Electromagnetic Flowmeter

The electromagnetic flow sensors are useful for measuring the movement of conductive liquids. They are true noninvasive measurements. The operating principle is based on Faraday’s Law of electromagnetic induction.

“If a conductor of length L (m) is moving with a velocity v (m/s), perpendicular to
a magnetic field of flux density B (Tesla), then the induced voltage e across the ends of conductor can be expressed as:

e = BLv

The magnetic field, the direction of the movement of the conductor, and the induced emf are all
perpendicular to each other.”

In the case of electromagnetic flowmeters, the conductor is the liquid flowing through the pipe, and the length of the conductor is the distance between the two electrodes, which is equal to the tube diameter. The velocity of the conductor is proportional to the mean flow velocity of the liquid.

The generated voltage does not depend on parameters such as pressure, temperature, viscosity, conductivity, etc. Only a minimal level of conductivity is required to give this signal a (very small) minimal power.

The accuracy of these meters can be as low as 0.25% and, in most applications, an accuracy of 1% is used.

The traditional magnetic flow meter consists of two units: the measuring probe and electronic converter-amplifier that transforms this mV signal into one or more standard analog or digital signals.

The electrodes are placed at positions where maximum potential differences occur. The electrodes are electrically isolated from the pipe walls by nonconductive liners to prevent short-circuiting of electrode signals. The liner also serves as protection to the flow tube to eliminate galvanic action and possible corrosion due to metal contacts.

The main body of a flowmeter and electrodes can be manufactured from stainless steel, tantalum, titanium, and various other alloys.

The measuring probe consists of two electrodes made of a non-magnetic material
and is positioned at the inside covered with an electric insulating layer. Commonly used insulating layer are Natural rubber or neoprene, PTFE (Teflon), PFA (Per Fluor Alkoxy), Ceramic.

Coil excitation

The magnet coils generate a magnetic field that depends on the form of the field
excitation signal. There are several types of flow meters coil excitation.

(a)    DC excitation: It is only applicable with liquid metals. DC excitation results in electrolysis and hence they are hardly ever used.

(b)   AC or Sinusoidal excitation: An alternating current of 50 Hz to 60 Hz in coils creates
the magnetic field to excite the liquid flowing within the pipe. The disadvantage of these applications is that the electronic noise causes the zero point to drift after a certain time. It is essential to manually readjust the zero point at regular time intervals.

(c)    Pulsed DC field: here the converter feeds the magnetic coils with a pulsed DC (low frequency square wave) current. Because the converter is provided with the necessary intelligence, it can independently control the zero point, so that the zero point is stable. This method has lower power consumption (5-25W).

(d)   Capacitive detection: Here the electrodes, which are in contact with the medium are replaced by capacitive plates and that function as the electrodes of a capacitor. For this model the minimum required conductivity is 100 times lower than for the model with the contact electrodes.

The installation of the flow sensor may occur in every position, as long as the measuring instrument is completely filled with the fluid. If the fluid contains solid particles or fats, the magnetic flow meter is best positioned vertically. When placing it horizontally, the heavier particles will precipitate and the lighter particles will come up.

Theoretically the direction of the flow is not important, as long as the correct electric connection is used.

At a low rate of flow (< 1 m/s) the desired accuracy cannot be obtained. The velocity can be increased by reducing the pipe.

The flow profile is not very important for the magnetic flow meter. In practice,

however, a straight pipe segment of 5 times the pipe’s diameter is recommended. The grounding is of vital importance. This is very important because the voltage on the electrodes amounts to only a few mV.

The pipes of electromagnetic flowmeters must be full of liquid at all times for accurate measurement. If the liquid does not make full contact with electrodes, the high impedance prevents the current flow; hence, measurements cannot be taken. Also, if the pipe is not full, even if contact is maintained between the liquid and electrodes, the empty portions of the pipe will lead to miscalculated flow rates.

Important features

·         Completely obstruction-free and hence no pressure loss.

·         Highly accurate, better than 1% FS for higher flow.

·         Wide span, good linearity.

·         Measuring principle not dependent on pressure, temperature or viscosity.

·         Expensive because of electronics.

·         Minimum conductivity required (5µS/cm).

·         Not dependent on the flow profile

·         No mechanically moving components, maintenance-free.

·         Ideal for contaminated liquids.

Turbine Flowmeter

This flow meter is named so as it measures flow by counting the rotation of a turbine that is positioned in the flow line. The fluid flowing through the pipe makes the blades of turbine rotate at a speed that is proportional to the flow velocity of the fluid. The number of revolutions is monitored by either a gear train or by a magnetic or optical pick-up.

The reader or a “pick-up” is installed perpendicular to the rotor. Two types of pick-up assemblies are commonly used: magnetic pick-up and no drag pick-up.

The magnetic pick-up consists of a permanent magnet with a coil wrapped around the magnet. When the turbine blades cut through the magnetic field, an alternating current, the frequency of which is proportional to the flow, is induced in the coil.

