Frequently Asked Questions

The 3-A Symbol

During the 1920s, the need for more stringent and uniform standards for dairy processing equipment became evident as the U.S. economy and consumers entered the modern era. Representatives of three interest groups-processors, regulatory sanitarians and equipment fabricators-recognized the need for cooperative action and introduced the first industry standards for equipment. These standards became known as '3-A' standards for the three interest groups that forged a common commitment to improving equipment design and sanitation. Unlike other types of standards, 3-A Sanitary Standards relate to the cleanability of dairy equipment.

In 1944, the U.S. Public Health Service offered full cooperation with the '3-A program'. This marked the beginning of a program to provide uniform equipment standards for the protection of public health.

In 1954, the first processing equipment bearing the new 3-A Symbol was unveiled. By display of the 3-A Symbol, processing equipment could be shown to meet certain material, design and fabrication standards for cleanability and inspection.

In 2003, '3-A' opened another era with the formation of 3-A Sanitary Standards, Inc., an independent not-for-profit corporation. One of the driving forces for organizing 3-A SSI was to modernize the consensus development process used to develop, revise and amend 3-A Sanitary Standards and 3-A Accepted Practices. Another key mission goal of 3-A SSI was to implement a new independent Third Party Verification inspection program designed to enhance the integrity of the 3-A Symbol program.

Today there are 68 3-A Sanitary Standards and nine 3-A Accepted Practices. More than 360 companies from across the U.S. and 22 other countries around the world hold authorizations to display the 3-A Symbol on various types of processing equipment.

Does the instrument or device require CSA Approval?

Electrical Equipment operating at 24 VDC is not part of CSA's scope of inspection according clause 16-222 of the Canadian electrical code for class 2 circuits in general used in general purpose applications (non-hazardous locations) . The only requirement is that the power supply must be CSA approved.

For applications in Hazardous Area Locations, (or Life Support systems) please consult the specific electrical code to verify the device and circuit installation meet requirements for CSA approval.

How Differential Pressure Flowmeters Work

Differential pressure flowmeters use Bernoulli's equation to measure the flow of fluid in a pipe. Differential pressure flowmeters introduce a constriction in the pipe that creates a pressure drop across the flowmeter. When the flow increases, more pressure drop is created. Impulse piping route the upstream and downstream pressures of the flowmeter to the transmitter that measures the differential pressure to determine the fluid flow.

Bernoulli's equation states that the pressure drop across the constriction is proportional to the square of the flow rate. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. At 10 percent of full scale flow, the differential pressure flowmeter accuracy is dependent upon the transmitter being accurate over a 100:1 range of differential pressure. Differential pressure transmitter accuracy is typically degraded at low differential pressures in its range, so flowmeter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flowmeters. Remember that of interest is the accuracy of the flow measurement system --- not the accuracy of the differential pressure transmitter.

Because of the nonlinear relationship between flow and differential pressure, the accuracy of flow measurement in the lower portion of the flow range can be degraded.

What is the EHDG?

The European Hygienic Engineering & Design Group (EHEDG) is a consortium of equipment manufacturers, food industries, research institutes as well as public health authorities and was founded in 1989 with the aim to promote hygiene during the processing and packing of food products.

The principal goal of EHEDG is the promotion of safe food by improving hygienic engineering and design in all aspects of food manufacture. EHEDG actively supports European legislation, which requires that handling, preparation processing and packaging of food is done hygienically using hygienic machinery and in hygienic premises (EC Directive 2006/42/EC for Machinery, EN 1672-2 and EN ISO 14159 Hygiene requirement).

What are Ground loops

Ground loops are created when the source of a signal and the driven instrument have their common inputs tied to the earth at each device or the common lead is used to carry AC current for some other device. AC current flowing in the common lead drops a small AC voltage due to the wire resistance. The voltage appears in series with the desired signal and creates "noise" on the desired signal. The Isolator breaks the common lead and the AC current can no longer flow.

