Dp Calibration

7/30/2019 Dp Calibration

1/12

1

ER 2152 Pressure

Calibration of a Differential Pressure Transmitter

Lab Instructions


2/12

2

COLLEGE OF THE NORTH ATLANTIC

SEAL COVE CAMPUS

INDUSTRIAL INSTRUMENTATION FUNDAMENTALS

ER 2152 Analog DevicesPRESSURE

CALIBRATION OF A D/P TRANSMITTER2007

DATE: ______________________

STUDENT: ______________________

MARK: ___________

COMMENTS:


3/12

3

OBJECTIVE:

To bench calibrate a DP Transmitter to a given range of pressure.

To experimentally determine the Transmitters Minimum Span

DISCUSSION:

Differential Pressure Transmitter.

The electronic differential pressure transmitter is a device that incorporates

a differential pressure sensor and an electronics package.Its purpose is to measure a range of pressures and then convert thepressure into a standard proportional electronic signal i.e. 4 to 20 madc.The pressure-sensing device is often a capacitive cell as in this lab. Readyour text / notes for an explanation of this and other pressure sensorswhich may be used.The Transmitters DP cell has two ports, one is marked H for high pressurethe other is marked L for low pressure.The L (low) port is often left vented to atmosphere with the H (high) port

connected to the process pressure under measure.The D/P cell is a differential pressure sensing device, that is, there must bea change in the pressure across the cell for there to be a change in theproportional madc signal transmitted by the transmitters electronicspackage.It must be noted that a change in the D/P across the cell can be eithercaused by a change in the pressure applied to the High side of the cell ascompared to the Low side of the cell or vise versa.

In this laboratory experiment the student will use a configuration where avariable process pressure is applied to the High side of the cell and iscompared to, or measured against, atmospheric pressure at 0 PSIG(pounds per square inch (gauge)) or 14.7 PSIA (pounds per square inch(absolute)) applied to the Low side of the cell.For example if a process pressure with an expected range of 10 PSIG to 30PSIG is to be measured and converted to a standard signal range of 4 to


4/12

4

20 madc, then the low side of the transmitter is vented to atmosphere at alltimes throughout the experiment.The differential pressure experienced by the transmitters cell will be:URV (upper range value) minus LRV (lower range value)(150 inches W.C. 0 inches W.C.) for a range of 150 inches W.C.

In many cases the low port of the transmitter will not be vented to

atmosphere but will be connected into the process at some pressure otherthan atmospheric and is beyond the scope of this laboratory experiment.

The differential pressure applied across the D/P cell deforms a diaphragm,this diaphragm is a common plate of a dual capacitive cell.In this way a variable differential pressure causes a proportional change inthe capacitance of the cell which is sensed in an electronic circuit, thatwhen applied to the transmitter electronics is converted to a proportionalmadc signal.

The minimum and maximum D/P, which can be applied to the cell, is afunction of the manufacturing parameters for that device, and like allsensitive measuring instruments can be destroyed through incorrectapplications or practices.Therefore the manufactures manual should be consulted for thistransmitter to discern the correct magnitudes of pressure that can safelyapplied to this device.

The Transmitters output must be calibrated to obtain a zero percent (4madc) to 100 percent (20 madc) output proportional the D/P transmitters

zero percent to 100 percent range of input pressures.In other words calibration of the transmitter is required to make thetransmitters percent input equal to the transmitters percent output.This is accomplished by adjusting screws located and clearly marked asZERO and SPAN on the transmitters outer casing.Under no circumstances adjust or turn any other screws on theTransmitter.Differing configurations of the Zero and Range adjustment are to be foundfrom manufacturer to manufacturer.


5/12

5

Equipment:

Process StationRosemount 1151DP4Exxxxxxx Transmitter24 Vdc Power Supply250 Ohm Resistor

Hand PumpElectronic ManometerFluke Multi-MeterCalibration screwdriver (Standard Flat)Associated tubing and wiring

PROCEDURE:

1. Have your Instructor provide you with a Rosemount1151DP4ETransmitter then determine the Transmitters' Range,Span, and Maximum working pressure (MWP), from the manufacturersmanual, or located on the tag attached to the device.

Transmitter make and Model # ________________________________

Transmitter Range ___________

Transmitter Span ___________

Transmitter MWP ____________

2. Draw below a connection diagram showing the calibration devices andwiring with polarity indication you will use for your bench calibration

circuit. A bench calibration is a procedure where the device is calibratedat a calibration bench using calibration devices to simulate the process,rather than calibrating the device in the field using the actual processitself as the input means.Refer to your previous notes or as drawn on the board. You will hand inthe diagram with your lab.


6/12

6

MANOMETER or GAGE

1 5 Volt

+ Reference or Vent

_ +

POWER SUPPLY (PS1)

Calibration Diagram3. Make certain you connect 24Vdc only to the circuit, as well

ensure that the positive terminal of the 24Vdc power supply isconnected to the positive terminal on the transmitter.You must include a 250-Ohm resistor in this serried circuit.Connect a hand pump with the vent screw open, to a pneumatic teefitting.Connect one of the remaining two ports of the tee to the High side ofthe transmitter and leave the Low side of the transmitter vented toatmosphere.Connect the remaining open port on the tee to a 200-inch electronicmanometer and zero the manometer.Contact your Instructor if you are unsure of this procedure.

D/P

A

PI

PUMP


7/12

7

Once you have connected your calibration circuit, electric, pneumaticand calibration devices contact your Instructor to inspect yourCalibration setup before you apply power to the cct.

