UNIVERSITY OF TECHNOLOGY, JAMAICA

Faculty of Engineering and Computing

Laboratory Report

Title of Experiment: Pneumatic to Current Converter

Experiment #: 2

Instructor:

Course Name: Instrumentation and Control

Programme: Beng3M

Date: March 7, 2011

Submitted by: Eric Shaw (0800912)

AIM:

To investigate the characteristics of current to pressure converter (I/P) and the pneumatic

control valve.

THEORY:

In the industrial world there are many variables within a process that are necessary for the plant to operate. There variables need to be monitored and controlled to enable operations at the plant thus we introduce control valves. These are devices that control a variable such as temperature, pressure, flow, level by fully or partially opening or closing in response to signals received from controllers that compare a "setpoint" to a "process variable" whose value is provided by sensors that monitor changes in such conditions. There are many different types of control valves. The main difference in the types of control valve is the way in which they are activated/operated, example electrically, use of hydraulic or pneumatically. Positioners are used to control the opening or closing of the actuator based on Electric, or Pneumatic Signals. These control signals, traditionally based on 3-15psi (0.2-1.0bar), more common now are 4-20mA signals for industry, 0-10V for HVAC systems, & the introduction of "Smart" systems, HART, Fieldbus Foundation, & Profibus being the more common protocols. These control vales can be calibrated for each industry or each process within an industry to meet that specific criterion. The pneumatic valve will be calibrated such that 4 mA corresponds to the minimum adjustment valve which fully close the control valve or 20 mA that fully opens the control valve. It would be adjusted to fall with the two extremes outlined which would partially open the valve. With this calibration

the signal can be converted so as to process control becomes automated.

P/I converters create an interface between pneumatic and electric measuring and control devices, for example, for connecting pneumatic transmitter to electrical equipment, such as controllers, computers or control systems. The input variable constitutes a standardized pneumatic signal, the output variable an electrical DC current signal, however it is normally interpreted as a DC voltage because of the device’s parallel setup with the measuring instrument.The fundamental operations of pneumatic to current converter are illustrated in Fig 1a below. The pressure transducer (1) converts the pressure p of the pneumatic input signal into an electrical DC voltage signal. The DC voltage signal which is proportional to the pressure is amplified in the measuring amplifier (3) to a defined level. Both the lower range value and span can be adjusted using potentiometers on the front panel. The constant DC voltage source (2) is used to supply the DC voltage at a constant level. Control equipment can be connected to the output circuit. In two-wire systems, the maximum permissible load impedance is UB = US – UA. US represent the minimumsupply voltage of the two-wire circuit. The converter requires a minimum natural voltage of UA = 12 V.

There are two types of pressure sensors namely Bourdon tube and Solid state pressure sensors. The Bourdon tube is a hard metal tube usually made of bronze or brass,is flattened and one end is closed. The tube is bent into an ark. The openings are attached to a header by which the pressure can be introduced to the tube. When this happens the tube will deflect when the pressure inside the tube is different from the pressure outside. If the pressure inside is greater than the outside pressure then the tube tends to straighten out while if the outside pressure is greater then the tube curves more. The solid state pressure sensor consists of integrated circuit technology that measures pressures differences. The basic sensing element is a silicon wafer

acting as a diaphragm that usually deflects in response to a pressure difference. The solid state pressure sensors are:

1. Sensitive in the range of 10-100mV per kilopascal.2. Respond to pressure difference in order of 10ms

They are normally used in home appliances such as dishwashers and washing machines.

DIAGRAMS:

Fig 1a

Fig. 1b showing Block diagram of the Pneumatic to Current setup

Fig. 1C showing pressure to current transducer

Fig. 1D showing schematic diagram of a pneumatic to current (P/I) transducer

APPARATUS:

Flow measurement trainer, multimeter, current supply, pressure gauge,

assortment of fittings

Fig. 2 showing Pneumatic to Current Converter

SPECIFICATION:

Input signal: 3-15 psi

Output signal: 4-20 mA

Accuracy: +/- 0.25% of span

Power Supply: 18V DC

METHOD:

A preliminary test was conducted to observe the performance of the instrument initially

over a specified range. The results were then recorded.

1. The appropriate pneumatic piping connections were made.

2. The power supply (18V) and the voltmeter were connected to the field terminal

connections of the converter.

3. The calibration pressure was then set to 3 psi and the required output registered on

the voltmeter was 100mV.

4. Over a span of 3-15 psi the output voltage was recorded and at 15 psi the

voltmeter registered 500mV.