A common mode voltage is a voltage that is connected to the + and - inputs of a device. Example - A small resistor in series with the + lead of a 138VDC battery and the - side of the battery is the "common" lead for the system. The resistor drops a small voltage proportional to the battery current. The Isolator can measure the small voltage across the resistor even though it is floating 138VDC above the system circuit common.

What is Ingress Protection?

Ingress Protection (IP) ratings are developed by the European Committee for Electro Technical Standardization (CENELEC) (NEMA IEC 60529 Degrees of Protection Provided by Enclosures - IP Code), specifying the environmental protection the enclosure provides.

The IP rating normally has two (or three) numbers:

  • 1. Protection from solid objects or materials
  • 2. Protection from liquids (water)
  • 3. Protection against mechanical impacts (commonly omitted, the third number is not a part of IEC 60529)

Example - IP Rating

With the IP rating IP 54, 5 describes the level of protection from solid objects and 4 describes the level of protection from liquids.

An "X" can used for one of the digits if there is only one class of protection, i.e. IPX1 which addresses protection against vertically falling drops of water e.g. condensation..

IP First number - Protection against solid objects

  • 0 No special protection
  • 1 Protected against solid objects up to 50 mm, e.g. accidental touch by persons hands.
  • 2 Protected against solid objects up to 12 mm, e.g. persons fingers.
  • 3 Protected against solid objects over 2.5 mm (tools and wires).
  • 4 Protected against solid objects over 1 mm (tools, wires, and small wires).
  • 5 Protected against dust limited ingress (no harmful deposit).
  • 6 Totally protected against dust.

IP Second number - Protection against liquids

  • 0. No protection.
  • 1 Protection against vertically falling drops of water e.g. condensation.
  • 2 Protection against direct sprays of water up to 15o from the vertical.
  • 3 Protected against direct sprays of water up to 60o from the vertical.
  • 4 Protection against water sprayed from all directions - limited ingress permitted.
  • 5 Protected against low pressure jets of water from all directions - limited ingress.
  • 6 Protected against temporary flooding of water, e.g. for use on ship decks - limited ingress permitted.
  • 7 Protected against the effect of immersion between 15 cm and 1 m.
  • 8 Protects against long periods of immersion under pressure.

IP Third number - Protection against mechanical impacts (commonly omitted, the third number is not a part of IEC 60529)

  • 0 No protection.
  • 1 Protects against impact of 0.225 joule
  • (e.g. 150 g weight falling from 15 cm height).
  • 2 Protected against impact of 0.375 joule
  • (e.g. 250 g weight falling from 15 cm height).
  • 3 Protected against impact of 0.5 joule
  • (e.g. 250 g weight falling from 20 cm height).
  • 4 Protected against impact of 2.0 joule
  • (e.g. 500 g weight falling from 40 cm height).
  • 5 Protected against impact of 6.0 joule
  • (e.g. 1.5 kg weight falling from 40 cm height).
  • 6 Protected against impact of 20.0 joule
  • (e.g. 5 kg weight falling from 40 cm height).

In order to comply with the stringent requirements for CE Marking machine makers today fits their machines with parts certified according EU (European Union) and international standards.

Measurement Accuracy

Meter accuracy's are expressed as % of full scale, and % of rate.

A % F.S. (full scale) definition means that you take the full range calibrated value of the meter and multiply it by the stated accuracy. If a meter has a ±1% F.S. statement and has a maximum flow rate of 100 GPM, at exactly 100 GPM, the register may vary from 99 to 101 GPM. This same meter at 10 GPM can have a register variation from 9 to 11 GPM (a 10% of actual flow rate error).

A % of rate definition means that the stated accuracy is applied to the entire range of the flow meter. For example, if the above meter had a ±1% of rate statement, it is allowed to have an error at 100 GPM of 99 to 101 GPM. However, at 10 GPM, the allowable register error is only 9.9 to 10.1 GPM.

Positive displacement meters, ultrasonic meters and rotameters are generally defined with % F.S. accuracy statement.

