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Controllers

Temperature controlled systems comprise four essential elements that

are joined to form a closed loop.

1. Load: Object which is required to be maintained at a constant

and specified temperature.

2. Heater or cooler: Provides heating or cooling to the system

3. Sensor: Measures the temperature of the system and feeds the

information back to the controller.

4. Controller: Compares the information it receives from the sensor

(PV, or process value) with the desired temperature (SP,or set value).

It then adjusts the power (MV, or manipulated variable) that is fed

to the heater or cooler, to compensate for any tendency of the load

temperature to drift up or down, closing the loop.

Types of Controllers

On/Off control: A simple control system in which the heater or cooler

is completely off as the temperature rises through the set point, and

on as the temperature falls through the set point.This type of control

continuously hunts around the set point, and should only be considered

when the control requirements are not critical.

PID control: Proportional-Integral-Derivative control.When more stable

control is required, it is necessary to slow down the rate of temperature

rise when approaching the set point in order to avoid overshoot. PID

control provides precise control and should be used for systems that

may have frequent and unexpected disturbances.

INSTRUMENTS …setting the standard, again and again

Technical Information

Thermocouples

What is a thermocouple?Thermocouple sensors consist of

two dissimilar metals, joined to produce a specific voltage at a given

temperature.This voltage is then measured and interpreted by a

thermocouple thermometer, and then displayed for a user to view.

There are many types of thermocouples, which are made of different

types of materials. Some of the more common types are J, K, and T.

Why choose a thermocouple? Choosing the correct thermocouple

type depends on the applications’ required temperature range and

accuracy.Thermocouples are used in most general-purpose applications

when precise accuracy is not a top priority.They offer a wide temperature

range and come in a large variety of configurations.

Thermistors

What is a thermistor?Thermistors are thermally sensitive resistors

which change electrical resistance due to temperature changes.They

have very predictable characteristics and offer long-term stability.

Why choose a thermistor? Thermistors have excellent accuracy

over the biological or ambient temperature ranges when compared to

thermocouples or RTDs. Response times are generally faster than other

types of probes. But, thermistors have a limited temperature range that

usually cannot exceed 300°F (150°C).

PID Control

Response to Response to

target values external disturbance

Set point

OFF

Infrared Thermometers

When choosing an infrared thermometer, consider the temperature range

required, size, material, environment, distance of the measured object, and

its spectral response. Portable infrared thermometers are most often used

for preventative maintenance, troubleshooting, and quality control. Infrared

temperature measurement is ideal for objects that:

• Are too far or too difficult to reach

• Require non-contamination from one reading to the next

• Are moving, rotating,or vibrating

• Have high temperatures up to 1400°F (760°C)

• Are scratched or damaged if contacted

• Have curved, distorted, or varying surface conditions

How do Infrared Thermometers Work?

All objects emit infrared energy.The hotter an object is, the more active

its molecules are, and the more infrared energy it emits.An infrared

thermometer houses optics that collect the radiant infrared energy from

the object and focus it onto a detector.The detector then converts the

energy into an electrical signal, which is amplified and displayed.

Emissivity:An object’s ability to emit or absorb energy. Perfect emitters

have an emissivity of 1.An object with an emissivity of 0.8 will absorb 80%

and reflect 20% of the incident energy. Emissivity may vary with

temperature and spectral response (wavelength).

Distance-to-target-size ratio:The infrared thermometer focuses

infrared energy from an object onto its detector at this ratio. For example,

a 4:1 distance-to-target-size ratio means that the infrared thermometer

will read a 1" dia area from 4" away.The object must fill the entire area

for accurate readings.

RTDs

What is an RTD? RTD stands for Resistance Temperature Detector.This

is the sensing technology that measures temperature by measuring the

change in electrical resistance across two metal wires.The resistance value

is then measured and interpreted by an RTD thermometer, and displayed

for a user to view.While RTD wire can be made of any metal, platinum

is the metal of choice due to its excellent repeatability, stability, and

resistance to corrosion and chemicals. RTDs are more accurate and stable

than thermocouples, but cannot be used to measure extremely high

temperatures. Compared to thermistors, RTDs are more rugged and

can measure higher temperatures

Why choose an RTD? RTDs are more accurate and stable than other

sensors, such as thermocouples, but they cannot be used to measure

extremely high temperatures. Choose an RTD sensor when accuracy

and repeatability are important, as long as you do not need to measure

extreme temperatures.

http://www.novatech-usa.com/Products/Laboratory-Equipment-Supply-Products Tel: (281) 359-8538 Toll Free:(866) 433-6682

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