Measuring Temperature ACADs (08-006) Covered Keywords Filled system thermometer, thermocouple, 3 wire resistance temperature detector, volatile fluid sensor. Description Supporting Material 1.1.2.2.2 1.1.2.2.3 If you edit other slides, use Arial (Headings) 36 flush left for titles and Arial (Body) 24 for body text.

Measuring Temperature Terminal Objective: Given the appropriate equipment and procedures, the I&C Technician will calibrate and maintain temperature instruments. Mastery will be demonstrated by successful completion of a Lab Performance Exercises and written Exam.

Describe the theory of operation of Filled System Thermometers Describe the theory of operation of a thermocouple Draw a diagram of a three wire RTD bridge circuit and explain it's operation Check a Volatile Fluid sensor for proper operation per lab instructions Given thermocouple tables or graphs, a millivolt meter, and a thermometer, determine the temperature of the measuring junction of a thermocouple within two degrees Given a known Resistance Temperature Detector (RTD), it's type and it's temperature coefficient of resistance, calculate the RTD resistance for a given temperature, then verify the results in the lab setting Read the objectives

a measure of the average kinetic energy of the atoms of a substance Class I - Liquid filled (excluding mercury) Class II - Vapor filled Class III - Gas filled Class V - Mercury filled a measure of the average kinetic energy of the atoms of a substance Temperature is

Scales The story of old man Fahrenheit (and a little too much mercury exposure) Daniel Gabriel Fahrenheit (1686-1736) was a German instrument maker who invented the first practical mercury thermometer. But he needed a scale. He went to his friend, a Danish astronomer by the name of Ole Romer who already had a system. Boiling water was 60. Nice and numerological. 60 minutes in an hour, right?. Zero was colder than it usually got in Denmark because Ole didn’t like negative numbers. Big landmarks on the scale were the freezing point, 7 1/2 and body temperature. 22 1/2. Old D.G liked it a lot, but decided to get rid of the fractions and so, multiplied everything by 4. So freezing was 30 and body temperature was 96. Then, for some reason known only to him he multiplied the whole scale by 16/15. So now freezing was 32 and body temperature 98. Before he could present his scale to scientists of the day, he had to come up with some reason why zero was zero. He discovered that this was the temperature of a mixture of water, ice and ammonium chloride. At some point he figured out that water boiled at 212, and as he got more precise, that body temperature was 98.6. So on the fahrenheit scale 100 means nothing, 96 used to mean something but doesn’t anymore and 0 is colder than it gets in Denmark. Celsius was a little better, but his scale had water boiling at 0 and freezing at 100. It was reversed after his death.

Fill fluid expands as temperature increases, increasing in volume Operating Principles Fill fluid expands as temperature increases, increasing in volume Liquid in glass thermometers are also limited by the ability of glass to handle temperature extremes Mercury becomes solid at minus 39 degrees C Alcohol doesn’t freeze until -150 C Your basic thermometer, liquid in glass Mercury isn’t used much anymore because of it’s toxicity Typically colored alcohol Ways of measuring Temperature liquid-in-glass thermometer thermocouple Resistance Temperature Detector Bimetallic thermistor infrared pyrometer optical pyrometer Class I - Liquid filled (excluding mercury) Class II - Vapor filled Class III - Gas filled Class V - Mercury filled

Mercury thermometers can range from -38F to 1110 F Alcohol thermometers range from -328 F to 1110 F Other thermometer fill fluids include benzene & ether

Filled system thermometers, Also called pressure thermometers ***The big advantage of the filled system is that it places the sensor in one place and the signal processing equipment on another. *** Convert a pressure generated by temperature into motion of a needle or switch Improves over the basic thermometer by allowing temperature sensing in one location and signal processing in another a. Class I (Liquid filled - except mercury) b. Class II (Vapor filled) c. Class III (Gas filled) d. Class IV (DELETED) e. Class V (Mercury filled) capillary sensitive to nicks and abrasions, kinking, bend radius Sensing element is a capillary tube filled with a liquid or gas which expands with an increase in temperature. This sensing element delivers a motion of physical change that is applied to the control element which either indicates, records, or by comparing the signal to a setpoint can be used to control the temperature of a process.

Class II (vapor filled) Sensing bulb partially filled with volatile fluid Common fluids include: methylchloride, ether, butane, hexane, propane, toluene, sulfur dioxide Based upon the principle that in a system containing only a liquid and its vapor, at a given temperature, a given pressure will exist in the system, regardless of system volume Actual temperature measurement occurs at interface between liquid and vapor May exhibit erratic operation when temperature being measured swings above and below ambient Offers good reliability, inherently accurate, non-uniform scales (non-linear) Has mounting requirements

Class III (gas fill) Utilizes perfect gas law Absolute temperature = constant x pressure x volume (Of course, in real life folume does not remain constant, and perfect gasses do not exist) Helium approximates perfect gas, but tends to leak and is not often used Nitrogen usually is used Compensation generally not necessary if a large bulb is used The Perfect gas law or Ideal gas law says, in part, that pressure will vary in direct proportion to temperature. It doesn’t in real life gases, but close enough for approximation.

