge is the part mounted in the test kit that senses and indicates the pressures in the backflow prevention assembly. The test kit is the whole combined apparatus of a differential pressure gauge, needle valves, test cock fittings, hoses and carrying case. A differential gauge cannot operate as a test kit without the use of needle valves, hoses and fittings.

The type of gauge used in backflow preventer test kits is called a "differential pressure gauge." A differential pressure gauge is one that reads the difference between two pressure areas and gives one reading of the difference between the two pressure areas. The output reading of the gauge can be in PSI or PSID. When you are using two hoses (high & low) during your field test, you are reading in PSID (pounds per square inch differential). When you are using one hose on the high side with the low side sensing atmospheric pressure, (like on a PVB) you are using your differential pressure gauge like a pressure gauge and you are reading in PSIG (pounds per square inch gauge).

All differential pressure gauges within a test kits are created with a certain level of accuracy. Accuracy is defined as the conformity of a gauge indication to an accepted standard of true value. Accuracy is expressed as a percentage of the full-scale reading of the gauge. A variance from a true reading is called the "error" and is expressed in a pressure reading. A 0-15 PSID differential pressure gauge that has a 2% accuracy means that the reading can be off ± 0.3 PSID from a true value (2% or 0.02 X 15.0 = .3) at any one reading and still be within factory tolerances. Gauges in electronic test kits can express a higher reading than the 0-15 PSID found in mechanical gauges. An electronic test kit may have two transducers with ± 0.1% accuracy and a full scale range of 200PSIG. Each transducer will have a permissible error of ± 0.2 PSI ( 0.1% or .001 X 200 = 0.2PSI). The error for each transducer is additive for a permissible error of ± 0.4 PSI. Electronic circuitry plus matching transducers can reduce the error to ± 0.1 PSID.

Test kits may start off with proper accuracy when built, but due to usage conditions and wear and tear, gauge readings may drift outside error tolerances resulting in inaccurate readings. When testing a gauge for accuracy, the gauge is compared to an accurate reference gauge in an ascending order (0 -15) and then in a descending order (15 - 0). The difference from a true reading in both ascending and descending readings at any one point, is the hysteresis. If the hysteresis of the gauge is not within requirements, then the gauge must be calibrated. "Calibration" is the act of returning the gauge to within permissible error tolerances. Some manufacturers express their accuracy in descending scale only. This means that their variance from a true reading is only checked on the descending scale (15-0) and not the combined readings in ascending and descending order.

Quite frequently, the word "calibration" is misused in our industry. Often local administrative authorities will state in their requirements that test gauges be "calibrated annually;" in fact, what they are really requiring is that annually, gauges must be tested, and then calibrated if accuracy is out of tolerance. Realize that calibration is a repair function, while accuracy verification is a maintenance function.

Design and Construction of Test Kits

The test kit contains several parts as we discussed, but the gauge is the heart of the test kit. There are two types of differential gauges. The first is the analog or mechanical gauge and the second is the digital or electronic gauge. Besides a gauge's accuracy, you must look at how a gauge is constructed. In the construction you must look at several characteristics. The first is the housing of the gauge body. Today we have gauge bodies made of plastic, brass or aluminum. Some materials are stronger than others, some corrode and leave particles inside the body. The maximum working water pressure of the gauge should be evaluated so the gauge is not damaged in normal use. Many gauges today have "overrange protection;" eg, they have the ability of the gauge to stop the travel of the elastic element in the gauge when excessive pressure is applied to the gauge. This is usually accomplished by some form of internal stop mechanism that ensures excessive pressure does not let the traveling parts inside the gauge move beyond their normal range.