Turbines, magnetic flow and oval gear meters are generally defined with a % of rate accuracy statement.

NEMA 4 and Other NEMA Ratings

What does NEMA 4 rated mean? In non-hazardous locations, there are several different NEMA ratings for specific enclosure "types", their applications, and the environmental conditions they are designed to protect against, when completely and properly installed. Computer Dynamics offers many industrial flat panel display systems configured for various NEMA ratings, including NEMA 4 type. The following provides an overview of the NEMA Types. For complete definitions, descriptions, and test criteria, see the National Electrical Manufacturers Association (NEMA) Standards Publication No. 250.

NEMA 1 - Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment and to provide a degree of protection against falling dirt.

NEMA 2 - Same as NEMA 1 including protection against dripping and light splashing of liquids.

NEMA 3 - Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, and windblown dust; and that will be undamaged by the external formation of ice on the enclosure.

NEMA 3R - Same as NEMA 3 excluding protection against windblown dust.

NEMA 3S - Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, and windblown dust; and in which the external mechanism(s) remain operable when ice laden.

NEMA 4 - Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt, rain, sleet, snow, windblown dust, splashing water, and hose-directed water; and that will be undamaged by the external formation of ice on the enclosure.

NEMA 4X - Same as NEMA 4 including protection against corrosion.

NEMA 5 - Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against settling airborne dust, lint, fibers, and flyings; and to provide a degree of protection against dripping and light splashing of liquids.

NEMA 6 - Enclosures constructed for either indoor or outdoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against hose-directed water and the entry of water during occasional temporary submersion at a limited depth; and that will be undamaged by the external formation of ice on the enclosure.

NEMA 6P - Same as NEMA 6 including protection against the entry of water during prolonged submersion at a limited depth.

NEMA 7 - Enclosures are for indoor use in locations classified as Class I, Groups A, B, C, or D and shall be capable of withstanding the pressures resulting from an internal explosion of specified gases, and contain such an explosion sufficiently that an explosive gas-air mixture existing in the atmosphere surrounding the enclosure will not be ignited. Enclosed heat generating devices shall not cause external surfaces to reach temperatures capable of igniting explosive gas-air mixtures in the surrounding atmosphere. Enclosures shall meet explosion, hydro-static, and temperature design tests.

NEMA 9 - Enclosures are intended for indoor use in locations classified as Class II, Groups E, F, or G, and shall be capable of preventing the entrance of dust. Enclosed heat generating devices shall not cause external surfaces to reach temperatures capable of igniting or discoloring dust on the enclosure or igniting dust-air mixtures in the surrounding atmosphere. Enclosures shall meet dust penetration and temperature design tests, and aging of gaskets (if used).

NEMA 12 - Enclosures constructed (without knockouts) for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against circulating dust, lint, fibers, and flyings; and against dripping and light splashing of liquids.

NEMA 12K - Same as NEMA 12 including enclosures constructed with knockouts.

NEMA 13 - Enclosures constructed for indoor use to provide a degree of protection to personnel against incidental contact with the enclosed equipment; to provide a degree of protection against falling dirt; against circulating dust, lint, fibers, and flyings; and against the spraying, splashing, and seepage of water, oil, and non-corrosive coolants.

What does NO or NC mean?

"Normally Open" or "Normally Closed" are classifications that refer to actuated (not manually operated) valves. For an actuated valve, the "normal" state refers to its position when the actuator is not energized.

Therefore a normally open valve is one that is open until the actuator is energized to close it; if power to the actuator fails, the valve opens or "fails open".

Likewise with a normally closed valve if power to the actuator fails, the valve closes or "fails closed".

The "normal" state of an actuated valve must not be confused with the "usual" position of the valve during routine operation of the system. A normally closed valve may be open continuously over a long period of time while a process is running, but it is still a normally closed valve. It just happens that this normally closed valve is usually open.

