Instrumentation Specialist

4-20 mA signals from instrument (transmitters) are analog input to control system.

4-20 mA signals to instrument (I/p & electro pneumatic positioner) are analog output from control system.

Volt free (24V) contact (NO/NC) by instruments (all type of switches, ex. Limit switch, press,temp,flow,level switch) are digital input to control system.

All powered signals (24V,48V,110V….) from system to instruments (SOV) are Digital outputs.

Cable tray is nothing but the way or media through which we lay the field cables in plant. There are two basic types

1) Ladder type(made in Rungs type construction)

2) Perforated Type(Solid sheath consist of Holes for ventilation). Basically discussing about tray support than we could say it depends on the site conditions. Only care has to be taken considering adequate space for laying cable, considering their bends. Etc.

The breakdown region of a p-n diode can be made very sharp and almost vertical diodes with almost vertical breakdown region are known a s zener diodes. A zener diode operating in the breakdown region is equivalent to a battery. Because of this current through zener diode can change but the voltage remains constant. It is this constant voltage that has made the zener diode an important device in voltage regulation.

Voltage regulator: The output remains constant despite changes in the input voltage due to zener effect.

AARH refers to average roughness height and is a measure of roughness of sealing surface of a flange. The # refers to ANSI class of flange/valve and is indicative of its pressure rating.

Cold junction compensation is done for temperature measurement using thermocouples. The t/c output is a function of temp. at hot junction as well as temp. at ref. junction. In order to measure hot junction temp. accurately, it is necessary to measure ref. junction temperature and compensate for its variation. This is done by modern digital instruments, by using a ref. junction temp. sensor to measure cold junction temp., and compensating for it using a look-up table corresponding to the t/c.

An electric potential is developed when a conductor is moved across the magnetic field. In most electrical machinery the conductor is a wire. The principle is equally applicable to a moving, electrically conductive liquid. The primary device of commercial magnetic meters consist of straight cylindrical electrically insulated tube with a pair of electrodes nearly flush with the tube walls and located at opposite end of a tube diameter. This device is limited to electrically conducting liquids. The magnetic meter is particularly suited to measurement of slurries and dirty fluids.

Following reasons justify for providing orifice tab:

1. Indication of orifice plate in a line

2. The orifice diameter is marked on it.

3. The material of the orifice plate.

4. The tag number of the orifice plate.

5. To mark the inlet of an orifice.

Burnout provides the warnsug feature of driving indicator at the end of scale, if the input circuit is open. A burnout resistor is provided which develops a voltage drop between the measuring circuit and amplifier. The polarity of the signal determines the direction of the servo drive upon an open circuit in the input.

Upscale burnout: R value 10 M

Downscale burnout: R value 22 M

Balanced draft boilers are generally used negative furnace pressure. When both forced draft and induced draft are used together, at some point in the system the pressure will be same as that of atmosphere. Therefore the furnace pressure must be negative to prevent hot gas leakage. Excessive vacuum in the furnace however produces heat losses through air infiltration. The most desirable condition is that the one have a very slight negative pressure of the top of furnace.

I think the question is wrong – it should read pressure switch and not proximity switch. Set and reset points are kept different to avoid chattering of switch contact at the set point and provide a stable switching action.

A diode is often used in parallel with a relay coil to dissipate energy stored in coil inductance when the coil is disconnected from the electric circuit. It is also known as freewheeling diode.

1. Emitter +ve of meter and base -ve output =Low resistance

2. Emitter -ve of meter and base +ve output =High resistance

3. Collector +ve and base -ve output =Low

4. Collector -ve and base +ve output =Low

Emitter: Collector = High resistance

PNP: Opposite Results

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.

OPEN LOOP : This is nothing but to sense the process signals from the field and to send it to the control room for operator observation.

CLOSE LOOP : This is measuring the process signals for operator’s action. Means Tx sense the process and send it to the control room. Where the operators takes action i.e. control action and that given to the final control element as per the process requirement.

According to the no. of cable occupancy in the cable tray and available tray size we have to choose it. they are available in foll. types 80, 150, 300, 450, 600 & 900.

In open tank the lower pressure side is vented to the atmosphere. All pressure is applied to the high pressure side. This vessel pressure is measured through high pressure side.

The constant voltage circuit consists of a rectifier, CR and a filter capacitor. It is followed by two stages of zener regulation. Abridge configuration is provided to lamp line voltage zener regulation. Regulation 1 and regulation 2 provides relatively provide constant current. Resistors form a bridge that may remoment line voltage effects.

Auto cut in of pump refers to automatic startup of a standby pump when a running pump stops or when level in a vessel being emptied becomes high.

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.