Question { Wood Group, 2489 }

How to calibrate and check thermal mass flow meter of Make thermal instrument company inc model 60?

Answer

A Thermal Instrument was flow calibrated at the factory for a certain specified medium/fluid. If their flow meters can be recalibrated in the field for fluids other than the one specified at time of manufacture, the calibration instructions are not posted on the web site.

Their web site does have a fairly long and involved calibration check for landfill gas application (methane generated in garbage dumps/landfills) under the FAQ: What is your procedure on checking the calibration of a landfill gas meter in the field? It involves on-site multiple pitot flow measurements in a traversing pattern to get a flow average, calculating m molecular weight of the gas mixture, calculating density from the ideal gas law, and then converting to velocity and finally calculating mass flow with the tube's cross sectional area. Simple, huh?

The analog output of the transmitter, the zero at 4.00mA and the span at 20.00mA, can be adjusted with a field calibration check and adjusted as necessary.

Typically, the flow rate can only be calibrated on a certified flow stand or flow cell; field calibration is not practical. Return to the factory is typical.



Carl Ellis Measure First

Question { 32980 }

How to convert 0-2500mmH2o into kg/hr?
Or how to convert mmH2O into kg/hr,t/hr etc

Answer

mmH2O is units of pressure, probably differential pressure in a case involving flow rate.


kg/hr is units of mass flow rate, kilograms per hour.



There is no inherent or implicit relationship between mmH2O and kg/hr. Therefore, there is no conversion between the two.

Differential pressure can be used to measure volumetric flow rate of liquids and gases and an inferred mass flow rate can be calculated for liquids with a known density and gases when there is static pressure and temperature compensation.


But not only do those those measuring conditions need to be met, but DP flow rate is dependent on a specific differential primary flow element and the characteristics of the fluid involved.


That means a measurement in mmH2O pressure drop in a specific design for a liquid differential flow meter could might be converted to a kg/hr mass flow rate (with appropriate static pressure and temperature for gas), but it would be specific only for that application and not transferable to any other application.

Any given DP measurement, say 2000 mmH2O, can represent 1.20Kg/hr in one flow meter and 45,000kg/hr in another flow meter. There is NO inherent relationship. Any correlation is application-specific.

Question { 3669 }

In dcs contol valve parameters showing like this












Gain=1 or 2 , ti=7 sec, td=20sec. But temp,pressure,level control valves we are using in which type parameter side using ti,td clearly tell me

Answer

Gain, ti and td are PID proportional control tuning terms/constants. PID controllers use PID tuning constants to calculate an output based on the error signal; final control elements (valves) do not do a PID calculation so valves are not involved with gain, ti and td. The controller is typically in the control room, the valve is somewhere out in the process.

A PID controller calculates the expected valve/damper position from the error signal and the tuning constants, the result of which is an output control signal ranging from 0-100%.

A valve positioner or valve I/P receives that control signal from a controller (4-20mA, Profibus) and supplies air or electric power to the actuator to achieve the correct position demanded by the control signal.

Ti is the term for reset or integration. It is used when the process itself is not an integrating process. Level, pressure, and pH are usually integrating processes. Sometimes temperature is an integrating process, sometimes not. When the process is non-self regulating/integrating process, the ti term is not used in the PID calculation.

Flow and temperature processes typically use the ti term for PID calculation.

An exception to the rule of PID being done in a separate controller is Foundation Fieldbus (FF), which allows function block PID execution in FF devices. In a case where PID control is executed in an FF valve positioner, then a valve would use the PID tuning constants.

Question { Edac, 3934 }

In field side transmitter voltage droping here no loose connection then what will happen?

Answer

The minimum required loop voltage depends on the voltage needed to drive 20ma (or 21.5mA if fail-safe high is used) through the total loop resistance.

Even if the power supply voltage sags/drops slightly, it is still possible that the loop will function properly, because it depends on the actual loop resistances and the lift-off voltage required by the transmitter

If there is not sufficient loop voltage, it will become apparent when the transmitter fails to go above a certain milliamp output, because there is insufficient voltage to drive more current through the loop's total resistance.

In some cases of low loop voltage, a field transmitter will cycle on and off, re-initializing each time it fails to push sufficient current because it is voltage-starved.

If the loop wiring is sized properly, then a voltage drop is likely due to
- a bad connection in junction box
- in some cases a ground loop from damaged wire insulation
- adding too many additional loads - more transmitters than the power supply is rated for
- failing power supply
- adding another device in series in the loop, like an indicator, recorder, or I/O module input that pushes the total loop resistance too high. Carl Ellis
Measure First

Question { Pakistan Steel, 3357 }

How to do ma trim in massflow meter?

Answer

There are thermal dispersion, coriolis, differential pressure multivariable and solids belt scale mass flowmeters from a variety of manufacturers.

Each device has a specific procedure for trimming the mA output, if that's what the output is (it could be digital or fieldbus protocol). The specific manufacturer provides instructions on trimming the mA output.