For an actuated valve to return to its "normal" position, there must be some force, often a spring, that moves the valve from its actuated state back to its normal state. If a valve is actuated to open and then actuated to close, it will "fail" in whichever position it is in when the activator is de-energized. This type of a valve is not considered "fail safe".

A normally closed valve might be used in an application such as the loading of gasoline transport trucks where in the event of a power or automation system failure, you would want to stop flow.

A normally open valve might be used in a situation where you want flow to continue in the event of a power or automation system failure. This might be the case for the cooling water system for a nuclear reactor or for an actuated valve on the water piping to the fire protection system.

What is the difference between proof pressure and over range pressure?

Over range pressure is the working pressure of the unit. That is the pressure unit can see on a daily basis without damage to the unit or compromise in repeatability. Proof pressure is the maximum pressure the unit can see on an occasional basis. Although there would be no physical damage to the unit , there may be a need to recalibrate the unit.

Burst Pressure refers to the pressure at which the device will mechanically fail (leak), and sustain permanent physical damage.

How Much Straight Run Is Enough?

When a fluid flowing through a pipe assumes a desirable flow profile, it moves uniformly with the greatest velocities near the center of the pipe. Improper flowmeter installation can disturb this profile and degrade measurement accuracy. Flow-profile distortion and swirl - the two most prominent types of fluid disturbance that affect a meter's flow coefficients - are typically the product of improper piping configuration.

Fluid profile distortion occurs when an obstruction - such as a partially open valve or a poorly mounted flange gasket - partially blocks the pipe. Swirl occurs when the fluid moves through piping bends in different planes. Swirl is far more difficult to correct than flow-profile distortion. Obstructions upstream and near the flowmeter can cause errors ranging beyond 50 percent.

Flowmeter manufacturers will recommend various lengths of straight pipe upstream and downstream of the flowmeter to attain a fully developed desirable flow profile. Long straight-pipe lengths can be avoided through the use of flow-straightening devices and flow conditioners. Flow-straightening devices include tube bundles, perforated plates, and internal tabs. These solutions reduce swirl, but not flow profile variations; some may even introduce a distorted profile. Flow conditioners can reduce swirl and also mimic a fully developed profile. A grated plate, for example, can introduce such a profile.


As a general rule of thumb straight lengths should be about the same as that required for an orifice installation with a beta ratio of 0.7 - see the table below

Types of Flow

There are in general three types of fluid flow in pipes

  • * laminar
  • * turbulent
  • * transient

Laminar flow

Laminar flow generally happens when dealing with small pipes and low flow velocities. Laminar flow can be regarded as a series of liquid cylinders in the pipe, where the innermost parts flow the fastest, and the cylinder touching the pipe isn't moving at all.

Shear stress depends almost only on the viscosity - µ - and is independent of density - ?.

Turbulent flow

In turbulent flow vortices, eddies and wakes make the flow unpredictable. Turbulent flow happens in general at high flow rates and with larger pipes.

Shear stress for turbulent flow is a function of the density - ?.

Transitional flow

Transitional flow is a mixture of laminar and turbulent flow, with turbulence in the center of the pipe, and laminar flow near the edges. Each of these flows behave in different manners in terms of their frictional energy loss while flowing, and have different equations that predict their behavior.

Turbulent or laminar flow is determined by the dimensionless Reynolds Number.

Reynolds Number

The Reynolds number is important in analyzing any type of flow when there is substantial velocity gradient (i.e. shear.) It indicates the relative significance of the viscous effect compared to the inertia effect. The Reynolds number is proportional to inertial force divided by viscous force.

The flow is

  • * laminar when Re < 2300
  • * transient when 2300 < Re < 4000
  • * turbulent when 4000 < Re

Types of Pressure

  • * Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
  • * Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted.
  • * Differential pressure is the difference in pressure between two points.

Difference between Compound Versus Absolute Pressure Ranges?

