Question { Nirma, 3993 }

How to Calibrate variable Head flowmeter (Orifice), here we
dont know High pressure and low pressure then how to apply
pressure on HP and LP side ?

Answer

The situation must be an installed, running system, with a receiver that displays a flow rate from the DP pressure signal, because no one just has a DP transmitter across an orifice plate for fun.

Every differential pressure primary flow element is assumed to be zero flow at zero DP. So the DP low side pressure 'zero' will always be zero DP, or when the 3 valve manifold has its process connection valves closed and the equalizing valve open.

The measured DP represents some flow rate on the square rooted curve of the normalized percentage of span.

In order to determine the DP span value, you measure the actual DP and note what the flow rate reads on the receiver (PLC/DCS/recorder/indicator/controller) at that DP.
Then you can calculate what the DP span value is for the max flow rate that receiver is configured for.

The receiver has some configured flow range: 0-500gpm? 0-1,000 bpd? 0-650pm? Find out what it is.

For example, say you measure a DP value of 700mm wc and that 700 mm wc shows a flow rate reading of 264.5 lpm on the receiver.

You have discovered that the receiver is scaled 0-500 lpm for that DP transmitter and the receiver does the square root extraction.

What is the flow rate as percentage of flow span?
264.5 lpm/500 lpm = 52.9% of flow rate span

52.9% normalized is 0.529

Since the flow rate is the square root of the normalized DP span, you need to square the normalized percentage of flow rate to find the equivalent percentage of DP span.

0.529^2 = 0.2798, or 27.98% (~ 28%) of the DP span.
If 700mm is 28% of span, 100% of span is 2500mm wc.

So the DP span for the transmitter is 0 (no flow) to 2500mm wc (max flow).

Carl Ellis
Measure First

Question { 2285 }

What is passive connection of instrument in different type
of I/O module

Answer

What is passive connection of instrument in different type of I/O module?

What is passive connection of instrument in different type of I/O module?

The output of a field instrument or analyzer that makes a measurement and provides a conditioned, high level signal like 4-20mA is characterized as either active or passive.

An active output is powered internally by the instrument. These are 3 or 4 wire instruments with a separate power wire or wires.

A passive output requires a DC power supply in series with its output in order to excite or power the output. This done to
- limit field wiring to 2 wires
- reduce the likelihood of a ground loop due to difference in ground potential between the field instrument and the receiver.

So, a connection of a passive (2 wire, loop powered) device has to include connections to a DC power supply, observing correct polarity at the field device and the receiver's analog input.

Carl Ellis
Measure First

Question { 5301 }

why we use by pas on a positioner? When can a by pass be
not used on a positioner ?

Answer

Pneumatic positioners equipped with a bypass option can be removed for maintenance without interrupting the valve service.

A position bypass is/was a common optional feature on pneumatic positioners that receive a pneumatic control signal (typically 3-15 psi, 20-100 kPa, or 0.2 - 1.0 bar) from a controller or I/P.

The bypass option is a pneumatic switch. The inlet is the control signal. The valve's outlet is either the positioner (normal mode) or the actuator (bypass mode).

If the positioner needs servicing, the bypass switch is changed from normal mode to bypass mode and the control signal will drive the valve actuator directly. The bypass assembly can typically be removed from the positioner with a couple bolts so that the bypass assembly remains connected to the process, while the positioner is removed for service.

Positioner Bypass operation does not work
- on valves with actuators whose input signal range does not match control signal range (a 1:1 output:input signal ratio). A valve actuator that requires 40-60 psi to actuate will not be actuated by a 3-15psi control signal.
- for split range valves
- for valves with control action (direct or reverse) the opposite of the control signal action.
- Electropneumatic positioners that use a 4-20mA input signal have no bypass option because there's no pneumatic signal to bypass to the actuator.