The decision is to whether one trims the output to a calibrated milliammeter or whether one does a 'loop calibration' in which the HMI value is trimmed to match the flow meter's output.

A positive means of flow shutoff, bubble tight valving, is necessary because thermal or pressure siphon flows as well as back flow leakage through a pump or 'leaky' valves can occur when the pump is turned off.

Carl Ellis
Measure First

Question { 3349 }

What is the difference between floating and fixed voltage?

Answer

For fixed voltage, (assuming you mean ground-referenced), one DC line or one AC phase (or neutral) is connected to ground (AKA earth).

The output of a floating power supply is isolated from ground/earth.

A floating voltage is generally produced from the output of a transformer, a power supply or a battery, unless the circuit powered is tied to a ground reference.

A voltmeter will not detect a floating voltage when measuring between the floating point and ground.

Carl Ellis
Measure First

Question { 2295 }

what is the difference between impact plate n coriolis type solid flow meters n how they works....n which one is better

Answer

One, the impact plate, is for solids. The other, coriolis, is for liquids and gases.

The flow of solids produces a mechanical deflection as the material strikes the flowmeter sensing plate before continuing

through the process unhindered.

Coriolis measures the phase shift the mass of liquid or gas creates in a bent tube that is vibrated at a known frequency.

Each is a mass flow meter for different media and you can not subsitute one for the other. So a comparison is irrelevant.

Carl Ellis
Measure First

Question { Bhole Baba Dairy Industries, 3146 }

How to connect another 24 volt dc power supply in circuit when previous power supply has become weak.what is the connection?

Answer

A second power supply is wired in parallel with the first, but redundant/parallel power supplies should be decoupled from each other with power diodes, where each diode is forward biased on the power supply (+) output with the diode (-) coupled in common to the load).

The diodes need to be rated to carry the full current load. If diodes are not used, a severe fault in one of the power supplies could cripple the output of the 2nd power supply.

The isolation diodes must be installed on a heatsink to maintain operating temperature at a safe level. The power dissipation on the diode is determined by the product of the current passing through the diode and its "on" diode voltage drop.

Some recommend the use of Schottky diodes rather than silicon diodes becaus Schottky diode has a lower voltage drop so it runs cooler.

There are commercial diode decoupling/isolation packages sold with appropriately sized heat sinks to keep the diodes within temperature spec.

Carl Ellis
Measure First

Question { IOCL, 4727 }

in current transmitter why we use 4-20ma ???

Answer

1) A current signal has the same current in all parts of the loop, unlike a voltage that drop voltage over distance, which provides reliability that a voltage signal does not.

2) A current signal is more resistant to noise pickup than a voltage signal is.

3) A live zero at 4mA allows for '2 wire operation', where the power for the loop uses the same two loop wires as the signal, a critically important feature for geographically dispersed industries like oil & gas, steel, paper and mining. 2 wire loop powered transmitters use about 3.5 or 3.6mA of current to operate the transmitter.

4) A live zero is a diagnostic detection of an open circuit

5) 4-20mA is a low enough current with limited energy that transmitters can be designed to operate intrinsically safe in hazardous areas with the appropriate barrier protection from faults in the safe area.

6) The DC current signal is suitable for superimposing a modulated FSK signal used by HART for digital communications that allow for configuration, troubleshooting, diagnostics or multivariable data transmission, all while retaining an analog primary variable

7) 4-20mA drives considerable sized loads at fairly low DC voltages, allowing for a relatively long distance signal path (1-2km depending on wire size). 20mA has a relatively low IR drop.

Question { Reliance, 4281 }

why gray colour is choosing to all instrument panel.why not choosing other colour?

Answer

The color Gray is used because it is a neutral color.

One consideration in the selection of color is heat dissipation.

IEEE published a paper by Ralph H. Lee at E. I. Du Pont (1975), The Effect of Color on Temperature of Electrical Enclosures Subject to Solar Radiation by Effect of Enclosure Color and Finish, which is summarized by the extract:

"Electrical enclosures have historically been painted with dark colors ostensibly to enhance the radiation of internally generated heat.

Outdoors, these dark colors are great absorbers of solar radiation, from which the enclosure incurs an appreciable temperature rise. The lighter colors absorb less solar radiation, incurring smaller solar temperature rise. However, it is demonstrated that these lighter colors radiate the lower temperatures of the equipment as well as the darker colors.

Hence, equipment painted with the lighter colors will operate at a lower temperature and be subject to less severe derating from the effects of solar radiation."

Many vendors offer custom painted enclosures.

Carl Ellis
Measure First

Question { 5525 }

why the output sugnal is 4-20 ma and not 4-25 ma?

Answer

@Marmkijani
The main reason for 4-20mA is that 2-wire field instruments need to use 3.5mA to power their own electronics in order to make a measurement and to control the regulated output.

4-20mA is also, as you pointed out, a range in which the energy in the circuit is low enough to qualify for Intrinsic Safety approval (when used installed properly with an approved DC power supply and I/S barrier).

