Temperature Measurement in Freezers, Autoclaves, and · PDF fileMeasuring air temperature can be more difficult than liquids because of several factors, with time response being the

© Burns Engineering Temperature Measurement in Freezers, Autoclaves, and Air

Temperature Measurement in Freezers, Autoclaves, and Air

Bill Bergquist, Sr. Applications EngineerJeff Wigen, National Sales Manager

Jeff Bill

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Heat transfer rate is much slower than in liquids and as a result:Time response is much slower than in liquidsStem conduction be more significant requiring a longer immersion lengthTemperature gradients are larger

• Large ductwork may require multiple sensors or an averaging sensor

Boundary layer consideration

Air Temperature Measurement Challenges

Measuring air temperature can be more difficult than liquids because of several factors, with time response being the most significant. RTDs and thermocouples respond slower to temperature changes in air and this can be either a plus or minus depending on the application. A storage freezer for example may require an even slower response to eliminate false alarms caused by loading and unloading product from the freezer.

Temperature Measurement in Freezers, Autoclaves, and Air© Burns Engineering

Protection • Ambient conditions• Physical damage• Hazardous atmosphere• Water, frost• Pressure changes• Sunlight and other

radiation sources

Placement• Location

Performance• Calibration• Stem conduction• Accuracy• Durability• Long term stability

PriceService life

Today’s Discussion

We will be primarily discussing the use of RTDs for air temperature however there are some measurements where a thermocouple is a better choice. Fast time response or durability may dictate the use of a thermocouple.

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Autoclave air temperature measurementPressure changeSteamDurabilitySealing

Protection

A steam autoclave is a difficult environment for an RTD or a thermocouple. The pressure change when a vacuum is applied to the chamber can also remove all the air from inside a sensor if the sensor and cable are in the autoclave. When steam is added, the vacuum inside the sensor pulls the steam inside and immediately causes low insulation resistance which then causes a low temperature reading. A sensor that is fully immersed in an autoclave requires a vacuum tight seal where the cable enters the metal sensor body. Also the potting and cable materials need to be rated for the temperature service and should be resistant to steam. For example, if silicone rubber is used for the extension cable it should be a variety called platinum cured.

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Autoclave air temperature measurementPressure change

• Sensor needs to be sealed to prevent escape of internal air– Accomplished with hose covered cable– Molded sensor assembly

Protection

Multiple sensor feedthroughEPDM hose covered cable

One method of sealing the sensor is to cover the cable with a hose. In this example an EPDM hose is attached to the sensor and a hygienic ferrule with hose barb connectors. This effectively seals the sensor and also provides a positive seal through the autoclave wall.

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Autoclave air temperature measurementSealing through autoclave wall

Protection

What doesn’t work so well

One possible solution:silicone rubber and SS braid

covered Teflon® hose

Here’s an example of a method to seal sensor cables through the autoclave wall using a cord grip fitting and a liberal dose of silicone rubber -- definitely NOT recommended! Another hose material that works well is Teflon®

covered with stainless steel over-braid and a silicone rubber outer cover. This design features a replaceable sensor which is sealed to the hose with compression fittings.

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Durability is a big problem with autoclave air temperature measurementSensors for smaller autoclaves depend on the cable jacket for pressure sealing and they are easily damaged. May want to consider additional protection.

Protection

Besides the pressure and sealing challenges, a high degree of durability is required to withstand frequent handling. Dropping the sensor on the floor can cause a resistance shift unless it is designed to withstand it. Sensors that depend on the cable jacket for sealing can be easily damaged especially if the sensor is forgotten when the cart is rolled out of a larger autoclave. The example part shown here uses a platinum cured silicone rubber cable jacket and a triple vacuum/moisture seal inside the handle. It is intended to be used as a load cell temperature sensor and offers a sharpened tip for piercing rubber vial seals.

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Autoclave air temperature measurementDurability - sensors attached to carts are sometimes forgotten resulting in damaged lead-wires or broken sensing elements when the cart is moved

• Thin-film sensing elements when packaged correctly can be extremely durable

Protection

Internal design of the sensor is the primary means of insuring durability. A thin film sensing element if packaged correctly can be extremely durable and testing has shown it can survive thousands of drops on a hard floor.

