Monday, July 2, 2012
Instrumentation Common Questions -2
EPC School 2012 Series
I for Instrumentation
Q: What is potential free contact? What is the significance and application of this contact?
Contacts having not potential. E.g. Relay contacts/ field switches contacts. They are used in logic circuits. A potential free contact is usually wired into an electrical circuit. However it must be ensured that the contact ratings are suitable for the service in which it is used.
Q: In split range control, whether the signal is splitted through I/P convertor or the convertor itself?
This can be typically achieved by two ways:
By connecting o/p of one I/P converter to two positioners adjusted suitably for split range operation of control valves. Taking two AO from DCS. Split range to be defined in DCS. Both I/P converters and positioners to be calibrated with input as 4to20 ma dc and 3to15 psi respectively.
Q: When do we use SOV of rating 110vac and 110vdc?
Primarily depends upon the availability of reliable power supply source.
Q: How is cold junction compensation in thermocouple carried?
This is typically performed in modern programmable instruments by means of measuring actual reference junction temperature using a temperature sensor mounted close to the ref. junction and compensating for the same using appropriate look-up table stored within the instrument’s memory.
Q: In which applications do we use 4 wire RTD?
Three wire is a better alternative. Primary objective of 3-wire and 4-wire arrangements is to eliminate effects of lead resistance on temp. measurement.
Q: What is the sensor used in coriollis mass flow meter to measure density?
Density is measured here by measuring the resonant frequency of a vibrating U-tube.
Q: What if thermocouple wire is opened in the field? What signal goes to DCS?
In most modern instruments the signal may be programmed to go to either maximum or minimum depending upon end user’s requirement.
Q: if the power supply connections to a two-wire transmitter get interchanged? What signal will go to DCS?
Usually there is a blocking diode to protect the transmitter against supply reversal and almost zero current signal should be transmitted.
Q: What are the possible reasons for the failure of barrier? !?!
Fuse blowing sometimes. Power circuits are most likely to fail.
Q: How can we say that the given RTD or Thermocouple is correct?
We can only measure sensor output (resistance / maillots ) accurately and look-up corresponding temperature in reference tables. The accuracy depends upon quality / condition of the sensor. Degraded sensors may not give accurate readings and must be replaced. To test a sensor, the sensor response may be tested using a high quality temperature calibrator and compared with reference tables.
Q: In some cases we have to select the cam position in a control valve for different application? How do we select that?
Refer to instruction manual for the positioner / control valve. The cams are often marked with limited amount of information, which may help an experienced person.
Q: What is the difference between a protocol and a field bus?
A protocol defines a standard method for communications. A fieldbus is a multidropping arrangement where multiple instruments communicate with special interface hardware using the same pair of wires and in most of the cases draw power from the same pair of wires.
Q: What happens if transmitter wires get shorted?
The barrier if installed limits electrical energy flowing into hazardous area. If there is no barrier, typically a fuse in the power distribution system will blow.
Q: What will happen if thermocouple wires get shorted?
A cold junction compensated instrument will typically indicate temperature of the location where the T/C wires are shorted.
Q: Why do we require loop-terminating resistor in any digital communication loop?
A minimum loop resistance is required so that modulated current signal produces a modulated voltage signal, which may be detected by the receiving equipment.
Q: What is adapter flange?
It is a transmitter part for allowing process connection to pipe/tube.
Q: How to calculate the safe distance between cables to avoid electromagnetic interference of each other ?
The design engineers / equipment manufacturers follow/publish certain guidelines w.r.t. different types of cables and the voltages/currents and types of signals carried by them.
Q: How Control loop should be tuned in process loop?
You may use Ziegler-Nichol’s method ( open loop / closed loop ) or special tuning software tools.
Q: What is the significance of single ended & differential ended input for PLC? Application wise comparison of these two types of inputs?
Differential inputs provide better common mode rejection and signal-to-noise ratio.
Q: What is Ground Loop? Preventive steps to avoid ground loop?
When ground wiring is not done properly, grounding of various points is not effective and potential differences exist between them resulting in currents flowing between them. This leads to measurement errors and is not desirable. It can be eliminated by proper ground wiring.
Q: In a globe type control valve, what is the importance of flow direction (top to bottom or bottom to top)?
Control valves must be installed as per direction marking provided by the manufacturers or instruction manuals. Though people tend to generalize, this is often misleading.
Q: What is ATEX directives / FM Approval / CSA approved / CE certified ? What is the importance of individual certification? Are all these certification required for each instruments? Which certification do we prefer?
ATEX/FM/CSA certifications generally refer to certification for suitability of instruments for use in hazardous area when installed in accordance with recommended guidelines. Any certification, which is locally acceptable as per statutory requirements, may be used. We typically accept American/European/Indian certifications/approvals in India.
Q: Advantages of tachometer as speed measuring device compared to inductive type proximity switches?
Some tachometers provide analog output with almost instantaneous response time. They are highly suitable for speed control in some applications.
Q: Application wise advantages of Inductive type proximity switches over capacitive type switches?
Inductive proximity switches are better suited for detection of conducting metal objects and are easily tested for proper operation. Capacitive switches are typically used for detecting non-conductive materials.
Continued....
I for Instrumentation
Q: What is potential free contact? What is the significance and application of this contact?