Difference between -30" Hg to 15 PSIG Range (Compound) and 30 PSIA (Absolute) Ranges:

The compound range transducer has a vent to atmosphere on the side of the measuring diaphragm opposing the "process" side connection. Therefore the compound range measures pressure relative to atmospheric pressure (Gauge pressure/vacuum). Therefore output is as follows:

  • 4 mA= 30" Hg process Vacuum (Relative to Atmosphere)
  • 12 mA= 0 PSIG (process pressure equals atmospheric pressure)
  • 20mA= 15 PSIG process pressure (relative to atmosphere)

The Absolute range transducer does not have a vent that connects to rear side of tranducer, it is sealed and not influenced by atmospheric pressure. Therefore it measures pressure based on (referred to) absolute pressure as follows:

4 mA= 0 PSIA, 12 mA=15 PSIA, 20 mA=30 PSIA

The difference here is that when you measure 15 PSIA in the process it will output 12 mA, regardless of what the atmosheric pressure is.

With the compound range, if you measure 15 PSIA in process the resulting signal will depend (a bit) on atmosphere pressure.

chemical compatibility

WARNING The information on material chemical compatibility has been supplied to Kobold Instruments by other reputable sources and is to be used ONLY as a guide in selecting equipment for appropriate chemical compatibility. Before permanent installation, test the equipment with the chemicals and under the specific conditions of your application. Ratings of chemical behavior apply at a 48-hr exposure period.

Kobold Instruments has no knowledge of possible effects beyond this period. Kobold Instruments does not warrant (neither express nor implied) that the information given is accurate or complete or that any material is suitable for any purpose.

DANGER Variations in chemical behavior during handling due to factors such as temperature, pressure, and concentrations can cause equipment to fail, even though it passed an initial test.

SERIOUS INJURY MAY RESULT. Use suitable guards and/or personal protections when handling chemicals.

current loop isolators

For industrial process control instruments, analog 4-20 mA and 10-50 mA current loops are commonly used for analog signaling, with 4 mA representing the lowest end of the range and 20 mA the highest. The key advantages of the current loop are that the accuracy of the signal is not affected by voltage drop in the interconnecting wiring, and that the loop can supply operating power to the device. Even if there is significant electrical resistance in the line, the current loop transmitter will maintain the proper current, up to its maximum voltage capability. The live-zero represented by 4 mA allows the receiving instrument to detect some failures of the loop, and also allows transmitter devices to be powered by the same current loop (called two-wire transmitters). Such instruments are used to measure pressure, temperature, flow, pH or other process variables. A current loop can also be used to control a valve positioner or other output actuator. An analog current loop can be converted to a voltage input with a precision resistor. Since input terminals of instruments may have one side of the current loop input tied to the chassis ground (earth), analog isolators may be required when connecting several instruments in series.

Depending on the source of current for the loop, devices may be classified as active (supplying power) or passive (relying on loop power). For example, a chart recorder may provide loop power to a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. (A 4-wire instrument has a power supply input separate from the current loop.) Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. (The battery is the active device).

Panel mount displays and chart recorders are commonly termed 'indicator devices' or 'process monitors'. Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).

The relationship between current value and process variable measurement is set by calibration, which assigns different ranges of engineering units to the span between 4 and 20 mA. The mapping between engineering units and current can be inverted, so that 4 mA represents the maximum and 20 mA the minimum.

Pulsation dampers

Pulsation dampers (aka pulsation dampeners) are normally applied immediately downstream of reciprocating, positive displacement pumps. No matter whether you have simplex metering pumps or large multi-headed machines, reciprocating positive displacement pumps produce pulsating flows which lead to pressure spikes. These pressure spikes may not be desirable in a given piping system, therefore there is a need for a pulsation dampener.

Why does a pump produce a pulsating flow? Considering a single-headed, reciprocating pump, the piston moves toward the pump head on the discharge stroke, pressure within the cylinder rises and product is forced through a check valve and down the discharge line. In order to accelerate the fluid to maximum velocity, each piston stroke must overcome the inertia of the columns of fluid in the suction and discharge pipe work. At the end of each stroke, this inertia must again be overcome to bring the fluid columns to rest. This cycle of alternate acceleration and deceleration is the primary cause of fluid pulsations or pressure spikes.