4. For each device you calibrate in any Laboratory exercise you arerequired to fill out and pass in a calibration sheet.

Calibration testing and documentation is very important in the Instrumentationoccupation and plant safety.

This information vital for a number of reasons some are:The device must operate with a high degree of accuracy andprecision and be repeatable in its data reporting over long periods oftime,An error in this device will lead to unsafe conditions within the plantsystem that may result in catastrophic life threatening conditions.

Testing and documenting performance with calibration sheets is onestandard used to ensure reliability and builds a history for that device.

Important: You must perform the AS FOUND check Prior toeach and any calibration.

If you calibrate the device before first testing and recording the ASFOUND data, the history of the devices performance data will be lost.

5. The range of values you will use for this transmitter will be that as youhad recorded in step number 1, use this range of values to fill outyour Calibration Report As Found and AS LEFT sections directlyacross from the INPUT engineering units rows only, not in the outputrows.Be sure to indicate the correct engineering units, in this case inchesof W.C., that is, inches of water column pressure.Notice that the data from step 1 has two numerical values, the LRVand URV, see the preliminary discussion for the meaning of these

terms.The LRV is the numerical value you place under both 0 percentcolumns and the URV is the numerical value you place under the 100percent column. All other intervening i.e. 25%, 50% and 75%, valvesyou are to compute and fill in under the appropriate columns androws for the five point calibration check.Almost all calibration devices used in industrial settings have the datapoints checked both upscale and downscale so to provide data as to


8/12

8

the devices agreement between rising values of pressure and fallingvalues of pressure.This gives indication of the devices Dead Band and Hysteresis.

6. Power up the circuit and close off the vent on the hand pump,pressurize the High side of the transmitter to your first data point.In the case that the first data point is 0 psig (atmospheric) then

leaving the pump vented will be the first data point.Read and record the current value of the transmitters output on yourCalibration Report under the first 0 percent column of the OUTPUTrow in the AS FOUND section of the report.

7. Raise the pressure input to the next value and repeat the process ofdocumentation.Continue with this process until you have filled out all the data fieldsin the AS FOUND section of the calibration report.

8. To Calibrate this transmitter to the values of the range you indicatedin step # 1, read carefully through the following discussion.The Transmitters zero percent, (LRV), is to be calibrated to themanufacturers stated LRV, and the Transmitters span is to becalibrated to the manufacturers stated (URV).

Locate the manufacturers instruction manual and with it locate thetransmitters ZERO and SPAN adjustment screws sometimes calledZero and Range adjustment screws.

This Lab specifies using the 1151DP4ERosemount Transmitter,its ZERO and SPAN adjustment screw is located on the externalhousing.Note that these screws each connect to a variable resistance(potentiometer) and can be turned indefinitely.That is the potentiometer is of a type that once fully adjustedclockwise or counter clockwise the screw may continue to turnwithout further varying the resistance for either direction.

The potentiometer has a maximum of 20 turns between minimum andmaximum resistance therefore turning the ZERO or SPAN screwsclockwise or counter-clockwise for 20 turns will cause thepotentiometer to be at either maximum or minimum.

9. Turn the ZERO and SPAN screws both 20 turns clockwise.Next turn both screws 10 turns counter clockwise to approximatelyadjust the potentiometer to the mid resistance point (50%).


9/12

9

10. Next you will apply the 0% (LRV) pressure to the transmitters Highside, in the case of this Lab this will be 0 inches W.C. and with thevent screw on the hand pump still in the open position and theTransmitters low side vented we are assured there is no differentialpressure acting across the transmitters DP cell.

11. Adjust the zero screw on the transmitter while observing the ammeterto cause the indication to be 4.00 madc, this is the transmitters LRVoutput.The acceptable margin or error for this lab exercise is 0.1 percent.Calculate below this error in the appropriate engineering units forboth input and output values.

Acceptable margin of error for units of input = ___________

Acceptable margin of error for units of output = ___________

12. Close the vent screw on the hand pump, the pressure indicated bythe manometer may jump to 1 inch W.C., if so loosen the vent plugonce again to see if the pressure falls back to 0 inches W.C. Thiscondition is normal, a value of I inch WC is an extremely low pressureand can be caused by the closing of the vent valve on the handpump.

Operate the hand pump while observing the manometer indication tocause the pressure applied to the High side of the transmitter toincrease to the 100 percent value (URV) indicated in step one andunder the 100% column of the calibration reports AS FOUND andAS LEFT INPUT sections.

13. Adjust the SPAN screw while observing the ammeters currentindication to cause the ammeter to indicate 20 madc which is the100% (URV) output value signal for the transmitter.

14. Re check the zero % calibration, you will find that it no longer iscorrect, this is due to the interaction between the zero and spanadjustments, and re adjustment of the 0% calibration point (LRV) willthrow off the 100% calibration point (URV).It will be necessary to repeat steps 12 and 13 many times, each re-calibration will result is a diminishing amount of error until the 0%


10/12

10

through to 100% input to the transmitter (pressure) exactly equals thetransmitters 0 % through 100 % output (4 to 20 madc current).A correctly calibrated device can be described as one where the %input equals the % output for all values between 0 and 100 percent.

15. Once you are satisfied with your calibration have your Instructor verifyyour work, then proceed to fill out the AS LEFT data in your

calibration report.

16. Power down your circuit and return all equipment and supplies totheir correct places.Answer all questions and turn in your calibration report with thisLaboratory exercise.


11/12

11


12/12

12

Documents

Dp Calibration