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Instrument Technician Interview Questions
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What is a Thermocouple?

A thermocouple is composed of two dissimilar metals, joined with each other at one end, which produce a small voltage as soon as heated (or cooled). This voltage is measured and used to identify the temperature of the boil metals. The voltage for any one temperature is distinctive to the mix of steels used.

Are There requirements Governing species of Thermocouple?

British requirements Specification, BS 1041, Temperature Measurement gives guidance for the selection and use of tools for measuring temperature.

ASTM standard E230 offers specifications because that the common industrial grades, including letter designations.

Why are there different species of thermocoples?

Thermocouples are obtainable in various combinations of metals, usually referred to by a letter, e.g. J, K etc.

Each combination has a different temperature range and is therefore more suited to details applications than others. Although that is precious noting that the preferably temperature varies v the diameter of the wire used in the thermocouple.

Why room straight lengths essential in installing a vortex/orifice devices?

Like most circulation devices, a Vortex circulation meter needs a fine developed and also symmetrical flow velocity profile, totally free from any distortions or swirls if that is come give an excellent accuracy and also repeatability.

To achieve an excellent accuracy and also repeatability the is standard practice to place the meter some distance from sources of turbulence. Usual sources that turbulence incorporate pumps, valves, transforms in line direction (i.e. Bends), changes in line dimension etc.

How are straight lenghts defined in vortex/orifice?

Most manufacturers provide the user through minimum distances for their certain products. These distances, referred to as right lengths, are indicated in pipeline Diameters (D).


For example, 10 D method place the circulation meter ten times the pipe’s within diameter far from the resource of turbulence. Due to the fact that turbulence both upstream and also downstream have the right to reduce accuracy, manufacturers carry out straight length requirements because that up and downstream the the meter.

Why do we use pipelines for transmitters in vapor service and also tubes in any kind of other service?

A) Pipe and also tubes deserve to be provided for steam line transmitters. However pipe walls being thicker provide an ext corrosion allowance.

B) For vapor services, through high temp and pressure demands IBR standard. Max. Working press is 153 kg/cm2 in ~ 38 degC because that 1/2” O.D tube as per style standard. So tube is no suitable.

What is auto reduced in of the pump?

Auto reduced in the pump describes automatic startup of a standby pump as soon as a running pump stops or once level in a ship being emptied becomes high.

What is Zero speed?

In some equipments a speed listed below a threshold value however close to zero is referred to as zero speed.

What is the purpose of measuring dissolved oxygen?

Dissolved oxygen is measured in boiler feeding water for enhancement of dosing compound to eliminate oxygen to avoid corrosion in steam boilers. In wastewater plants it is measured to estimate high quality of effluent. (oxygen demand)

What is SPDT relay?

The term SPDT refers to single-pole-double-throw relay contact.

How Cold Junction Compensation is done because that thermocouples?

Cold junction compensation is done for temperature measurement utilizing thermocouples. The t/c output is a duty of temp. At warm junction as well as temp. At ref. Junction.

In order to measure warm junction temp. Accurately, that is crucial to measure ref. Junction temperature and also compensate for its variation.

This is done by contemporary digital instruments, by utilizing a ref. Junction temp. Sensor to measure up cold junction temp., and compensating because that it making use of a look-up table corresponding to the t/c.

What is differential tracking system?

In differential tracking system collection point alters with readjust in procedure value in one direction. E.g. Over there is pilgrimage of OMS in MEG plant on high oxygen circulation (with diff tracker).

Trip worth is say 10% more than normal procedure value. Now suppose procedure value is reducing 보다 trip collection value will also reduce maintaining 10% difference. If process value enhanced than trip collection value will not increase and system will certainly trip once trip collection value is reached by process value.

How spectacle blind occurs?

A spectacle remote is a remote plate and also a plate v line dimension bore associated together and installed in between pipe flanges. This blind may be reversed to blind/deblind a line. It additionally provides a visual clue as to whether a line is blinded in ~ that place or not.

How press limit switch occurs?

A pressure switch generally consists of a spring-loaded plunger actuated by procedure pressure to run a microswitch. The pressure at which the microswitch operates is dependent upon feather force, which may be varied by an adjusting nut.

Where need to we use “ open to alert “ and also “Close come alarm “?