The most important part of a gauge is its elastic element. The elastic element is the elastic component inside the gauge body that moves in response to a pressure change. The ability of the elastic element to make the same motion time and time again in response to a pressure change is what helps determines the gauge accuracy and repeatability. The more precise the motion of the elastic element, the more precise the gauge is. There are presently three different types of elastic elements that are used in backflow test kit gauges: a diaphragm, a bellows, or a transducer. Diaphragms or bellows are used in mechanical gauges; transducers are used in digital or electronic gauges. Let's evaluate bellows and diaphragm elastic elements next. (Transducers are discussed in the next section on electronic/digital gauges).

Bellow Elastic Element
The bellow type of elastic element is used in several differential gauges. The bellow mechanism consists of two separate but joined accordion shaped bellows that move back and forth in response to pressure changes exerted on their exterior within a gauge body housing. There is a high and a low side bellow. The bellow unit is mounted inside a brass pressure housing which encloses both sides of the bellows and has openings for hose attachment. The bellow unit consists of a balanced pair of bellows with an internal range spring, a center plate, overrange valves, a temperature compensator, and a torque tube assembly. Metal copper beryllium bellows are mounted on opposite sides of the center plate. The outer ends of the bellows are rigidly connected internally by a stem passing through an angular passage in the center plate. The bellows and center plate are sealed and permanently filled with a non-corrosive low freezing point liquid mixture. The temperature range is approximately -50 - +220 degrees. The pressure from the backflow prevention assembly is applied to the outside of the bellows from the high and low pressure connections. An additional bellow fold is attached to the end of the high pressure bellow. This is the temperature compensator. As the temperature conditions of the liquid fill inside the bellows changes, due to conditions like exterior temperature, the fill fluid could expand or contract in volume. To be sure this does not affect the accuracy of the gauge readings, this extra bellow fold can handle the expansion and contraction of the fill liquid. As in all of the different types of elastic elements, the movement of the element is critical to the workings of the gauge. How long the bellow maintains its original shape and elasticity is what assures the repeatability and life of the elastic element. There are two types of bellows elastic elements used: either a welded bellow, or a hydro formed bellow. The welded design has two individual pieces of beryllium copper sheet folded together, and then they are welded into a rigid round-shaped bellow with squared edges at each fold. The hydro-formed type of bellow starts with a piece of copper beryllium tubing that is heated, then when pressure is exerted on the inside of the tubing, it stretches the tube into an accordion-shaped bellow. The ability of the bellow to move back and forth is at the heart of the bellow-type elastic element. A bellow's strength and ability to maintain its shape determines its lifespan.

Once the bellows are constructed, they are matched, to ensure their ability to work uniformly in conjunction with each other. In other words, as higher pressure is applied on the outside of the bellows, the high side will push the bellow in, and the corresponding low pressure side will move out. So, it becomes important for the two bellows to react identically to pressure changes. If one or both bellow units becomes stressed or worn, either during use or during its construction, it can affect the linear movement of the bellow, and shorten its lifespan. The differential pressure range of a bellow type gauge is determined by the force required to move the bellows through their normal travel. In order to provide various pressure ranges and uses, a range spring is incorporated inside the bellow unit which accurately balances the differential pressures applied.

Since bellows are essentially two fluid filled accordion shaped balloons, to be sure that they are not squeezed too hard by either over pressure from the backflow prevention assembly or because of a pressure spike, an overrange feature is included. Should the bellows be subjected to a pressure greater than the pressure range of the gauge, the bellows will move through its normal range of travel plus a small amount of "over travel" until the valve mounted on the center stem seals against its corresponding valve seat. This stops the movement of the bellows and ensures that they stay fully supported and cannot be ruptured regardless of the over-pressure applied. Since there is a valve seat on either the high or low side, protection is ensured against an overrange in either direction.