Carl Ellis
Measure First

Question { 4174 }

HOW CAN I CONNECT ROSEMOUNT 3300 LEVEL TRANSMITTER TO
Allen-Bradley SLC500

Answer

The 3300 is a 2 wire, loop powered guided wave radar level instrument. So it needs 16-42Vdc for power; typically a 24Vdc power supply is used. Copper wire cable, shielded, twisted pair is used. If the radar is in a hazardous area, then either the installation is designed for explosion proofing or Intrinsic Safety (which uses a barrier or I/S isolator).

The SLC500 PLC needs a 4-20mA current analog input card. Usually the instrument power supply is separate from the rack power supply.

The wiring is shown in the manual, but it is traditional 2 wire loop powered wiring.

Carl Ellis
Measure First

Question { 10650 }

how to calibrate bubbler type level transmitter of
rosemount make and how to set air flow according to the
range.. give one calibration procedure with one example

Answer

A bubbler system uses a pneumatic differential relay as a constant flow regulator.  Most bubbler systems include a variable area flowmeter, a rotameter, to indicate the air/gas flow rate and a needle valve for adjusting the flowrate.  

Typical air/gas flowrates for bubblers are 8-30 cc/s (1 to 4 SCFH), a flow rate suitable to create a constant stream of bubbles from the dip tube.

The only 'calibration' of the pressure transmitter in a bubbler system is setting the range of the transmitter's 4-20mA output with a zero-to-span range that takes into account the maximum liquid column head pressure that the transmitter will 'see', which takes into account the specific gravity of the liquid.

Conversion to physical level units (accounting for specific gravity) is done in the receiver (indicator/PLC/DCS/PAC/controller).

Carl Ellis
Measure First

Question { 6172 }

what is the thermocuple compensation circuit

Answer

I disagree with answer #1

>other metals create junction (screw, conducting wire of
another metal)

This does NOT create an error according the Thermocouple Law
of Intermediate Junctions, which states that a 3rd metal
inserted between the two dissimilar metals of T/C junction
will have no effect upon the output voltage as long as the
connection junctions are isothermal, that is, both
connection junctions are at the same temperature.

So compensation is not needed for a 3rd metal because the
error in the positive and negative legs are of opposite
polarity, so the error is self correcting.

There is no practical method of compensating for
non-isothermal conditions so T/C compensation does not refer
to that.

>using ckt that may contain RTD at cold junction, eliminate
known emf at cold junction using electronic ckt

The original question is poorly worded because "thermocouple
compensation" refers to correction of non-linearities of
thermocouple's EMF with respect to the accepted temperature
value. That is done mathematically with a polynomial or in
some cases, with a look-up table, neither of which is an
electronic circuit.

The electronic circuit used with ALL thermocouples is a cold
junction circuit. ALL temperature measurements done with
thermocouples require cold junction compensation. The cold
junction measurement does not 'eliminate' anything, it
provides the required difference between the freezing point
of water and the temperature of the terminal block,
necessary to use standardized tables, all of which are
referenced to the freezing point of water. Without cold
junction compensation, the error of a thermocouple
measurement is the cold junction temperature difference.
That's not uncertainty, that's ERROR.

Industrial applications typically use a thermistor to
measure the temperature of the connection block at the
analog input. That temperature or its EMF equivalent is
'added' to the EMF measurement of the thermocouple.

Cold junction compensation can be done with a 2nd
thermocouple in series inserted into an icebath which still
done for laboratory and high accuracy measurements. A 2nd
T/C qualifies as a circuit, too.

Answer #2 is correct insofar as it refers to cold junction
compensation.

Carl Ellis

Question { 3371 }

Hi Frnds , If Suppose 1 Pair cable of 1.5 sq mm is used for
Transmitter and instead of 1.5 sq mm if i used 0.5 sq mm
cable will the resistance change will occur and does it
affect our transmitter reading. plz mail ur answer to
ntnkamble@gmail.com plz immediately mail ur reply

Answer

It depends on how much voltage the field transmitter needs to operate and the total loop resistance of the loop circuit .