A) where probability that an open up circuit fault is higher we usage “open come alarm” and also where probability that a closeup of the door circuit fault is greater we usage “close come alarm” strategy. “Open come alarm” strategy is usually preferred.

B) an option should be such the whenever component/ instrument/ device fault occurs, device should offer alarm.

What is the difference between offset and also hysteresis?

A) An counter is difference between a preferred value and an yes, really value. The phenomenon that a dependent change assuming a different collection of values when the equivalent independent change is raising than when it is to decrease is well-known as hysteresis.

B) offset is fixed prejudice from preferred value. Hysterisis is difference between collection and reset.

“ push switch with setup of 0.5 kg/cm2 (head pressure included) was found in calibration report . What is the meaning of head pressure?

Head pressure refers to the push exerted by liquid inside the advertise pipe connecting the press switch come the vessel. This press acts in enhancement to the actual process pressure in the vessel. Push acting on move = process pressure + head pressure.

Why and also where execute we use seal pots and also condensate pots in level transmitters?

Seal pots or condensate pots are offered to carry out buffer volume of fluid for impulse lines wherein wet foot measurement is used. Little quantities that draining from these lines to produce very tiny changes in head pressure and therefore to reduce errors compared to the situation when no seal pots room used.

Also in many seal pots it is possible to fill liquid right into the impulse line once the heat is fully drained.

How the sizes of proximity switches space selected?

 The size of proximity switches room selected based on sensing distance and mounting requirements.

What is orifice sizing and how it is done?

Orifice sizing is done using adhering to information:

1) circulation measurement range

2) matching diff. Pressure

3) piping dimensions

4) insanity locations

5) liquid properties in ~ operating conditions (like density/temp/pressure etc.) traditional formulas space used.

What is regulate valve sizing?

Control valve sizing refers to calculation of circulation capacity that a valve because that a given fluid, operation conditions and also piping arrangement.

What is galvanic isolation?

The hatchet galvanic isolation ad to electric isolation of two circuits utilizing a transformer or one optoisolator. Yet a transformer provides a much better isolation.

Why is deadband ( Reset and collection ) values preserved in proximity switch or push switch?

Set and also reset points space kept different to prevent chattering that switch call at the set point and provide a secure switching action.

Explain briefly on Thermocouple Types

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Type B

Type B thermocouples can be supplied up to 1600°C with brief term excursions as much as 1800°C.

They have actually a low electrical output, thus are hardly ever used listed below 600°C. In reality the output is essentially negligible approximately 50°C, as such cold junction compensation is not usually forced with this type.

Type E

Type E thermocouples are regularly referred to together Chromel-Constantan thermocouples.

They are regarded as much more stable than kind K, thus often used where a greater degree the accuracy is required.Note – Constantan is Copper-Nickel.

Type J

Type J thermocouples degrade rapidly in oxidising atmospheres over 550°C. Your maximum constant operating temperature is about 750°C though they have the right to with stand brief duration excursions come 1000°C.

They are typically not used below ambient temperature as result of condensation creating on the wires leading to rusting that the iron.Note – Constantan is Copper-Nickel.

Type K

Type K are the many widely offered thermocouples in the Oil & Gas, and refining industries because of their wide variety and low cost. Castle are periodically referred to together Chromel-Alumel thermocouples. Note that above about 750°C oxidation leads to drift and the need for recalibration.

Type N

Type N thermocouples can handle higher temperatures than type K, and offer better repeatability in the 300 to 500°C range. They provides many advantages over type R & S in ~ a tenth of the cost, as such prove to be well-known alternatives.

Type R

Type R thermocouples cover similar applications as type S yet offers boosted stability and a marginal increase in range. Consequently, form R tend to be provided in choice to type S.

Type S

Type S thermocouples have the right to be continually at temperatures up to 1450°C. They deserve to with stand brief duration excursions as much as 1650°C. They need protection native high temperature atmospheres to stop metallic vapour ingress come the tip leading to reduction that emf generated.

Protection commonly available is high purity recrystallised alumina sheath. Because that most commercial applications, thermocouples are housed in a thermowell.

See more: How Many Tablespoons Is 3 Cloves Of Garlic, 6 Cloves Of Minced Garlic How Many Tablespoons

Type T

Type T thermocouples are rarely used in commercial applications, and also lend themselves more to usage in laboratory situations.