There are other parts that connect to the elastic element that make the gauge operate. The bellows are connected to the drive arm. The drive arm takes the linear movement of the bellows and conveys the motion to a rotary motion through the torque tube assembly. As the bellows move in a back and forth motion, the drive arm, which is connected in the middle between the two bellows, twists. The torque tube shaft is welded to the tube at the end. Movement of the bellows is transmitted to the in-board end of the torque tube by means of the drive arm. Because the outer end of the torque tube is sealed to the center plate, it must twist when subjected to torque from the drive arm. The torque shaft is supported within the torque tube at its outer end, but is rigidly attached to the tube and the drive arm at the inner end. There is a needle bearing that supports the inner end of the torque tube and the ball bearing that is attached to the drive arm. This allows for a minimum of friction because they are bathed in the fill liquid. The torque tube shaft only rotates a total of 8 degrees as the bellows move back and forth through their full range of travel.

Lastly, there is the indicating mechanism. This is the part of the gauge that translates the motion of the elastic element into readings (scale face and pointer). We know the linear movement of the bellows causes the torque tube shaft to move a maximum of 8 degrees. If you look at the pointer on the dial you see that the movement has a 270 degree sweep on the scale face. The concept behind the bellow/torque tube design is to provide a very precise movement of the torque tube by the bellows which sense the pressure changes. Through the use of a jeweled movement, the 8 degree movement of the tube is amplified through a gear and pinion to the pointer which gives a full 270 degree movement of the pointer. The torque tube comes from the drive arm/bellow assembly through the back of the indicator case. Here it is attached to the follower arm. As the torque tube rotates through its 8 degree travel, the follower arm turns the segment gear to translate the movement to a full 270 degree motion of the pointer on the scale face.

Diaphragm Elastic Element
The diaphragm type of elastic element utilizes a single rubber diaphragm which has high and low pressure applied on the opposite sides of the diaphragm. A stainless steel stem passes through the center of the diaphragm, which is spring-biased, allowing it to move linearly in proportion to the differences between two pressures. As in the bellow design, the range spring allows the manufacturers to use this gauge to produce many different pressure readings. (In the backflow prevention business we presently are standardized on the 0 - 15 PSI differential gauge.) The rubber diaphragm is supported on both sides with diaphragm retainers. These retainers are stainless steel plates that hold the diaphragm onto the center stem and help maintain a constant shape of the center portion of the diaphragm. The part of the diaphragm not covered by the diaphragm retainer is in contact with the water in the gauge body. The diaphragm retainers also provide overrange protection. If an excessive pressure is placed on the diaphragm, the retainers will bottom out on the gauge body, ensuring that the stem does not travel too far, which could damage the diaphragm. The last important part of this diaphragm mechanism is a magnet placed at the end of the diaphragm stem. As the diaphragm moves in conjunction with the pressure changes, the magnet travels in the same path as the diaphragm stem. This diaphragm stem is mounted in a sealed gauge body. There is a separate body holding the indicator mechanism (The scale face and pointer) which is attached to the gauge body. Unlike the gauge body, the indicator mechanism body has no fluid flowing through it. Connected to the pointer is a stem, which is connected to a follower magnet, which rotates when the primary magnet on the diaphragm stem moves with the diaphragm. There is no direct connection between the indicator mechanism body and the elastic element or gauge body. The motion of the diaphragm moves the primary magnet causing the follower magnet to move when the two magnets interface. This movement of the follower magnet causes the pointer to move on the scale face.

Indicator Mechanism
Besides the elastic element, another feature to evaluate is the indicator. This is the portion of the gauge that translates the movement of the elastic element to the scale face and pointer of the gauge so we can take a reading. Indicators are either directly connected or indirectly connected to the movement of the elastic element. Those that are directly connected (as in the bellow design), as the name implies, cause the needle on the scale face to move immediately to a pressure change. Indirectly connected indicators (diaphragm type) have a magnetic coupling between the elastic element and the pointer on the scale face, so the movement of the pointer is directly related to the ability of the magnetic coupling to cause the needle to move when the elastic element moves. You will usually see a slightly slower scale face movement of the pointer on a diaphragm type of elastic element as the indicator (indirect) can only move when the two magnets collectively move. The bellow type will usually show a more direct movement as the bellows are directly attached through the torque tube (direct) to the indicator mechanism.

