1. Process Control Loop and Calibration Basics

2. Learning objectives ● Develop an understanding of calibration and why it’s important ● Review the related quality standards and regulations ● mA control loop basics ● Understand mA and voltage input indicators ● Hands on lab: mA loop calibrators ● Hands on lab: Verifying a mA input indicator ● Transmitter basics ● Hands on lab: Fluke-744 Calibrator overview

3. What is calibration? Calibration is: ● Comparison of a measured value to a traceable measurement standard ● Adjust to agree with a standard if necessary Reasons to calibrate: ● Product quality, quality standards ● Regulatory compliance ● Personal safety

4. Why calibrate: Product quality ● Processes working to same standards ● Processes stay working to same standards ● Product quality stays consistent ● Company saves money, stays competitive

5. Quality standards ISO-9000, Quality practices ● ANSI-ASQC-9000 ● Ford Q9000 IS0-17025 ● Proficiency/accreditation based regulation ● FoMoCo is a big advocate/driver in the US Six Sigma corporate initiatives ● Pioneered by Motorola o Adopted as a standard in industry

6. Compliance and regulatory standards E.P.A. Emission regulations O.S.H.A. 1910 Safety systems F.D.A. cGMP regulations Consumer protection D.O.T Custody transfer, weight and volume N.R.C Environmental protection, human safety A.P.I Voluntary, American Petroleum Institute

7. Electrical safety, 4-20 mA, 24V, No harm? ● Normal instrumentation signals are 4-20 mA and voltages 24 V and below ● Older instrumentation systems can be 10-50 mA o 48V and 96V supplies ● Many instruments require line power, 120 Vac ● Many instrumented systems switch 120, 240, even 480 Vac. o Process heating circuits o Pump controls ● Never assume low voltage: always take the same precautions you would take working with high power electrical circuits

8. Process loop introduction Process loop and control signals take many forms ● Analog: o 4-20 mA signal, most common o 3-15 PSI, old control system technology, still in use o 10-50 mA signal, old technology, isolated use ● Digital equivalents o Foundation Fieldbus o ProfiBus o HART o ControlNet – DeviceNet o Ethernet IP

9. Current Loop 4 to 20mA analog (2 wire) signal for communication between instruments, controllers, and indicators Controller PLC, DCS Indicator 2200 ºC 2 2 5 2 zer o span Instrumentation Transmitter SENSOR CONTROL ELEMENT Control loop basics 2200 ºC

10. Current loop devices ● Transmitters o Temperature, pressure, flow, analytical ● I to P, 4-20 mA input, 3-15 PSI output ● Control Valves ● PLC and DCS analog Inputs ● Indicators ● Controllers ● Flow computers ● Chart recorders or Dataloggers ● PLC: Programmable Logic Controller ● DCS: Distributed Control System

11. ZEROSPAN ● 4-20 mA (DC) signal is proportional to sensor input or PV ● Series circuit dictates the current at one location must be identical to other locations ● Big advantage sending in sending mA signals over long distances compared to voltage or pressure signals 2200 ºC 4 to 20 mA 2 Wire Transmitter Sensor Input ● Temperat ure ● Pressure ● Flow ● Frequenc y ● PH Readout / Controller DCS / PLC / Recorder 24 V Loop Supply 250 ohm input shunt Example current loop +–

12. Control valves ● Primary application, Flow control ● 4-20 mA or 3-15 PSI input ● Normally open or closed o Normally open fails open with loss of power o Normally closed fails closed with loss of power ● Actuation style and flow control characteristics can be customized Normally ClosedNormally Open Closed Open

13. Scaled current indicators, analog D’Arsenval movement most common ● Opposing magnetic fields cause needle deflection o 4-20 mA typical deflection range ● Custom scales according to pertinent application o Examples:  0-100%, 0-100 PSI, 0-300 F, etc. ● Require care to accurately interpret measurements

14. Scaled current indicators, digital mA signal applied to A/D* converter ● 4-20 mA typical deflection range o Other common ranges 0-20 mA, 10-50 mA ● Custom scales according to pertinent application o Examples:  0-100%, 0-100 PSI, 0-300 F, etc. o Connected in the output loop of a transmitter *A/D; Analog to Digital

15. Voltage devices ● Transmitters o Select few with 1-5 V output in place of 4-20 mA ● Indicators o Many operate on a 1-5 V signal taken an IR drop across a 250 ohm resistor ● Controllers ● Flow computers ● PLC and DCS analog Inputs ● Chart recorders 4-20 mA signal with 250 ohms IR drop: Ohms law states: E=I*R Voltage = Current X Resistance

16. Voltage devices ● Requires close proximity between measured process and input device to avoid lead resistance (IR) losses ● Sometimes used in low power applications where power consumption is critical o Remote transmitters powered by solar power.