Every 2 wire loop powered transmitter has a specification for the voltage required to drive a given loop resistance. Sometimes that is a formula, sometimes a load line graph, sometimes a statement like "minimum liftoff voltage is 17.7V", or rarely, a vague statement of maximum loop resistance (common with active analog outputs, rare with loop powered transmitters)

Excessive wire resistance in a 2 wire loop powered transmitter circuit can affect the operation by
- preventing the transmitter from turning on and operating
- allowing partial operation at lower current outputs, but failing to operate at higher current outputs. That failure can be either an output that just never exceeds a certain value or a transmitter that cycles on-off-on-off an never initializes.

If there is sufficient voltage to run drive the total loop resistance, then the transmitter could function normally.

You need to calculate the wire resistance over its total wire run length (out and back) and know what the receiver's analog input resistance is. Then look up the specification for how much resistance the transmitter's can drive and see whether your loop resistance is lower than the maximum.

Carl Ellis
Measure First

Question { 6155 }

There is a problem with our HART475 it is not communicating
with smart devices error becomes "NO DEVICE FOUND"we have
insert 250ohm resistance as per requirement but invain plz
tell me the solution?

Answer

1) How do you know that the device talks HART? Many field transmitters talk Brain, Honeywell DE, Foundation Fieldbus, Profibus or 4-20mA only.

Have you confirmed the HART functionality on that device?

2) Point-to-point (communicator-to-transmitter) is usually done with the device at address 0 and with the communicator polling address 0.
Is the communicator polling address 0?
Is the device at address 0?

3) On many transmitters, HART can be disabled. Is HART enabled on this transmitter?

4) The transmitter has to be powered, initialized and in its operating mode in order to communicate via HART. Sometimes transmitters are not powered, but when connecting from the a marshalling panel, it is obvious, because the transmitter is remote location. Is the transmitter powered and operating?

5) Are connections across the dropping resistor or across the transmitter's (+) or (-) terminals? Connecting across a pressure transmitter's 'test' terminals will not work properly for HART communication.

6) A loop isolator module that is not specifically designed for HART will strip off the HART signal, but pass the 4-20mA current signal. If a loop isolator is installed in the loop, is it designed for HART operation.

7) If the power supply ripple and noise exceeds the HART specification, the power supply noise will interfere with HART or prevent communication entirely.

8) The 475 can have 3 terminals at the top where the banana plug field cable plugs in. 2 terminals are for HART, 2 terminals are for Foundation Fieldbus (FF). If the cable is plugged into the 2 terminals for FF, HART communications will be blocked by not having a cable in the correct port.

Carl Ellis
Measure First

Question { 7502 }

what do you mean by Loop checking ? What are the cold loop
check and hot loop check? what are the difference between
these two loops?

Answer

Answer:
A Cold Loop check is cold because it is done without
powering the loop.

A cold check consists of checking wire continuity from the
field instrument through junction boxes and through DCS/PLC
marshalling panel terminals to the control panel. Once
continuity has been confirmed, then a Hot Loop Check is
performed on the Loop's I/O.

A Hot loop check is 'hot' because it is done with the loop
powered up.
A Hot loop check consists of
- simulating a process input at the field instrument and
then checking the instrument's output and the response to
the process signal input at DCS/PLC.
- simulating outputs at the control end and checking for
correct response at the field end.

Carl Ellis
Measure First

Question { 5704 }

How we will check whether the proximity, which we have is
either NPN or PNP, if no data sheet or manual or any
details is available to us ???

Answer

Often identification data is printed on proximity (or prox) sensor's label but it is not uncommon for the label to damaged, missing or unreadable.

Many prox sensors are DC powered, 3 wire devices that conform to a standard wiring color code:

brown is (+) supply voltage, typically 24Vdc
blue is (-) supply voltage common or return
black is the sensor's switched output

To test a 3 wire sensor to determine whether it is a PNP or NPN type you need a DC power supply and a DC voltmeter.