Two dissimilar metals bonded together Metal A has a lower coefficient of thermal expansion than metal B As temperature increases, metal B expands more than metal A Frequently used in home thermostats, oven thermometers, mercury switches, indicators

Bi-metal thermometers Lollipops Some can be calibrated, most can not except for zero

Convert several numbers Solve for when the fahrenheit scale equals the celsius scale (answer, -40)

Seebeck Effect A circuit formed from two dissimilar metals joined at both ends, develops an EMF (voltage) proportional to the difference in the two junction temperatures. So, if the temperature of one junction is kept at a known value, the temperature of the other junction can be determined by the amount of voltage produced. Seeback Effect: Two dissimilar metal wires, when twisted into two junctions, create a current flow which is proportional to the difference in the two temperatures that the junctions detect Red is negative Type K - Chromel-alumel Chromel is 90% Nickel, 10% Chromium Alumel is 95% Nickel, 2% Magnesium, 2% Aluminum Cold or reference junction, by convention this is referenced at zero, (0 celsius) Hot or measurement junction. All thermocouple voltages are given relative to 0 degrees Celsius. Fairly linear. Not too accurate Show a thermocouple chart basic skill of I&C tech: Using a voltmeter, measure the mv out of a thermocouple and give the temperature. You can’t just hook a junction across a meter since this creates another junction at the connection point to the meter.

Peltier Effect Reverse of the Seebeck Effect

Modern Peltier coolers use semiconductor materials rather than dissimilar metals to produce greater cooling effects.

Law of Homogeneous Circuits (also known as the law of intermediate temperatures) If thermocouple wire is homogeneous (all thermocouple wire between T1 and T2 and If temperature at T1 is known, and temperature at T2 is known, then the EMF will be known and will not be affected by temperature along the wire

Law of Intermediate Metals Thermocouple wire Thermocouple wire Non-thermocouple wire The algebraic sum of the thermo electromotive forces (EMF) in a circuit composed of any number of dissimilar metals is zero if the circuit is at a uniform temperature. -or- You can use non-thermocouple wire as long as both intermediate junctions are at the same temperature without affecting the total EMF

How to take a thermocouple reading with a DVM Wrong way! (unless you are going to mathematically compensate for ref. junction temperature using thermocouple tables, or the DVM is set up to do self-compensation) Reference junction created at meter

How to take a thermocouple reading with a DVM Right way! The ice bath can be eliminated by knowing the temperature of the reference junction because the opposing thermoelectric voltage generated by the non-zero temperature of the reference junction can simply be added to the voltage reading. So you need to know the temperature at your meter connection, then using a thermocouple table, look up the mv reading for that temperature and add that many mv to your meter reading. Some meters do this for you. (Fluke 700 family)

Reading a thermocouple Read the millivoltage for the unknown measuring junction temperature Obtain the millivoltage for the reference junction temperature from the applicable table. (reference junction is where the TC wire goes to copper) Algebraically ADD the two millivoltages The sum may then be converted to temperature directly from the same table. This is the unknown measuring junction temperature The calculations are performed automatically whenever a thermocouple reading device is used. Usually done with a resistive temperature device

At Palo Verde we use type K thermocouples – Chromel/Alumel Polarity of thermocouple wire : all thermocouple leads have a red lead which is the negative lead

Resistance Temperature Detector Electrical resistance of certain metals increase / decrease in a repeatable manner as temperature increases / decreases No compensation or reference junction needed Slower, but more accurate and more linear than thermocouples The most commonly used metals for RTDs are Platinum, Copper, Tungsten and Nickel. At PV we use Platinum

Resistance Temperature Detectors Most RTDs at Palo Verde are 100Ω at 32F We have a few 200 Ω RTDs Thermoresistive temperature measuring devices Principle of operation: A change in temperature causes the electrical resistance of a material to change. The resistance change is measured to infer the temperature change. There are two types of thermoresistive measuring devices: resistance temperature detectors and thermistors Platinum is the most common metal used for RTDs Thermistors A thermistor is similar to an RTD, but a semiconductor material is used instead of a metal. A thermistor is a solid state device A thermistor has larger sensitivity than does an RTD, but the resistance change with temperature is nonlinear Thermistors cannot be used to measure high temperatures either, compared to RTDs. In fact, the maximum temperature of operation is sometimes only 100 or 200 oC What’s this called?