17. Transmitter basics ● Primary function: accurately “transmit” a signal proportionate to the measured PV ● 4-20 mA (common),1-5 V (rare) analog outputs ● HART, Fieldbus, ProfiBus digital outputs ● Analog and smart versions o HART smart versions can be mA, digital output or both ● Analog versions are fixed range o Basic signal conditioner technology ● Smart versions can be digitally re-ranged o Microprocessor controlled PV: Primary variable, e.g.: temperature, pressure, flow … HART : Highway Addressable Remote Transducer

18. ●Converts low level, non-linear thermocouples, RTD signals to linear 4 - 20 mA or DC voltage ●Millions in service, require service and calibration ●“A temperature controlled current regulator” Typical transmitter, temperature ZEROSPAN Sensor Input ● Thermocoupl e ● RTD ● Thermistor +– Thermocouple Transmitter 0 to 300 Deg C 24 V Loop Supply

19. Pressure Transmitter 3 to 15 PSI ●Converts pressure to linear or square root responding 4 - 20 mA or DC voltage ●Millions in service, require service and calibration ●“Pressure controlled current regulator” Typical transmitter, pressure Sensor Input ● Gage ● Absolut e ● Different ial +– 24 V Loop Supply

20. I to P transmitters I to P converts a 4-20 mA signal to a 3-15 PSI signal ● Often used with control valves ● Used as a bridge between 4-20 mA loop and 3-15 PSI pneumatic technology ● Typically operate from a 20 PSI or greater pressure supply ● “A current controlled pressure regulator” 4-20 mA Current input Pressure Output Supply Pressur e ~20 PSI

21. Transmitter terms Input span ● The values that correlate to 0% and 100% of measured process ● The values when applied to the input correlate to an output of 4-20 mA if there are no errors Output span ● For a 4-20 mA device: 20 mA – 4 mA = 16 mA span Re-span ● Change the setpoints of a transmitter for 0% and 100% Dampening ● A setting that causes a delay in the output mA signal for a change in the applied PV

22. Transmitter terms Commissioning ● Configuring a transmitter and the installation and verification of its performance o Setting the span, tag number, dampening and transfer function Turn-down ratio ● The ratio between the factory calibrated span and the commissioned span once installed Tag ● An identification number assigned to a location in a loop that is attached to the transmitter installed there Transfer function ● The relationship between applied signal and the output o Linear, square root or other

23. Transmitter performance terms Linearity error ● The deviation of performance between 0 and 100% Offset error ● Error at zero that effects the entire measurement range Span error ● Gain error, error at full span

24. Specification definitions Reference uncertainty ●Performance immediately after being calibrated Stability ●Error due to the effects of time Temperature coefficient error ●Additive errors when used outside a specified temperature range Total uncertainty ●Performance over a specified period of time with the effects of temperature and other adders included ●Sometimes referred to as total performance Error % of span ●((Nominal – Actual) / Span) * 100 ●Most common error calculation method for instrumentation Error % of actual ●(Nominal – Actual) * 100

25. Error = Indicated - Ideal Example: IdealIndicated Error

26. A measure of the consistency or repeatability of a series of measurements. Example: Ideal Precision

27. The degree of conformity of an indicated value to a recognized, accepted standard value (or ideal value). Example: Ideal Accuracy

28. The smallest change in quantity a given instrument can detect or provide. Example: Ideal Resolution does not equal accuracy! Resolution

29. NATA or NIST Traceability ● Unbroken chain of calibration. ● Documented proof.

30. Test uncertainty ratios ● The Test Uncertainty Ratio (TUR) is the ratio between the accuracy of a calibration device to device under test (DUT). ● Ideal ratio, 10X or better o 10:1 ratio is ideal but impractical ● Typical accepted ratio, 4:1 o Based on outdated MIL-STD-45662A ● Ratios of as small as 1.5:1 with guard banding o Test the DUT to 80% of specification  Provides same confidence level as 4:1

31. As Found / As Left Test As found ● The verified condition of a device prior to adjustment o If the test results are within the specified values no adjustment is required Adjustment if necessary As Left ● The verified condition of a device after adjustment o If the test results are within the specified values no further adjustment is required. o If test fails, readjust, re-run As Left until the test passes o If the device fails As Left after several re-tries, it may be defective and need replacement

32. Calibration Documentation A documented calibration requires: ● Tag, model and serial number of the device ● Person performing the task ● Calibration device and its certification information o When calibrated and when due for re-calibration ● Date and time performed ● Test tolerance ● The As Found data o Applied value, measured value, error calculation ● As Left data if adjustment is performed

33. Documentation example

34. Summary ● Quality and regulatory forces mandate calibration ● Normal electrical precautions are necessary for working on low voltage 4-20 mA circuits ● 4-20 mA devices are the most common devices to be calibrated ● Indicators with a 4-20 mA input can be custom scaled ● The mA output of a transmitter is proportional to the PV input being measured ● Total uncertainty is the combination of a number of different error types ● Documentation is necessary proof of a successful calibration