How to measure the temperature?

Temperature is, after time, the physical quantity most often measured. What would the weather be without indicating the temperatures?

In addition to ambient temperature, temperature is also measured in many other environments. Whether it is the operating temperature of an industrial installation, the temperature of the rising and falling pipes of radiators, the storage temperature of medicines or the core temperature of a yogurt: contrary to weather forecasts , the temperature values must be precise and correct in these areas. Only in this way is compliance with standards guaranteed and damage to valuable goods can be avoided.

The right basic knowledge and practical advice prepare you well for your measuring tasks.

Physical basis of temperature as a measured variable

When a physicist speaks of temperature, he designates a quantity which measures the energy inherent in bodies. A body has this energy due to the random movements of its atoms or molecules. When the particles move faster, the temperature rises. Temperature is therefore a state quantity. In common with other quantities such as mass, heat capacity, and others, temperature describes the energy contained in a body, or, as we often say in physics, a system.

Or very briefly:

The input of thermal energy causes an increase in the speed of the particles: the temperature rises

The extraction of thermal energy causes a decrease in the speed of the particles: the temperature drops

If a body no longer has thermal energy, its molecules are in a state of rest. This state is impossible to achieve in reality. It is designated as absolute zero because there is no poorer energy state. It is assigned the value of 0 K (Kelvin). This is why the temperature in Kelvin is always a positive quantity.

It is quite possible to measure temperature directly in units of energy. But the indication of temperature in degrees has a long tradition and is firmly anchored in physics. This is why we have kept this tradition until today for practical reasons.

To note:

Temperature is shown in Kelvin (K) and measured in degrees Celsius (° C) or Fahrenheit (° F) (in the US and other countries) for everyday use.

The temperature differences are always indicated in Kelvin by the experts.

• Conversion: 1 K º 1 ° C = 9/5 ° F
• Conversion formulas according to DIN 1345:
• tC = 5/9 (tF - 32) = TK - 273.15
• TK = 273.15 + tC
• tF = 1.8 tC + 32

How to measure?

What is the general structure of a unit of measure?

1. The probe: houses the sensor. There are different shapes of sensors for different applications.
2. The probe tube with the sensor: converts the physical measurement value into an electrical signal.
3. The connection line: connects the measuring device to the probe (sensor).
4. The measuring device: converts the probe signals into displayed values (A / D conversion).
5. Resolution: the smallest still readable subdivision of the unit of measurement.

The resolution

In this example, the display shows 22.3 ° C, so the resolution is 0.1 ° C. If the display showed 22.34 ° C, the resolution would be 0.01 ° C. In digital devices, the last digit on the display may jump +/- 1> unit. The smallest unit is called a digit.

Example: Display of 22.3 ° C

Display -1 digit 22.2 ° C; display +1 digit 22.4 ° C.

The poorer the resolution of a measuring device, the more a digit jump can influence the accuracy of the measuring device.

Sensors: operation

Choosing the right sensor is crucial for the accuracy of the measurement result. But there is no one ideal sensor for all applications:

A large measuring range usually means limited accuracy.

Extremely fast probes are generally not suitable for measurements in daily work requiring a robust device.

The construction form determines which probe is best suited for a specific measuring task.

Thermocouple sensors

Electrical voltage measurement junction / reference junction.
The measured value of the reference junction is converted to 0 ° C.
Sufficient response time must be observed.

Platinum resistance sensors

Measuring principle: positive temperature coefficient of metals.

Report: temperature and electrical resistance.

Reference value 0 ° C at a resistance of 100 ohms by a coiled platinum wire.

Thermistor sensors

Temperature sensors based on ceramic mixture of oxides.

Negative temperature coefficient: CTN.

Without cold junction compensation - ideal for cold rooms and freezers.

Types of probes

What is the task of a probe?

Temperature probes capture the temperature of a medium and transmit it to the sensor. To do this, the sensor material must first adapt to the outside temperature. This is why it is never the temperature of the medium that is measured, but only the temperature of the probe or the sensor.

The probe takes some time to harmonize with the material being measured. A probe has adopted the temperature of the material measured when its temperature is the same as 99% at the outside temperature. This period of time is called time t99.

What forms of construction of probes are there?

There are different forms of construction of the probes depending on the intended application. Just as one uses, for example, another knife to cut bread than to cut meat; there are also different probes to measure the temperature.

Immersion / penetration probe

Harmonization of probe and liquid temperatures.

Reduced agitation response time.

t99 approx. 0.5 sec in the ideal case.

Room sensor

Reduced heat transfer between the air and the sensor.

The sensor is free; good for the impact of air flow.

t99 ideally approx. 7 sec.

Contact probe

Strong temperature jump, like air has an insulating effect.

Removal of air insulation by the contact thermometer.

t99 approx. 3 sec in the ideal case.

Avoiding Measurement Errors - Tips for Measuring Correctly

Penetration and immersion measurements

During penetration or immersion measurements, the temperature probe is inserted directly into the measurement object. The measurement is finished when the time t99 is reached.

The most frequent measurement errors

If the temperature probe is cooler than the measuring object, energy in the form of heat is withdrawn from the measuring object in the vicinity of the sensor. If it is hotter than the measuring object, the object receives heat input. It is also necessary to pay attention to the mass ratio of the probe and the medium: the greater the ratio, the greater the quantity of energy withdrawn from the object. As this energy extraction no longer makes it possible to measure the real temperature of the object, too great a mass of the probe can cause measurement errors.

Tips for correct penetration measurement

• The depth of penetration or immersion should be 10 to 15 times the diameter of the probe.
• When measuring immersion in liquids, the liquid must be kept in motion.
• Ideally, t99 is reached after about 0.5 seconds.

Contact measurement

In surface measurements, the probe head is placed vertically on the surface. Here it is important to make sure that neither the contact surface of the probe head nor the measuring object is uneven, as this can distort the measurement.

Tips for measuring surface temperature

• Hold the probe perpendicular to the surface so that its tip rests fully on it.
• Do not move the probe during the measurement.
• Apply a constant and sufficient downforce.
• Use low mass contact probes.
• Ideally, t99 is reached after about 3 seconds.

Ambient temperature measurement

To measure the temperature of the moving air, the measuring probe is simply held in the environment to be measured. Ideally, use a room sensor with a free sensor to obtain a short reaction time. The measurement result can be optimized by moving the probe in air at a speed of 2 m / s during the measurement.

Tips for measuring room temperature

• Use a room sensor with a free sensor (no penetration or contact sensor).
• Move the probe during the measurement at a speed of 2 m / s.
• Keep the probe away from your body.
• Use a radiation protected probe.
• Ideally, t99 is reached after about 7 seconds.