Set the meter to DC voltage to a range greater than the supply voltage (typically 24Vdc), like 50/100/200/600V

The black meter probe should be in the 'common' meter socket
The red meter probe should be in the DC 'volts' socket

Connect the sensor's brown wire to the power supply (+)
Connect the sensor's blue wire to the power supply (-).

Turn the power supply on.

Ensure that the prox sensor is not triggered by presence to an object.

Connect the black meter probe to the blue sensor wire.
Connect the red meter probe to the black switched signal output sensor wire.

If the meter is connected to a PNP device, the meter will read 0V.
If the meter is connected to an NPN device, the meter will read 24Vdc.

Force the sensor to change state/output by detecting an object.
- If it is an inductive proximity switch, place a piece of metal in front of the sensor.
- If it is a photoelectric sensor, block the photoelectric beam.
- For an ultrasonic sensor or a capacitive sensor, you can just use your hand to make the sensor output. Some prox sensors use an LED that lights up when the sensor detects an object.

With an object present, a PNP device's output will change and the voltmeter readout will read 10V to 30V. This is also known as a "sourcing" device.

With an object present, an NPN device's output will change and the voltmeter readout will be 0V. This is also known as "sinking."

Another test to check for PNP:
Connect the power supply (+) to brown, (-) to blue.
Connect the red meter probe to the brown (+) sensor wire.
Connect the black meter probe to the sensor's black signal output wire.

When sensing no object, the signal output will be 10V to 30V.

When sensing an object, the signal output will be 0V.

Another test to check for NPN:
Connect the power supply (+) to brown, (-) to blue.
Connect the red meter probe to the brown (+) sensor wire.
Connect the black meter probe to the sensor's black signal output wire.

When sensing no object, the signal output will be 0V.

When sensing an object, the signal output will be 10V to 30V.

Question { Gasco, 4191 }

capillary type tranmitter using remote seals and capillary
tubes filled with liquid of known of known density so if
you wanna callibrate it just close block valve just before
capilary @ high side & connect druck apply it's calibrated
range pressure as normal calibration ensure low side block
valve must be closed & if you asking commissioning so this
is not a DP type pressure tx so simple if you wanna put in
servie this tx just open high side block valve & than low
side block
same way i do it but still transmitter showing -13 ma.just
tell me what i want to do ?

Answer

A 2 wire, loop powered transmitter can not output a negative current, like -13mA. The 4-20mA output is always positive.

However, a remote seal DP transmitter used for level will 'see' a negative pressure with nothing in the vessel and the LRV (lower range value) for the transmitter will be a relatively negative value, close to the (negative) span of level range.

A DP transmitter with remote seals used for level measurement will ALWAYS have a zero level reading that is a relatively large negative number because of the head pressure of the fill fluid in the capillaries.

The reason for the negative value is that the fill fluid in the high side capillary (which connects to the lower flange) pulls away from the transmitters high side diaphragm as it presses down on the lower seal by the force of gravity.

Oppositely, the fill fluid in the low side capillary (which connects to the upper flange) exerts a positive static pressure on the transmitter's low side diaphragm because the seal is up high and the fill fluid presses down on the low side transmitter diaphragm, again by the force of the acceleration of gravity.

A differential transmitter works by subtracting the low side pressure from the high side pressure:

High side minus low side = differential

As an example, in a vessel 7m tall, the transmitter has two remote seals, each 4 meters in length. The upper seal has to be above the liquid level in the head space above the liquid. The medium is water, SG = 1.00.

Assume a fill fluid with a Specific Gravity of 1.00, 4m of high side capillary and 4m of low side capillary. With nothing in the vessel (zero level), the applied pressures would be:

high side pressure = approx - 3.75m H2O (negative 3.75 meters, water column)
low side pressure = approx 3.75m H2O (positive 3.75 meters, water column)

high side minus low side = differential
-3.75m minus 3.75m = - 7.5m (negative 3.75m minus 3.75m = negative 7.5m)

The LRV (lower range value) would be approximately -7.5m H2O.