The no drag pick-up consists of an oscillator that transmits a high-frequency carrier wave to the coil of the pick-up. The rotation of the turbine modulates the carrier wave depending on the velocity of the rotor. In both cases pulses proportional to the flow are produced.

Every turbine flow meter is characterized by the K-factor, a coefficient that for a specific flow of a particular fluid shows how many pulses per liter are generated by the flow meter.

The K-factor can be calculated from:

K = 60*f/Qv

where

Q_V à the flow [liter/minute]

f à frequency [pulse/s]

Kà K-factor [pulse/liter].

Ideally, the value of K-factor should be constant i.e. relationship between the meter output and the flow rate should be linear. In reality, however, the driving torque of the fluid on the blade is
balanced by the influence of viscous, frictional and magnetic drag effects.

Since these vary with the flow rate, the shape of the K-factor curve depends on
viscosity, flow rate, bearing design, blade edge sharpness, blade roughness and the nature of
the flow profile at the turbine leading edge. In practice, all these influences have differing
effects on the meter linearity and thus all turbine meters, even from the same manufacturing
batch, should be individually calibrated.

The linear relationship of the K-factor is confined to a flow range of about 10:1 – sometimes
extending up to 20:1.

At low flows, the poor response of the meter is due to turbine rotor bearing friction, the effect of fluid viscosity and magnetic drag on the rotor due to the use of a magnetic pick-up. The humping section of the curve flattens as the viscosity decreases – with resultant increase in accuracy.

Practical installation

-The turbine flow meter requires equalization pipes at upstream and downstream sides of the instrument. The length of the horizontal pipe segment on both sides depends on the flow conditions. As a rule of thumb equalization pipe of 10D upstream and one of 5D downstream is recommended.

– The accuracy of the instrument is strongly influenced by the quality of the blades and the friction of the rotor against its axis.

– Inertia of the rotor can greatly influence the response time, especially when dealing with gases.
– To perform the digital-to-analog conversion, a frequency-to-voltage converter can be used, which transforms the pulses into a standard electrical signal.

– To stop any contamination that might block the turbine, a filter can be placed in front of the turbine flow meter if desired.

– Providing a bypass is efficient for continuous processes as it allows the replacement and cleaning of the flow meter (and of the filter) without interrupting the process.

Characteristics

Advantages

-          An extensive selection of ranges is available, for gases as well as for liquids.

-          A high level of accuracy, (0.2-0.3%), is attainable under specific circumstances because of the digital output.

-          Excellent repeatability (± 0.05 %).

-          Wide rangeability up to 20:1

-          Wide range of temperature applications from -220 to 600 °C

-          Measurement of non-conductive liquids.

-          Suitable for very low flow rates.

Disadvantages

-          Very sensitive to wear, especially with highly contaminated fluids and at high speeds.

-          Linear only in a limited area, which reduces the measuring range.

-          Not suitable for high viscous fluids.

-          Viscosity must be known.

-          10 diameter upstream and 5 diameters downstream of straight pipe is required.

-          Not effective with swirling fluids.

-          Only suitable for clean liquids and gases.

-          Relatively expensive.

Voltage references

The ability of a voltage reference or regulator to maintain a constant output under varying external conditions is characterized in terms of performance parameters such as line and load regulation, and the thermal coefficient. In the case of voltage references, output noise and long-term stability are also significant.

Line and Load Regulation

Line regulation, also called input, or supply regulation, gives a measure of the circuit’s ability to maintain the specified output under varying input conditions.

where ∆Vo is the output change resulting from a change ∆Vi at the input. Line regulation is expressed in millivolts or microvolts per volt.

Load regulation gives a measure of the circuit’s ability to maintain the specified output voltage under varying load conditions.

Temperature Coefficient

The temperature coefficient of output voltage gives a measure of the circuit’s ability to maintain the specified output voltage Vo under varying thermal conditions.

It is usually expresses in mV/⁰C.

Besides line and load regulation, thermal stability is the most critical performance requirement of voltage references due to the tendency of IC components to be strongly affected by change in temperature. Consider the silicon pn junction, which forms the basis of diodes and BJTs. Its forward-bias voltage V_D and current I_D are related as

where V_T is the thermal voltage and Is the saturation current.

where k =1.38×10^-23 J/K is known as Boltzmann’s constant, q = 1.602 x 10^-19 C, T is absolute temperature.

TC = V_T/T = k/q = 0.086 mV/⁰C

Temperature coefficient for diode forward voltage drop V_D is -2.1 mV/ ⁰C.

Two types of voltage references are popular - Zener voltage reference and Bandgap reference.

Zener Voltage Reference

A zener diode is widely used as a voltage reference. While true zener breakdown occurs below 5V, avalanche breakdown occurs at higher voltages and has a positive temperature coefficient. Zener breakdown truly has negative temperature coefficient but nowadays avalanche breakdwon is referred to as zener breakdown only.

A combination of normal PN diode with negative temperature coefficient is used to compensate for temperature instability as shown below.

However to get maximum temperature compensation, odd values of output voltage is produces like 4.7, 5.3, 6.2V etc.