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Freezer temperature measurementFrost

• Requires waterproof construction• Slows time response

Mounting considerations• Can be difficult to remove for

calibration while freezer is in service• Placement in warmest or coldest part of

chamber? Survey the interior to determine best location

• Consider frost build-up• Lead wire materials susceptible to

cracking if inside – minimize movement

Protection

Freezers present another host of issues for accurate stable temperature measurement. Frost is the biggest problem and can slow down the response time if it covers the sensor. If not properly sealed, water from frost melt during maintenance can enter and cause insulation resistance failure. Sensors that are placed entirely inside the freezer should be waterproof and not simply water resistant.

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Freezer temperature measurementExamples of waterproof designs that work well in freezers down to -196°C

Protection

Transition and cable rated for cold environment

Both of these sensors are designed for complete submersion in water and both the cable and potting materials are rated for use down to -196°C.

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Freezer temperature measurementStandard RTD with epoxy seal (water resistant) may work okay initially but it will not seal out water that will be present during defrosting or calibration. They may look the same but will perform differently

Protection

Water resistant RTD Waterproof RTD

Appearance is not a good indicator of “waterproofness”. Both of these sensors are 3/16” diameter, and have evidence of an epoxy potting. The only way to determine which is suitable for freezer service is to review the specifications.

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Time response in freezers may need to be adjusted to eliminate false alarms when loading or unloading the chamber.

Can use a delay feature in a transmitterAdd Teflon sleeve over the sensorPlace sensor in a bottle filled with glycol or other fluid

Performance

Time response can be adjusted to allow loading or unloading product without setting off an alarm. Three possibilities shown here are a Teflon® sleeve, sensor installed in a bottle filled with glycol, and an example of a response time adjustment in a smart transmitter setup.

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Ambient air temperature measurementSunlight

• Other radiationFlow rateStem conductionBoundary layer considerationsTemperature gradientsDurability

Protection

Example of simple radiation shield

Ambient air measurement

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Ambient air temperature measurement sensorsWall mount (1)Heavy duty (2)Low cost (3)

Protection

1

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Ambient air temperature measurementRecommended immersion for ductwork

• A couple common rules of thumb you might see– 10X thermowell diameter– Two thirds of duct size– Neither work very well

Room air temperature measurement• Interior wall farthest from heat source• Insulate between sensor and wall or inside junction box

Performance

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Measuring room air temperature with ceiling mounted RTDsA series of RTDs were placed in a clean room ceiling extending 3.5” below the finished ceiling. Connection head was located above the ceiling in unconditioned space. There was a 30°F difference above and below the ceiling with above being warmer. Measured temperature with the array of sensors was higher than with a hand-held temperature standard placed next to them. This was a classic case of stem conduction causing an error in measurement. Solution was to increase length of the RTD immersed in the room. A 90 degree bend was added to keep the sensor within the 3.5” distance from the ceiling. The extra immersion length eliminated the stem conduction.

Application Note

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Measuring room air temperature with ceiling mounted RTDsSolution was to increase length of the RTD immersed in the room. A 90 degree bend was added to keep the sensor within the 3.5” distance from the ceiling. The extra immersion length eliminated the stem conduction.

Application Note

90° to 100°F

70°F

ceiling

3.5”

RTD connection head

SolutionOriginal configuration

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Averaging air temperature measurementUseful for large ductwork where boundary layers and large temperature gradients existSensing element is stretched out for full length of the sensor providing an average temperature measurement

Performance

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Ductwork temperature with averaging RTDBendable sensor allows for covering large area of duct for an average temperature measurement

Averaging RTD for Room Air

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Ductwork temperature with averaging RTDBoundary layer in ducts

• For best accuracy place sensor outside of boundary layer• Low velocity = thicker layer

Averaging RTD for Room Air

flow

Stagnant air boundary layer

Stagnant air boundary layer

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Protection

A connection head offers protection to the sensor from moisture, physical damage, and houses a terminal block or transmitter. Numerous styles and materials from plastic to aluminum are available. Some carry ratings for use in hazardous atmospheres.

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Hazardous atmosphere

Protection

Hazardous atmospheres require an RTD or thermocouple and connection head assembly that carries an appropriate rating.

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Error BudgetSensor accuracy

– Interchangeability– Matching to transmitter– Thin film or wire wound– Repeatable

Measurement accuracy– Installation– Time response– Control system– Repeatable

Performance

Accuracy of the sensor and accuracy of the measurement are most times quite different. Sensor accuracy is determined mostly by the manufacturing interchangeability and the style of sensing element. The wire wound style has the widest temperature range and lowest drift. Measurement accuracy includes the sensor accuracy and the installation effects. In addition, time response can be a large factor in the measurement accuracy.

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InterchangeabilityInterchangeability refers to the “closeness of agreement” between an actual sensor R vs. T relationship and a predefined R vs. T relationship.