The transmitter needs to be ranged properly, with the appropriate lower range value for zero level.

Carl Ellis
Measure First

Question { 2772 }

what is instrument termination

Answer

Termination involves cutting and stripping the instrumentation wire, dressing the shield/screen with insulation on one end, installing crimp ferrules and connecting the wires to the screw or spring terminal terminal blocks.

Tradecraft includes maintaining the integrity of shield/drains at field instruments, junction boxes and marshalling panels.

Carl Ellis
Measure First

Question { SCS, 6040 }

Consider a control valve action,the inlet air pressure is only 6 ksc but
physically how it can be operate against 100 ksc line fluid pressure?

Answer

The reference to 6 ksc pressure means that the valve uses a
pneumatic actuator.

The ability of a pneumatic actuator to work against the line
pressure depends on the force that it can exert on the
valve's plug/cage/trim against the line pressure's force and
the bench setting of the spring.

A maximum of 6ksc of air inlet pressure can be exerted on
the total area of the actuator's diaphragm.

If the diaphragm area * air inlet pressure > plug area *
line pressure force + spring force, then the actuator will
move the valve plug. If not, the line pressure will move
the actuator.

A standard step in sizing a valve assembly with an actuator
is to check that the actuator can close against the maximum
line pressure and the spring setting at a given air supply
pressure. All vendors provide sizing charts for this purpose.

Question { 6486 }

A DP Endress and Hauser flow transmitter is always tripping
pump at low flow. set point is 74 cubic meter/hr. Thorough
impulse line flushing and zero calibration has been done,
yet problem persists. What do you think is the problem and
how do you rectify this fault?

Answer

The stated question assumes that the design criteria for a
low flow alarm SP at 74m^3/hr is valid. It is not clear
whether the problem is limited to the instrumentation
without a clear statement that the pump has been determined
to be operating properly. Since this is an instrumentation
exam, I'll assume the problem is in the instrumentation.

If the instrumentation is the problem, then the DP's
indicated DP/flow rate has to be lower than the alarm SP.
What can cause a DP transmitter to indicate a lower
DP/flowrate than it should?

If the equalizing valve is not tightly closed or leaks at
the valve seat, the DP will be lower than it should be as
the DP approaches zero, depending on the leak rate.

Damaged orifice plates read low, resulting in low indicated
DP readings.

Lower line pressure than the "design" line pressure will
reduce the indicated DP reading.

Someone changed out the orifice plate (OP)with an OP with
bigger bore or re-installed the correct OP backwards.

Since DP operation is "high side pressure minus low side
pressure equals indicated DP", a leak anywhere on the high
side will reduce the high side DP pressure and result in a
low indicated DP. That could be
- leak in the high side impulse line or its fittings
- manifold valve flange gasket leak on the high side or on
the process side
- vent leak
(however liquid leaks are usually visually evident)

In a mis-configured system, the PLC/DCS might have square
root extraction in addition to square root extraction on the
DP transmitter, which would cause a low flow rate indication
at the PLC/DCS.

Tell 'em Carl sent you.

Question { Bapco Bahrain Petroleum Company, 4430 }

Explain " FAIL SAFE " feature in temp. measurement.

Answer

The fail-safe feature in temperature measurement uses some means to detect an open circuit in the sensor input wiring. An open circuit could be a broken wire, loose connection or a failed sensor.

Should an open circuit be detected, the output signal representing temperature will drive either full scale up usually to a value higher than 20.0mA or full scale down to a value lower than 4.0mA in order to flag, alarm or indicate a failed input circuit.

The failsafe feature and setup for upscale or downscale under fault conditions is sometimes referred to as "thermocouple break". In heating applications, upscale break is usually chosen because an indicated very high temperature on a controller's input will drive a controller's output down to 0% output (assuming reverse control action), preventing an overheat, fire or potential damage situation.

Some failures, such as a shorted thermocouple extension wire or sensor drift can not be detected in with open circuit detection.

Carl Ellis
Measure First