The "live zero" is nice and convenient as a diagnostic for an open circuit fault, but it was not the main reason for 4-20mA; powering field instruments was the main reason.

Question { 5525 }

why the output sugnal is 4-20 ma and not 4-25 ma?

Answer

Historically, the 4-20 mA electronic control signal replaced the 3-15 psi, 20-100 kPa or 0.2-1.0 bar pneumatic control signals and its competitor 10-50 mA.

The ratio of contorl signal span to its 'live zero' is the same in all cases: 4:1

The ratio of span to the 'live zero' for 3-15 psi is:
span = 12 psi (15-3)
live zero = 3
ratio span:live zero = 12:3 = 4:1

The ratio of span to the 'live zero' for 20-100 kPa is:
span = 80 psi (100-20)
live zero = 20
ratio span:live zero = 80:20 = 4:1

The ratio of span to the 'live zero' for 0.2-1.0 bar is:
span = 0.8 bar (1.0-0.2)
live zero = 0.2
ratio span:live zero = 0.8:0.2 = 4:1

The ratio of span to the 'live zero' for 10-50 mA is:
span = 40 mA (50-10)
live zero = 10 mA
ratio span:live zero = 40:10 = 4:1

The ratio of the span of the active control signal to its live zero is identical in all cases.

Carl Ellis
Measure First

Question { OPaL, 4458 }

Where should we terminate the shield and why?

Answer

Shielding reduces electrostatic or capacitive coupled electrical noise in signal or communications cable

Electrostatic or capacitive coupling is proportional to the capacitance between the noise source and the signal wires. The magnitude of the interference depends on the rate of change of the noise voltage and the capacitance between the noise circuit and the signal circuit.

Electrostatic noise can be addressed by installing an electrostatic shield (also called a drain) around the signal wires. The currents generated by the noise voltages prefer to flow down the lower impedance path of the shield/drain rather than the signal wires.

The shield/drain must be of a low resistance material such as aluminum or copper. For a loosely braided copper shield (85% braid coverage), the screening factor is about 100 times or 20 dB. For a low resistance multi layered screen, this screening factor can be 35 dB or 3000 times.

The shield should be insulated to prevent inadvertent contact with multiple ground points, which could result in circulating currents.

The shield should never be left floating because that would tend to allow capacitive coupling, rendering the shield useless.

A single point connection to ground attempts to minimize the possibilty of ground-loop current that will flow between grounds at different potentials. A shield grounded at both ends can form a ground loop which can cause a processor to fault if those grounding points are at different potentials.

Vendors may specify grounding the shield at either the field end or the receiver end. When the shield/screen is grounded at one end only, the grounding is usually made in equipment panel/cabinet at the end with the power supply or the receiver. The field device end is left ungrounded and the shield/screen insulated.

At sites with equipotential grounding where the grounding is the same potential between grounding points, some vendors (Profibus DP's RS-485) recommend grounding the shield at both ends.

Cabling that runs through intermediate junction boxes shall have the integrity and continuity of shield connection maintained in the junction box without connecting the shield to a ground in the junction box.

The signal wire shield is never connected to the common side of a logic circuit (this would introduce noise into the logic circuit).

For high frequency RF noise rejection, a capacitor is inserted in series between the shield's drain wire and the ground connection.

For some communication network cables, the shield connections are unique to the particular cabling system. In some such cases, a dc short to ground is not needed because a low-impedance ac path to ground and a high-impedance dc path to ground are provided internally at each node. Follow the specific instructions in the publication provided for the specific communication network cabling system.

At any shield grounding connection, do not strip the shield back any further than necessary to make a connection.

Carl Ellis
Measure First

Question { ONGC, 5158 }

my temperature transmitter in field showing 700 degree , whereas in dcs showing 459 degree, how to resolve this?

Answer

1) Most likely, the 4-20mA scaling (what the 4-20mA represents) is not the same on the transmitter as it is on the DCS.

The scaling has to match, that is, be the same on both the transmitter and the DCS analog input.

2) A ground loop will cause an offset. The offset can be negative or positive, depending on the polarity of the ground loop.

Eliminating ground loops can be a major task, so most times, a loop isolator is used to isolate the transmitter from the DCS, which eliminates the ground loop.

Carl Ellis
Measure First

Question { Aramco, 3115 }

Controller not working in auto mode?...what are the problem can occur...Pls explain

Answer

Question:
Controller not working in auto mode?...what are the problem can occur...Pls explain

Answer:
If a controller is not in automatic mode it is either broken or in manual mode.

In manual mode the controller's output is fixed at some percentage output and the controller is not adjusting its output to maintain the setpoint (SP).

While running at a fixed output, the process's process variable (PV) will generally stabilize at some value above or below the setpoint. PV changes will be dependent on external influences, like load changes.

However, if the output reaches an extreme, like 100% or 0%, the process can be put in jeopardy of going into a hazardous state from the uncontrolled input into the process or removal from the process of energy or mass, whatever the controller is controlling. Presumably, safeties are in place to shutdown the process should those dangerous conditions result from manual mode.

Carl Ellis
Measure First