Performance

ASTM E1137 and IEC 60751 are the two most commonly used standards that define a nominal R vs. T relationship. All sensors are manufactured with 0°C as the starting point. Variations in sensors result in the tolerance increasing as the temperature diverges from 0°C.

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Standard Tolerance Defining Equation¹ASTM E1137 Grade A ± [ .13 + 0.0017 | t | ]ASTM E1137 Grade B ± [ .25 + 0.0042 | t | ]IEC 607512 Class AA2 ± [ .1 + 0.0017 | t | ]IEC 60751 Class A ± [ .15 + 0.002 | t | ]IEC 60751 Class B ± [ .3 + 0.005 | t | ]IEC 607512 Class C2 ± [ .6 + 0.01 | t | ]

Note 1: | t | = absolute value of temperature of interest in °CNote 2: These tolerance classes are included in a pending change to

the IEC 60751 standard.

Performance

These equations can be used to calculate the interchangeability at any temperature. Note that the temperature t is an absolute value in °C. The resultant is the interchangeability in ± °C.

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-4

-3

-2

-1

0

1

2

3

4

-300 -200 -100 0 100 200 300 400 500 600 700 800

Temperature (°C)

Tole

ranc

e (±

°C)

IEC Class B

ASTM Grade B

IEC Class A ASTM Grade A

ASTM Grade A

IEC Class A

ASTM Grade B

IEC Class B

Performance

Note that the ASTM standard has slightly tighter tolerances for the two grades of sensors. All RTDs are built with the tightest tolerance at 0°C and as the temperature diverges from 0°C the tolerance increases. The vertical line on the graph represents 0°C and the tolerance on the y axis is expressed in ± °C from nominal.

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Stability

Definition: The state of being resistant to change or deterioration.

Performance

Change from

Initial ( °C)

0 200 400 600 800 1000

0.4

0.3

0.2

0.1

0.0

500 °C

400°Co

200°C

Long Term Stability (Drift)

Hours at temperature

Stability or long term drift is an important consideration in selecting an RTD for best accuracy. As you can see from the graph as temperature goes up the drift becomes much more significant.

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Sensor typesElements

• Wire wound– External wound– Coil

• Thin FilmSingle or Dual

Performance

RTDs are made using either a wire wound or thin film sensing element. There are performance differences in durability, stability, and temperature range. Wire wound typically have a wider temperature range and better long term stability.

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Sensor types

Performance

Wire Wound Thin Film

Element Resistance

Accuracy 0°C/200°CRepeatability

Time Response

Temp. Range

Vibration

Long Term Stability

100 ohms

± 0.13°C/0.5°C

0.1°C

4.0 Sec.

-200 to 500°C15 g’s

.1°C

100, 1000 ohms

± 0.26°C/1.0°C

0.1°C

6.0 Sec.

-50 to 200°C20 g’s

.5°C

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Cost of inaccuracy

Performance

• Process Fluid: Water• Flow Rate: 50 GPM• Control Temperature: 100 °F• Energy Cost: 2.9¢ / KW-hour

Annual Cost of Energy Per °F Error

$1846 / year

High accuracy insures product quality and efficient use of your energy dollar. Selecting the best sensor and installation pays dividends.

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TransmittersLead wire > 250 feet (+0.16°F/100 ft)Accuracy

MatchingLead wire

Robust signalRFI/EMI

Performance

Adding a transmitter can improve accuracy when a long run of lead wire is required. They also provide a more robust signal that is less susceptible to interference from electro-magnetic or radio frequency interference.

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RTD time response in water moving at 3 fps. In air times are about >10x slower depending on air velocity

Performance

Stepped

Tapered

Direct Immersion RTDs

1/4” - 1/8”

1/4”

1/2” - 1/4”

DIAMETERSTIME RESPONSE

2.5 seconds

4 to 6 seconds

22 seconds

26 seconds

Thermowells

6 to 8 seconds

A tapered thermowell will have 3 to 4 times slower response than the ¼” diameter direct immersion sensor. This can be a big factor in accuracy for processes that are changing temperature rapidly. The sensor needs to be fast enough to keep up with the process. Using the same assembly to measure air temperature will show at least 10x slower response.

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Thermocouple time response in water moving at 3 fps. In air times are >10x slower depending on air velocity

Performance

Stepped

Tapered

Direct Immersion RTDs

1/4” - 1/8”

1/4”

1/2” - 1/4”

DIAMETERSTIME RESPONSE

2 seconds

2 to 4 seconds

18 seconds

22 seconds

Thermowells

3 to 5 seconds

Thermocouples are a good choice where fast response is needed for air temperature measurement. They can be many times faster than an RTD depending on the configuration.

