Capacitive Level Sensor Design

APPLICATION:  “MEASUREMENT OF LEVEL IN A LIQUID FILLED TANK”

There are different sensors available for level measurement, depends on particular application.Read about different sensors.

I will discuss about the practical designing of a Capacitive Level Sensor.

Before going into details of the sensor parameters, let us understand the basic principle behind the working of this sensor.

• Basic concept of capacitive sensor:

Fig1: Capacitive sensor

•  Capacitance is nothing but capability of storing charge!!
•  Assuming that the basic capacitive circuit phenomenon is known, the circuit is explained. When the capacitance plates are connected to a battery, the electrons on plate2 are attracted to the positive side of the battery while electrons on the plate1 are repelled by its negative side.
• This causes a flow of electrons until a battery potential is developed across the capacitor
• Larger the capacitance,more current will flow to charge it.
• Total current flow gives the size of capacitor.

Based on my design, a simulator is designed. You can click on the following link:

http://coep.vlab.co.in/?sub=33&brch=91&sim=449&cnt=602

U can actually simulate your problems using this and make your own design. If you have any problem regarding this, u can contact me, i will try my level best to solve your problem.

Read more for the design details!

PRACTICAL ISSUES:

• Capacitance change is produced depending on the material being measured and level electrode selection.
• If a higher dielectric material replaces a lower one, the total capacitance of the system will increase.
• If the electrode is made larger by increasing surface area, capacitance output increases.

In a nut shell, your Capacitive sensor looks like this!

Fig2 : Pipe in Pipe construction

This is a Pipe in pipe Capacitive Probe structure, also called as Concentric probe structure.

Capacitance ‘C’ is given by,

Equation1

Assumptions:

As h >>  r2(inner radius of outer cylinder)

Equation 2

So,

By making the distance between the two plates or rods minimum, sensitivity can be increased.

Where, A= overlapping area between the rods or plates. (F/m). Therefore,

By keeping the‘d’ extremely small, sensitivity increases.

But practical limit is i.e., at atmospheric pressure; it is about 3kV/mm

As the level rises in tank, vapors with dielectric constants close to 1 are displaced by the process materials which have higher K value.

This results in an increase in the total capacitance C of the system which is calibrated in terms of  level.

WHY PIPE IN PIPE STRUCTURE SHOULD BE USED??

By using concentric probes, distance‘d’ is fixed. Also gain of the instrument is both linearised and increased, where gain is the capacitance change per inch change in level.

I will explain this using an example:

Consider you have designed ur capacitive level sensor as Outer metal body as one plate of capacitor and another probe inserted in liquid as another plate , liquid acting as a dielectric medium.

If later on an agitator is introduced in the liquid tank and the center probe is shifted by ‘x’ distance.

What will happen?!!

The distance between the plates ‘d’ will change… Consequently the cross-sectional Area will change.. and in turn your Calibrated Capacitance will change. So you will have rework  the changes altogether and recalibrate the sensor.. This will be painful!!

So, simple and smartest way is to go for Concentric Pipes, wherein the distance  ‘d’ is fixed! So only parameter that will change the capacitance is its overlapping area!! So designing becomes simpler.

As per the standards, specified in Liptak and omega.com sources, it is desirable to keep the level probe capacitance differential

• Terminal capacitance (TC): It is measured when the tank is empty.
• Also, ratio should be kept between 4 and 0.25.

For finding this,

• Estimate the dead or the gland capacitance value

Delta C, can be found out if‘d’ is known.

l = length of the electrode

d1,d2 = diameter of the electrodes is known.

Therefore,

When there is a change in the level by ‘x’ units then the new capacitance will be,

Where,

So,for finding differential capacitance or  over the level range of x feet

• Criterion established for a good installation:

should lie between 4 and 0.25

• While calibration zero, span adjustment should be done.

• Material of construction:

1. Stainless steel for non-conductive.
2. Teflon coated SS for both conductive and non conductive services
3. Higher alloys, ceramics, PVC, Kynar and other plastic coatings are also available.