Scale Face & Pointer
The part of the indicator we are most familiar with, is the scale face and the pointer. Some considerations are:

  • How easy is the scale face to read?
  • How high is the pointer off the scale face? If the pointer is mounted too high, a reading error called parallax error can occur.
  • How wide is the pointer - does it cover the numbers? This can make readings on the scale face difficult.
  • Are there pointer stops on the scale face? Pointer stops are, as the name implies, a small pegs that stop the pointer from moving beyond its normal travel.
  • The lens on the scale face. Most lenses today are a plastic material so that they will not break when bumped or hit. The lens must be sealed to the indicator to ensure that water does not get under the lens, which can fog the lens. (The lens also helps to keep out dirt and debris from the indicator mechanism inside.)

In all the mechanical gauges, it is important to consider what might restrict the motion of the elastic element (bellow or diaphragm); for example, dirt and debris inside the critical moving areas of the gauge. Dirt can be injected from the backflow preventer if the test cocks are not properly flushed. Debris can also get into the gauge body from corrosion of the gauge housing materials. Gauge bodies can be made of different materials such as brass, plastic or aluminum. Some of these materials can corrode, leaving particles inside the gauge body, similar to rust. These particles can also restrict the motion of the elastic element, again giving false readings to the indicator.

Another characteristic of the mechanical gauges to be aware of is friction error. This occurs when the elastic element moves in response to pressure changes, but the movement is not accurately conveyed to the scale face pointer. This error can be caused by restriction or friction along the normal movement of the internal stem. This type of friction error can be overcome by lightly tapping on the outside of the gauge body while taking readings. This tapping will help assure that you are getting a correct reading.

Transducer Elastic Element
The digital or electronic gauge has a different set of criteria than the mechanical gauge. The elastic element used in an electronic gauge is a transducer. A transducer is an electronic pressure sensor. A transducer is a sealed electronic component that contains a small piece of glass or similar material in the form of a pedestal that has a resistor bonded to it. When pressure is applied to the transducer, the resistor will measure the deflection of the glass pedestal from the pressure applied. This deflection is turned into an electronic signal sent through the electronic circuitry that translates this signal into a pressure reading on the LCD read out. A key element that allows this transducer to send an accurate signal is voltage. There must be adequate voltage so that the signal produced can be accurately conveyed from the transducer to the LCD readout. If proper voltage in the test kit is not maintained, then the signal can be scrambled and provide an inaccurate reading. Transducers do not have overrange protection and care must be taken to not over-pressurize the gauge. If the transducers are over-pressurized, the pedestal inside the transducer may crack and will have to be replaced.

It is important to also evaluate the burst strength of the transducer. The burst strength is the pressure at which the glass pedestal within the transducer will crack. There are two separate transducers used in each electronic gauge. There is a high and low side transducer that measures the pressure on each corresponding side. The electronic circuitry calculates the difference between the high and low transducer and displays the difference on the LCD readout. There is one version of electronic test kit that has a printer built into the test kit so you can print your test results on site rather than just the usual LCD indicator. The gauge case that holds the transducers must also be evaluated to assure that water from the test environment does not get into the electronic circuitry. All electronic test kits are water-resistant but not water proof. This is an important distinction, because the electronic circuitry can develop heat, which, if the case is sealed, will cause condensation to form which could damage the circuitry. The same problem could happen if water from the test environment was splashed inside the electronic circuitry. However, since there are no moving parts in an electronic test kit gauge, there is no need to worry about friction error while testing, as can be the case with mechanical gauges.

Needle Valves
The needle valves control the transfer of pressure from one area to another. You could have the most accurate gauge in the world but if your needle valves are not able to hold pressure, you could generate inaccurate data. Needle valves are categorized into two types: soft or hard-tipped needle valves. The hard-tipped needle valve utilizes a stainless steel stem that closes against a softer brass seating surface. If any scale build up grows on the seat or stem, the needle valve may not shut off easily and may have to be cleaned. The soft-tipped stems have either an elastic tip on the stem or a soft seat for the stem to seal against. If the seals are damaged, the needle valve may not shut off completely and have to be repaired. When using needle valves with a soft tip, it is important to ensure that the technician does not over-tighten the valve when closing it. This over-tightening could deform the soft seal and cause unneeded stress on the needle valve - causing it to fail prematurely.

