Process Variables. Process Variables. Temperature Strain Level Flow Pressure

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1 Process Variables Process Variables Temperature Strain Level Flow Pressure

2 Process Variables Weight Viscosity Vibration ph Conductivity Process Variables Density Distance Position Altitude Acceleration

3 Process Variables Colour Radiation Force Humidity Specific Gases Process Variables The number one variable that is measured and controlled in the manufacturing environment

4 Temperature Processes, as well as humans, are sensitive to changes in temperature. Temperature Before we start, SAFETY FIRST

5 Temperature SAFETY Temperature SAFETY

6 Temperature SAFETY Temperature SAFETY Due to the possibility of explosions use the safety barrier circuitry.

7 Temperature The four most common devices to measure temperature are: 1. Thermocouple 2. Resistance Thermal Device 3. Thermistor 4. Infrared Pyrometer / IC Sensors What is a thermocouple? It consists of two dissimilar metals, joined together at one end, which produce a small unique voltage at a given temperature.

8 are available in different combinations of metals or calibrations. Each calibration has a different temperature range and environment, although the maximum temperature varies with the diameter of the wire used in the thermocouple.

9 Choosing a thermocouple type Because thermocouples measure in wide temperature ranges and can be relatively rugged, they are very often used in industry. The following criteria are used in selecting a thermocouple: Choosing a thermocouple type Temperature range Chemical resistance of the thermocouple or sheath material Abrasion and vibration resistance

10 Choosing a thermocouple type Installation requirements (may need to be compatible with existing equipment; existing holes may determine probe diameter). Temperature Basic Principle of the Thermocouple Seebeck Effect When two wires composed of dissimilar metals are joined at both ends and one of the ends are heated, there is a continuous current which flows in the thermoelectric circuit.

11 Temperature Metal A Metal C Metal B Temperature If the circuit is broken at the center, the net open circuit voltage (Seebeck voltage) is a function of the junction temperature and the composition of the two metals. All dissimilar metals exhibit this effect.

12 Temperature Metal A E AB = Seebeck voltage Metal B eab= SC * T Temperature Cu Fe Cu C Voltmeter Reference Block

13 Temperature Why use them? (benefits) CHEAP! Temperature Very Wide Temperature Use -450ºF +4200ºF

14 Temperature Interchangeable to some degree Medium to fast response time Fair linearity Tip sensitive Small to large packaging Temperature Why not to use them? (problems)

15 Temperature Long term stability poor to fair Accuracy medium Repeatability poor to fair Sensitivity - low Temperature The most common combinations are: E, J, K R, S, T

16 Temperature Type E Nickel Chromium vs. Constantan -100 deg. C to 1000 deg. C +/- 0.5 deg. C Temperature Type J Iron vs. Constantan 0 deg. C to 760 deg. C +/- 0.1 deg. C

17 Temperature Type K Nickel Chromium vs. Nickel Aluminum-Silicon 0 deg. C to 1370 deg. C +/- 0.7 deg. C

18 Temperature Type R Platinum Rhodium vs. Platinum 0 deg. C to 1000 deg. C +/- 0.5 deg. C

19 Temperature Type S Platinum Rhodium vs. Platinum 0 deg. C to 1750 deg. C +/- 1 deg. C Temperature Type T Copper vs. Constantan -160 deg. C to 400 deg. C +/- 0.5 deg. C

20 Temperature Curves

21 Junction Types Grounded The thermocouple wires are physically attached to the inside of the probe wall. This results in good heat transfer from the outside, through the probe wall to the thermocouple junction. Junction Types Ungrounded The thermocouple junction is detached from the probe wall. Response time is slowed down from the grounded style, but the ungrounded offers electrical isolation of 1.5 M1/2 at 500 Vdc in all diameters.

22 Junction Types Exposed This type offers the best response time, but is limited in use to noncorrosive and nonpressurized applications Material Maximum Temperature Application Atmosphere 304 SS 900 C (1650 F) Inconel 1148 C 600 (2100 F) Oxidizing Hydrogen Vacuum Very Good Very Good Good Good Very Good Very Good Inert Very Good Very Good

23 Polarity In the thermocouple industry, standard practice is to color the negative lead red. The negative lead of bare wire thermocouple is approximately 1/4" shorter than the positive lead, and the large pin on a thermocouple connector is always the negative conductor. Extension Wire Thermocouple alloy wire must always be used to connect a thermocouple sensor to the instrumentation to assure accurate measurements.

24 RTD s Resistive temperature devices capitalize on the fact that the electrical resistance of a material changes as its temperature changes RTD s RTDs rely on resistance change in a metal, with the resistance rising more or less linearly with temperature

25 The heart of the RTD is the resistance element. Two varieties are: semi-supported wire-wound fully supported bifilar wound glass thin film RTDs operate from about -250 to 850 C

26 THERMISTORS Thermistors are based on resistance change in a ceramic semiconductor; the resistance drops nonlinearly with temperature rise. Thermistors have a more restrictive span, being commonly used between -40 and 150 C

27 RTD and Thermistor Problem Thermistors and RTDs share a very important limitation. They are resistive devices, and accordingly they function by passing a current through a sensor. Even though only a very small current is generally employed, it creates a certain amount of heat and thus can throw off the temperature reading. Selection, selection, selection Temperature Sensor Attributes Criteria Thermocouple RTD Thermistor Cost-OEM Quality Low High Low Temperature Range Very wide -450ººF +4200ººF Wide -400ººF +1200ººF Shot to medium -100ººF +500ººF Interchangeability Good Excellent Poor to fair Long-term Stability Poor to fair Good Poor Accuracy Medium High Medium Repeatability Poor to fair Excellent Fair to good Sensitivity (output) Low Medium Very high Response Medium to fast Medium Medium to fast Linearity Fair Good Poor Self Heating No Very low to low High Point (end) Sensitive Excellent Fair Good Size/Packaging Small to large Medium to small Small to medium

28 INFRARED An infrared thermometer measures temperature by detecting the infrared energy emitted by all materials which are at temperatures above absolute zero, (0 Kelvin). In many process situations, the energy is in the infrared region. As the temperature goes up, the amount of infrared radiation and its average frequency go up.

29 Different materials radiate at different levels of efficiency. This efficiency is quantified as emissivity, a decimal number or percentage ranging between 0 and 1 or 0% and 100%.

30 To function properly, an infrared measurement device must take into account the emissivity of the surface being measured. This can often be looked up in a reference table. Infrared sensors, though relatively expensive, are appropriate when the temperatures are extremely high. They are available for up to 3,000 C (5,400 F), far exceeding the range of thermocouples or other contact devices.