Dielectric constant table

Installation related issues:

• Standard Teflon or Kynar coated probe which can be used upto 20ft (6m)
• For lengths exceeding 20ft, flexible Teflon- coated cable can be used.
• If level span > 6m so there is not enough head room to install a rigid probe and the process fluid has tendency to coat so dual cable or dual wire assembly is recommended.

The capacitance equation can be approximated as,

since ‘d’ is fixed and dielectric of air will remain constant as 1.

As this equation demonstrates, the more material in the tank, higher the capacitance in the output will be.

• To find a relation between capacitance and level,

1. Capacitance of the probe when the tank is empty,

L = length of measurement probe

k = constant

2. When the probe is totally submerged,

k1 =  dielectric constant of the substance stored in the tank.

3. When the probe is partially submerged, its total capacitance is expressed as,

C_M = capacitance of the portion of the probe which is submerged

L_S = length of the measurement probe which is submerged in the substance

L – L_{S =} length of the measurement probe which is above the surface of the substance

Block diagram of the sensor working:

Based on the above design and considerations, a simulator is designed. You can click on the following link:

http://coep.vlab.co.in/?sub=33&brch=91&sim=449&cnt=602

U can actually simulate your problems using this and make your own design. If you have any problem regarding this, u can contact me, will try to solve your problem.

ACTUAL IMPLEMENTATION:

Configurable parameters:

1.     Height of the tank:

2.     Span of measurement:

3.     Service:

4.     Temperature:

5.     Agitation(if any):

6.     Accuracy:

7.     Resolution:

 

Installation related issues:

1. Electrode Location:

Mounting positions should be carefully considered.

They must be clear of the inflow of material as impingement during filling cycle can cause serious fluctuations in the capacitance generated.

Vertical mounted electrodes must be clear of agitators and other obstacles and far enough from vessel wall to prevent “bridging” of the material between electrode and vessel wall.

2. Where the level probe must be removable for maintenance purposes without opening up the process, external chamber type installations are used.

3. If the level span is large, a weighed cable instead of solid probe is used or cable is tied to the bottom of the bin using a turn-buckle.

4. Standard Teflon or Kynar coated probe which can be used upto 20ft (6m)

5. For lengths exceeding 20ft, flexible Teflon- coated cable can be used.

6. If level span > 6m so there is not enough head room to install a rigid probe and the process fluid has tendency to coat so dual cable or dual wire assembly is recommended

Measurement in which unit?:  nF/pF?

Environment:

Specify the container or vessel being used in measurement and its where is it being used.

Special considerations or Compensation related issues:

1. Temperature:

The dielectric constant of some materials varies with temperature which affects the capacitance measured by the dielectric constant are less affected by temperature variation.

Correction:

The effect of changing temperature and changing dielectric cannot be quantified; if the temperature or dielectric changes, it is recommended that the level transmitter be calibrated at each temperature and dielectric constant value to quantify the effect of the changes

2. Composition:

The dielectric constant of the measured material must remain constant throughout its volume. Mixing materials with different dielectric constants in varying ratios will change the overall dielectric constant and the resultant capacitance generated.

3. Conductivity:

Large variations in the conductivity of the measured material can introduce measurement error.

Correction:

The proper electrode selection can minimize the effect. Thick wall electrode insulation is recommended in this case.

4. Material build up:

The most devastating effect on the accuracy of the capacitive measurements is caused by the buildup of conductive materials on the electrode surface. Non-conductive buildup is not as serious since it only represents a small part of the total capacitance. Latex, carbon black, fine metal powders are examples of materials that produce conductive coatings.

Correction:

Periodic cleaning after certain days, weekly or monthly should be done depending on the system criticality. This is one of the solutions to this problem. Proper maintenance of the sensor is required where the system is critical and accuracy is major concern.

SOURCES :www.omega.com; www.capsense.com/capsense-wp.pdf

Electronics and Measurement by A.K.Sawhney 

Process Measurement and Analysis by Bela G. Liptak