Generally, the application of pulsation dampers is based on one or more of the following criteria:

  • * to prevent potential pipe hammer / vibration
  • * to reduce the load on the pump itself
  • * to minimize or eliminate pulsations for the benefit of downstream instrumentation
  • * or to minimize or eliminate pulsations as a dictate of process

Straight runs

Straight runs before and after the flowmeter should not be too short in order to avoid disturbances, which can cause the flowmeter to read incorrect values. We recommend at least 10-15 diameters upstream and 5 diameters downstream.

The reason for this procedure is to achieve a stable flow profile inside the pipe and by doing so, get a true reading. Please be aware of the fact that it is practicly impossible to predict when the flow is stable after disturbances in the piping, so this serve as a guideline only. The straight runs must be free from valves, bends or increasing or decreasing diameters.

Any of these disturbances must be placed before and after you start counting the straight runs.


Permanent pressure loss is a term every system engineer, designer, or technician should be aware of. Whenever a piece of equipment or pipe is added to a flow system, pressure is lost. This pressure loss makes the pump or compressor work harder to generate the same flows in the system. If too much pressure loss exists, the system will simply stop flowing. This may be of concern if you are working with both low and high pressure systems. Every bit of pressure loss is equal to extra energy used (electricity, steam, or natural gas) to pump or compress the fluid, i.e. more money to operate.

In the case of flowmeters, a loss is incurred because a piece of straight pipe would not have as much loss as the flowmeter. The loss is also permanent. Permanent pressure loss should not be confused with pressure drop. Meters such as differential pressure-types have a pressure drop inside the meter section. The pressure measured upstream of the meter will be greater than the pressure just downstream of the meter. As you move further downstream of the meter, the pressure recovers to a level not quite as high as the upstream pressure. The difference between the upstream pressure and the downstream recovered pressure equals the permanent pressure loss.

Fluid velocity also plays an important role in permanent pressure loss. The faster the fluid is moving, the greater the pressure loss. Therefore, a permanent pressure loss value must always be associated with a certain flow rate. Meter manufacturers often state the permanent pressure loss at the maximum stated velocity of the meter.

There are many different meter types and all have different characteristics of permanent pressure loss. Some meters have no restriction in the pipe, so therefore no permanent pressure loss. In other words, they incur the same loss as a straight piece of pipe. For example, magnetic meters and ultrasonic meters generally have no permanent pressure loss.

Other meters have a very high loss. These meters have physical restrictions due to the nature of the meter. Examples of high loss meters include curved-tube type coriolis flowmeters and positive displacement meters.

Permanent pressure loss is just one of the characteristics to consider when evaluating a flowmeter. A meter with a low loss is not necessarily better than a meter with a high loss. Every characteristic of the meter technology must be weighed according to the needs of the application.


When the flow has passed through the flowmeter, the pressure is trying to get back to its original pressure, normally in 10-15 pipe diameters downstream. Also at that point the flow profile will again become fully developed. The permanent pressure loss will be somewhere between 0.96 to 0.51 of pressure drop.


Explosion Proof

Generally speaking, "explosion proof" is the more commonly used method for detector/sensor assemblies for fixed gas detection systems, where higher voltages and power requirements may be encountered, and the installation is permanent. Intrinsically safe method can also be used for permanent installations where the detector/sensors are relatively low power devices. Almost all portable instruments use the "intrinsically safe" method.

Intrinsically Safe

An "intrinsically safe" classification and design means that an electronic circuit and it’s wiring will not cause any sparking or arcing and cannot store sufficient energy to ignite a flammable gas or vapor, and cannot produce a surface temperature high enough to cause ignition. Such a design is not explosion proof, nor does it need to be. For permanent installations, such an installation may include intrinsically safe barriers that are located outside the hazardous location, and limit the amount of energy available to the device located in the hazardous area.

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