Contacts having not potential. E.g. Relay contacts/ field switches contacts. They are used in logic circuits. A potential free contact is usually wired into an electrical circuit. However it must be ensured that the contact ratings are suitable for the service in which it is used.
Q: In split range control, whether the signal is splitted through I/P convertor or the convertor itself?
This can be typically achieved by two ways:
By connecting o/p of one I/P converter to two positioners adjusted suitably for split range operation of control valves. Taking two AO from DCS. Split range to be defined in DCS. Both I/P converters and positioners to be calibrated with input as 4to20 ma dc and 3to15 psi respectively.
Q: When do we use SOV of rating 110vac and 110vdc?
Primarily depends upon the availability of reliable power supply source.
Q: How is cold junction compensation in thermocouple carried?
This is typically performed in modern programmable instruments by means of measuring actual reference junction temperature using a temperature sensor mounted close to the ref. junction and compensating for the same using appropriate look-up table stored within the instrument’s memory.
Q: In which applications do we use 4 wire RTD?
Three wire is a better alternative. Primary objective of 3-wire and 4-wire arrangements is to eliminate effects of lead resistance on temp. measurement.
Q: What is the sensor used in coriollis mass flow meter to measure density?
Density is measured here by measuring the resonant frequency of a vibrating U-tube.
Q: What if thermocouple wire is opened in the field? What signal goes to DCS?
In most modern instruments the signal may be programmed to go to either maximum or minimum depending upon end user’s requirement.
Q: if the power supply connections to a two-wire transmitter get interchanged? What signal will go to DCS?
Usually there is a blocking diode to protect the transmitter against supply reversal and almost zero current signal should be transmitted.
Q: What are the possible reasons for the failure of barrier? !?!
Fuse blowing sometimes. Power circuits are most likely to fail.
Q: How can we say that the given RTD or Thermocouple is correct?
We can only measure sensor output (resistance / maillots ) accurately and look-up corresponding temperature in reference tables. The accuracy depends upon quality / condition of the sensor. Degraded sensors may not give accurate readings and must be replaced. To test a sensor, the sensor response may be tested using a high quality temperature calibrator and compared with reference tables.
Q: In some cases we have to select the cam position in a control valve for different application? How do we select that?
Refer to instruction manual for the positioner / control valve. The cams are often marked with limited amount of information, which may help an experienced person.
Q: What is the difference between a protocol and a field bus?
A protocol defines a standard method for communications. A fieldbus is a multidropping arrangement where multiple instruments communicate with special interface hardware using the same pair of wires and in most of the cases draw power from the same pair of wires.
Q: What happens if transmitter wires get shorted?
The barrier if installed limits electrical energy flowing into hazardous area. If there is no barrier, typically a fuse in the power distribution system will blow.
Q: What will happen if thermocouple wires get shorted?
A cold junction compensated instrument will typically indicate temperature of the location where the T/C wires are shorted.
Q: Why do we require loop-terminating resistor in any digital communication loop?
A minimum loop resistance is required so that modulated current signal produces a modulated voltage signal, which may be detected by the receiving equipment.
Q: What is adapter flange?
It is a transmitter part for allowing process connection to pipe/tube.
Q: How to calculate the safe distance between cables to avoid electromagnetic interference of each other ?
The design engineers / equipment manufacturers follow/publish certain guidelines w.r.t. different types of cables and the voltages/currents and types of signals carried by them.
Q: How Control loop should be tuned in process loop?
You may use Ziegler-Nichol’s method ( open loop / closed loop ) or special tuning software tools.
Q: What is the significance of single ended & differential ended input for PLC? Application wise comparison of these two types of inputs?
Differential inputs provide better common mode rejection and signal-to-noise ratio.
Q: What is Ground Loop? Preventive steps to avoid ground loop?
When ground wiring is not done properly, grounding of various points is not effective and potential differences exist between them resulting in currents flowing between them. This leads to measurement errors and is not desirable. It can be eliminated by proper ground wiring.
Q: In a globe type control valve, what is the importance of flow direction (top to bottom or bottom to top)?
Control valves must be installed as per direction marking provided by the manufacturers or instruction manuals. Though people tend to generalize, this is often misleading.
Q: What is ATEX directives / FM Approval / CSA approved / CE certified ? What is the importance of individual certification? Are all these certification required for each instruments? Which certification do we prefer?
ATEX/FM/CSA certifications generally refer to certification for suitability of instruments for use in hazardous area when installed in accordance with recommended guidelines. Any certification, which is locally acceptable as per statutory requirements, may be used. We typically accept American/European/Indian certifications/approvals in India.
Q: Advantages of tachometer as speed measuring device compared to inductive type proximity switches?
Some tachometers provide analog output with almost instantaneous response time. They are highly suitable for speed control in some applications.
Q: Application wise advantages of Inductive type proximity switches over capacitive type switches?
Inductive proximity switches are better suited for detection of conducting metal objects and are easily tested for proper operation. Capacitive switches are typically used for detecting non-conductive materials.
Continued....
Labels: common interview instrumentation questions, control, engineering, EPC School, instrument, instrument questions, instrumentation, Interview, interview questions, Questions, Rahul Kapoor, transmitters, valves
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