Calculate Temperature using an RTD Where Rt2 = Resistance @ temp T2 in Ω Rt1 = Resistance @ temp T1 in Ω α = temperature/resistance coefficient (F or C) T2 = measurement temperature (F or C) T1 = reference temperature (F or C) usually 0C or 32F

Two Wire RTD The RTD is one leg of a wheatstone bridge

Three Wire RTD

Four wire RTD

Thermistors Solid state device Cheap Similar to RTD except resistance goes down as temperature goes up. Less linear than RTD Often used in heat detection and compensation circuits Higher sensitivity to small changes in temperature

How can you tell if a thermocouple or RTD is in a thermowell? Answer; You can’t. Don’t guess. Drawings help. Know for sure before you take it out. Tell diesel lube oil story about thermowell. Talk about advantages and disadvantages of thermowells. Temperature lag, etc. How can you tell if a thermocouple or RTD is in a thermowell? Thermowells

Other Methods of Temperature Calibration Methods to calibrate a temperature device Baths: water, ice, oil, sand Dry Block temperature calibrators - Jofra Comparison with a contact or non-contact temperature measuring device: containment temperatures, QSPDS, etc Issues with pipe contacts and thermal transmission Pyrometry relies on a quantitative measurement of the radiation which is emitted from an object. The main advantage of pyrometers is that they work without physical contact with the hot object. The two types of pyrometers which will be used in this lab are the optical pyrometer and the infrared pyrometer. Infrared pyrometers use a radiation detector which, when pointed at an object, detects the amount of infrared radiation impinging on the detector. The temperature of the detector is measured (usually with a thermopile or other electronic device), and the infrared radiation emitted from the source is inferred. An optical pyrometer works by comparing the visible radiation that is emitted from a radiation source to the visible radiation emitted from a filament wire. The current supplied to the filament wire is adjusted until the wire "disappears", inferring that it is at the same temperature as the object whose temperature is being measured. The temperature of the filament wire is a known function of the supplied current, and therefore the temperature of the object is inferred.

Read about mod at end of temperature section in handout Discuss Plant Mod 2807626 Read about mod at end of temperature section in handout Discuss plant impact of mod This is required by a TCS action item

Instrument Loops Identify common instrumentation signals Explain the operation of a basic measurement loop Explain the operation of a basic control loop Read the objectives

Common Instrument Signals Current 4-20 milliamps Voltage 0-10 Volts DC Pneumatic 3-15 psig Why use instrumentation at all? Why not just have local indicators everywhere? Remote sensing, the ‘control room’ concept, the big picture The difference between measurement and control. Difference between manual and automatic control. There are no ‘standards’ in instrument signals, only commonly used signals. Why use 4-20 ma? Live zero 0-20 ma is also a common instrument signal On a 4-20 to 0-10v converter, if the current loop opens, output of the converter goes to -2.5VDC 0-10VDC - A foxboro standard 1-5 VDC is also a commonly used standard with a live zero and works well with the 4-20ma 3-15 psi is nearly a universal air signal

A pressure control loop using a pressure transducer and sensing element, a 4-20ma input loop to a converter, a controller which receives and puts out a 0-10VDC signal, an output card which sends a 4-20 ma signal to a valve, an air signal output to a valve and a final control element which is a valve regulating the input pressure to the process. What is the process here? It’s the tank pressure, since that’s what we’re controlling.

Basic Pressure Loop Between the sensing element and the transducer the signal is part of a bridge circuit and is usually not measured. Between the transducer and the detection circuitry the signal us usually a current loop of 4 to 20 ma. Between the converter and the indicator or whatever else you put downstream, the signal is always a 0-10VDC signal if it’s in the Foxboro system A level loop may be exactly the same as a pressure loop

Basic Flow Loop Square root extraction is always needed if differential pressure is used to measure flow.

Basic Temperature Loop

Sump level control using a float type level switch which turns a pump on or off. Crude on-off control

An oven or heater controller An oven or heater controller. Temperature is measured, compared to a setpoint and an output signal is generated which controls a heater.

Level control Wet or dry reference leg current loop converter controller output card current to pneumatic converter valve controlling input to the tank

Odds & Ends

There are many ways to destroy test equipment.

Check voltage before you check contact status on Ohms

DVMs are especially sensitive Excessive voltage Excessive current Leads on wrong test point Tell microwave oven story

Control your test leads

Check your mini-grabbers

Don’t trust your holding screwdriver, either to hold the lead or to keep you from getting shocked

When replacing a transmitter, beware When replacing a transmitter, beware! You are typically using a 3-valve manifold as your pressure boundary.

Know the pressure rating of your test tubing. Polyflow can take about 200 psi

The End