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Thermocouple time response for exposed junctions in air moving at 45 mph or 66 ft/sec.

.003 seconds for .001” diameter wire10 seconds for .125” diameter wire

Performance

Exposed junction

An exposed junction thermocouple made with small guage wire provides the fastest response time.

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Insulation resistance is the first and most important electrical check to make on an RTD

Low IR can cause a low temperature measurement due to shunting between the sensing element wires Most IR failures are due to moisture and/or contaminants that may have entered the probe

Calibration

This is a typical wire wound sensing element. They are about 1” long and 1/16” diameter and are potted inside a stainless steel sheath. If moisture gets into the sheath and sensing element the result can be a shorter path for the excitation current and the result is a low resistance measurement.

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Test methodLower resistance = lower measured temperatureTest at 50 VDCIR should be >100 megohms at 25°C

Insulation Resistance

IR decreases with an increase in temperature so at room temperature a value much higher than what is really needed for an accurate measurement is required. Industrial grade RTD accuracy is not significantly affected until the IR drops below a few megohms. The measurement is made by touching one lead of a megohmeter to the leads and the other to the probe sheath. Some industrial grade probes are tested to higher levels to insure maximum performance at high temperatures.

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Using an Ice Bath, check resistance at 0°C

Ice Point Check

Crushed ice made with purified water is packed into an insulated container. Purified water is added to fill in the gaps. If the ice floats, you have added too much water. Adding a stirring feature to keep the water flowing around the ice minimizes temperature gradients within the bath. Each probe should be immersed at least 4”. Do not use the probe to beat a hole in the ice. You may damage the sensing element. Use a scrap probe or similar rod to form the holes.

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Life cycle cost accountingInitial cost Operating efficiencyProduct qualityMaintenance costsEnergyDown timeTroubleshootingProduct lossOverhead and inventory

Price

Goal is to minimize the total life cycle cost of the measurement point while maximizing performance. Numerous costs need to be considered from the sensor cost to the cost of carrying a spare in inventory.

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Considerations:Application/EnvironmentConstruction styles

• wire wound• thin film• moisture seals

DurabilityOriginal specificationsFewer calibration cycles req’d

Service Life

USS Constitution 1797 to present

There are many RTDs that have been in service for 20+ years. When giving careful consideration to all the selection factors a long life can be expected and at the lowest life cycle cost.

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Total Air TemperatureIn aviation, stagnation temperature is known as total air temperature

and is measured by a temperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the aircraft. As the air is brought to rest, kinetic energy is converted to internal energy. The air is compressed and experiences an adiabatic increase in temperature. Therefore total air temperature is higher than the static (or ambient) air temperature.

Total air temperature is an essential input to an air data computer in order to enable computation of static air temperature and hence true airspeed.

From Wikipedia: http://en.wikipedia.org/wiki/Total_air_temperature

Other Air Temperature Measurements

The RTD is used to measure temperature used for critical aircraft power settings and airspeed determination.

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Deicing capabilityDurability

• Bird strike• Hail • Vibration


Photos from Rosemount and Goodrich Aerospace

These are a few examples of aircraft total temperature sensors. Besides providing an accurate and repeatable measurement they need to withstand abuse from hail, birds, vibration, and particles found in the air.

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Boeing 747


Sensor location

There are two sensors mounted just below the windshield on a 747.

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Airbus A320 engine inlet


Photo from AAIB Bulletin No: 11/2004

An example of an engine inlet temperature sensor on an Airbus A320.

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Business Jet


Sensor location

Other jets have the total temperature sensor located at the four o’clock position just below the cockpit. They provide temperature for true airspeed calculations.

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Autoclave• Durability• Sealing for pressure changes on probe and cable exit

Freezers• Frost• Location for desired temperature• Cable and probe temperature limits

Ambient air• Frost• Placement

• Account for temperature gradients• Maintenance• Accurate measurement

Summary

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Watch for upcoming RTDology® events

Temperature Basics

Surface vs. Immersion Temperature Measurements

Improving Accuracy in BioPharma Process Measurements

View presentation notes from previous sessions on our website at: www.burnsengineering.com/RTDology

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Documents

Temperature Measurement in Freezers, Autoclaves, and · PDF fileMeasuring air temperature can be more difficult than liquids because of several factors, with time response being the