Hose & Fittings
Another item to evaluate in the test kit are the hoses. The hoses utilized in test kits today use an SAE (Society of Automotive Engineers SAE-J513 - 1/4" flare) fitting with a rubber gasket in the hose. The rubber gasket in the hoses may require replacement if worn out due to excessive use.

Hoses sometimes have particle filters attached to the hoses. Filters can help prevent debris from getting into the key moving areas of the elastic element. Filters should be periodically cleaned or replaced to ensure that they do not become plugged and restrict the flow of water within the test kit.

The fittings that come in a test kit should allow you to adapt to all sizes of test cocks - from 1/8" - 3/4" size. The fittings should be sturdy and reusable without excessive wear.

Carrying Case
The last item we need to evaluate is the carrying case. The case is the box or container that the test kit is stored in. The main purpose of the case is to protect the gauge and other accessories of the test kit from damage due to environmental conditions. The hoses and fittings can also be place inside some of the cases. The case should be durable to ensure no damage happens to the gauge during transport. The case should have an insulating foam to minimize any shock from handling and or transportation. The insulation should also be a closed cell variety to assure that it does not soak up any water.

Care & Maintenance of Test Equipment

Test kits are no different from any other tool and must be cared for and maintained. Be sure your gauge in your test kit is within factory accuracy tolerances by having it tested periodically - annually, at the very least. A complete accuracy verification of the differential pressure gauge in your test kit cab be conducted by a gauge lab that has its equipment traceable to NIST. The gauge labs reference gauge should be at least 4 times more accurate at test pressure than the gauge they are checking.

A tester should also take the gauge in to a gauge lab any time there are unusual symptoms.

When a test kit is sent in for its annual check-up, besides accuracy check on the gauge, it should also be subjected to a pressure test. Pressure should be applied to the test gauge, needle valves and hoses to assure that there are no leaks in the connections between these parts. Needle valves need to have a pressure test conducted to ensure that they are drip-tight against the working pressure of the gauge. The hoses should also be submitted to a pressure test to ensure that the hose and end fittings are holding working pressure. If leaks develop at the gaskets in the end of the hose, the gauge lab should replace them. If the hoses become stiff or cracked, this also is cause for replacement. Some hoses may contain filter elements. The filters must be periodically cleaned to ensure that they are not restricting the flow of water. There are several types of filter elements, but most can be cleaned by soaking in a vinegar solution. Any leakage of these accessories could produce inaccurate data.

When testing a backflow prevention assembly, it is important to always flush the test cocks so no dirt and debris is flushed into the gauge. When pressurizing your test kit, always take care to pressurize it slowly to ensure that a pressure spike does not cause harm to the elastic element, which could effect gauge accuracy. The same procedure should be followed when opening and closing the bleed valves on the test kit.

When the test kit is not being used, it should be drained of all water from the gauge, needle valve and hoses by opening all needle valves. Leaving needle valves closed will hold water in the test kit and hoses, just as a thumb on a straw holds liquid in the straw. Water left sitting in the gauge, needle valves hoses or any connecting tubing, could cause internal damage.

The test kit should not be subjected to temperature conditions, internal or external, that exceed its minimum or maximum temperature requirements. When a test kit is not being used, it should also be stored in its carrying case to minimize any unnecessary shock or vibration that could affect its accuracy condition.

When evaluating a test kit, it is important to understand the different types on the market. Be sure to evaluate all the elements of a test kit: the gauge elastic element, indicator, needle valves, hoses & fittings and carrying case. Know the differences so you can knowledgeably select your tool.