5. Procedure 3 was continuously repeated and the zero screw was used accordingly

to adjust the lower range limit

6. In adjusting the upper range limit, the span screw was used until at 15psi the

current registered 500mV.

7. The post calibration results were then recorded.

RESULTS:

Input Current (mA) Output Pressure (psi) Valve stem Position (inch)

Flow Rate (% of Max)

4 3 0 08 6 0.198 40

12 9 0.386 6016 12 0.576 7720 15 0.757 8520 15 0.757 8516 12.1 0.568 7512 9 0.371 58

8 6 0.189 404 3 0.032 22

Table 1.1 showing input current, output pressure, valve stem position and % flow rate

% Input Current (mA) %Output Pressure (psi) %Valve stem Position (inch)

Flow Rate (% of Max)

25 25 0 050 50 26.15587847 4075 75 50.99075297 60

100 100 76.08982827 77125 125 100 85125 125 100 85100 100.8333333 75.0330251 75

75 75 49.00924703 5850 50 24.9669749 4025 25 4.227212682 22

Table 12 showing % input current, % output pressure, % valve stem position and % flow

rate

Graph 1.1 showing Input current verse output pressure

Graph 1.2 showing output pressure verse valve position

Graph 1.3 showing valve position verse flowrate

The ipt was linear because the values of input compare to output was similar.

The control valve was linear because flow capacity increases linearly with valve travel.

This was also proven when the flow capacity increases exponentially with valve trim travel.

Equal increments of valve travel produce equal percentage changes in the existing Cv.

The linear flow characteristics are usually specified in those process systems where

the majority of the pressure drop is taken through the valve. The piping effects have

a tendency to push the linear characteristics towards the quick open characteristics.

With an Equal Percentage characteristic, the change in flow per unit of valve position

is directly proportional to the flow occurring just before the change is made.

In describing the average sensitivity of the control valve ,20% flowrate increase per 25% valve stem height increase.

DISCUSSION:

During the preliminary test, it was noticed that the variance between the output voltages

for a specific input pressure was at a maximum of 10mV. After calibration, where the

span and zero screws were adjusted to set the standard of 15psi to yield 500mV and 3psi

to yield 100mV; the variance between the output voltages for a specific input pressure

drops to a maximum of 7mV. Even after calibration, there were still variances from the

expected standard. This occurs because of internal friction within the instrument and

connecting wires and possible errors experienced while conducting the experiment. The

possible errors experienced while conducting the experiment were:

1. The pneumatic to current converter consist of a spring that is responsible for the

response of the output voltage to the input pressure. For a specific input pressure,

there are two different output voltages because the spring does not return to its

original position.

2. Overshooting an input pressure and then trying to correct it, increased hysteresis.

3. Sensitivity of the pneumatic instrument. When there was a movement of the knob

to change the pressure, there was a delay in the response of the gauge pressure

pointer that indicates the pressure change.

4. The use of analogue meters allows for human errors while reading values as

opposed to a digital multimeter.

After calibration there was improvement in the accuracy of the instrument as well as

the linearity when compared to the uncalibrated values.

Voltage Span = 500mV – 100mV

Voltage Span = 400mV

Pressure Span = 15psi – 3psi

Pressure Span = 12psi

Therefore,

mV/psi = 400mV / 12psi

mV/psi = 33.33mV/psi

Thus,

The linear relationship between pressure and voltage is

y = 33.33x

where x = measured value which in this case is the input pressure.

When applying the equation, we get

Input Pressure [psi] Linear Voltage [mV]3 1004 1335 1676 2007 2338 2679 300

10 33311 36712 40013 43314 46715 500

Table 3 showing the linear relationship of input pressure and linear voltage.

Graph showing the Input Pressure versus Output Voltage for the Linearity Test

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16

Pressure (psi)

Vo

ltag

e (m

V)

Accuracy = (max. variance of output/ span) * 100

Accuracy = (7mV / 400mV) * 100

Accuracy = 1.75%

The accuracy of input pressure, P to Voltage, V is ± 1.75%

When comparing Table 2 and Table 3, the values of table 2 are not far off from that of

table 3. Hence, it can be said that the calibrated values are accurate. Also, it can be said

that they are precise as in there was a repeatability of the values. As it regards to linearity

of the instrument

CONCLUSION:

The relationship between the input pressure and the output current delivered and to

observe the overall operation of the prescribed instrument was examined. Additionally, it

was observed that when the pressure values were increased and then decreased the output

voltage yielded were slightly different for the specified range.

REFERENCES:

Johnson D. Curtis. Process Control and Instrumentation Technology 7th edition.