As Zener to avalance transition occur at 5.6 volt (and we use avalanche diode as voltage reference), a input voltage higher than 6.2V (5.6+V_D) is required.

                       

  

Bandgap voltage reference

For circuits which are powered by low voltage supply, bandgap reference offers an exciting choice. They can offer much lower voltage output as they utilize PN junction bandgap voltage which is 1.2V for Silicon.

The value of V_T is 0.086mV/⁰C and it has positive temperature coefficient. This voltage can be used to compensate temperature related variation of V_BE by amplifying the value to +2mV/⁰C.

This is the scheme behind Bandgap reference.

The actual circuit of a bandgap reference may look as below.

GATE INSTRUMENTATION (IN) Question papers

With GATE 2020 around the corner, first of all I would extend my good wishes to all the GATE 2020 aspirants. I am sharing links for questions papers which will be helpful for aspirants in practicing the type of problems asked by IITs/ IISC in the past.



GATE 2007 INSTRUMENTATION PAPER

GATE 2008 INSTRUMENTATION PAPER

GATE 2009 INSTRUMENTATION PAPER

GATE 2010 INSTRUMENTATION PAPER

GATE 2011 INSTRUMENTATION PAPER

GATE 2012 INSTRUMENTATION PAPER

GATE 2013 INSTRUMENTATION PAPER

GATE 2014 INSTRUMENTATION PAPER

GATE 2015 INSTRUMENTATION PAPER

GATE 2017 INSTRUMENTATION PAPER

GATE 2018 INSTRUMENTATION PAPER

Pipe Threads- NPT vs BSP threads

Pipe is a hollow structure designed to provide an enclosed pathway for fluids to flow, usually
manufactured from cast metal or plastic. This section discusses some common methods for joining pipes and pipe ends with instruments together.

Tapered thread pipe fittings

·         Thread fittings are more preferred for smaller pipe sizes.

·         A very common design of threaded pipe fitting is the tapered pipe thread design.

·         The intent of a tapered thread is to allow the pipe and fitting to “wedge” together when engaged, creating a joint that is both mechanically rugged and leak-free.

Several different standards exist for tapered-thread pipe fittings. For each standard, the angle of
the thread is fixed, as is the angle of taper. Thread pitch (the number of threads per unit length) varies with the diameter of pipe fitting.

The most common tapered thread standard for general-purpose piping is the NPT, or National Pipe Taper design. NPT threads have an angle of 60° and a taper of 1°47’.

NPT pipe threads must have some form of sealant applied prior to assembly to ensure pressure tight sealing between the threads. Teflon tape and various liquid pipe “dope” compounds work well for this purpose. Sealants are necessary with NPT threads for two reasons: to lubricate the male and female pieces (to guard against galling the metal surfaces), and also to fill the spiral gap formed between the root of the female thread and the crest of the male thread.

Another tapered-thread standard is the BSPR, or British Standard Pipe Tapered. BSPT threads have a narrower thread angle than NPT threads (55° instead of 60°) but the same taper angle 1°47’.

Parallel-thread pipe fittings

·         One popular parallel-thread pipe standard is the BSPP, or British Standard Pipe Parallel.

·         Like the BSPT (tapered) standard, the thread angle of BSPP is 55°.

·         Sealing is accomplished by means of an O-ring which compresses against the shoulder of the matching female fitting:

Difference between coaxial line and waveguide

	Coaxial Cable	Wave-guide
1.	It consists of center conductor and outer conductor.	Waveguide consists of single metallic walls acting as conductor. There is no center conductor in the waveguide.
2.	The fundamental or dominant mode wave in a coaxial line is TEM.	TEM wave can not propagate through it. Wave transmission takes place using TE or TM modes.
3.	There is no concept of cutoff frequency.	Each waveguide has a finite cut-off frequency.
4.	The coaxial lines have PTFE as di-electric and have relatively higher loss.	As waveguide is air filled there will be less loss compare to coaxial line.
5.	The power handling ability of coaxial-cable is relatively lesser.	Waveguide can handle higher power compared to coaxial cable because waveguide is filled with air as dielectric and air has higher break down voltage.
6.	The bandwidth of coaxial line is broad.	The bandwidth of waveguide is smaller.

Analog Voltage Divider or voltage division or voltage ratio circuit using Op-amp

Operational amplifier is a powerful tool to perform different mathematical operations on analog signals. In this post we will discuss about circuit to implement voltage ratio or voltage division using operational amplifiers.

Let us design a circuit to implement the following expression

Vout = (V1-Vref)/ (V2-Vref)

We will use the property of "logarithm" to implement division. And voltage difference circuit to offset Vref.
We know that log(A) - log(B) = log(A/B).
In our problem,

A = V1 - Vref

B = V2 - Vref

First stage is a difference amplifier which subtract Vref from V1 and V2 respectively. This gives us voltage Va and Vb.

The second stage performs logarithm of input voltage to obtain Va' and Vb'.

The final output voltage will be Va'-Vb'.

By setting suitable value of gain factor (-kT/q) and resistance ratio in previous stages, we can get the required expression of voltage division at the out.