Electrical temperature measurement
Resistance thermometers and thermocouples
Temperature transmitters
Widerstandsthermometer und Thermoelemente
How does a resistance thermometer work?
The electrical resistance of a resistance thermometer's sensor changes with the temperature. As the resistance of measuring resistors to EN 60751 (2009-05) increases with rising temperature, we refer to it as PTC (Positive Temperature Coefficient). Pt100 or Pt1000 measuring resistors are normally used for industrial applications. The thermometers based around EN 60751 are defined in DIN 43735.
What does "Intercrystalline corrosion" mean?
IC ("Intercrystalline Corrosion") is a form of corrosion that can occur in most alloys at the appropriate conditions. It is also known as "grain disintegration" or "chromium depletion" The corrosion takes place along the grain boundaries. In steels alloyed with chromium, the chromium contained in the material combines on heating (often while welding) with the carbon to form chromium carbide. Thus the chromium is no longer available for corrosion protection (formation of a passive layer) in the heated area. This occurs particularly in high-carbon steels. With corrosion-resistant steels, such as 1.4571 (AISI 316Ti), the binding of carbon with titanium or niobium to niobium or titanium carbide (stabilised steels) or lowering the carbon content, e.g. 1.4404 (AISI 316L) acts against IC. These measures prevent the harmful reduction of chromium content along the grain boundaries.
What do the Zones in explosion protection mean?
Gases:
Zone 0 (Category 1): permanent or long-term danger of explosions
Zone 1 (Category 2): hazardous atmosphere occasionally exists
Zone 2 (Category 3): explosive atmosphere exists only rarely, and then only for a short time
Dusts:
Zones 20, 21, 22 with the same meanings
What do the designations of Temperature Class mean?
The ignition temperature is the lowest temperature at which an inflammable mixture of gases can ignite at a flame, a hot surface or otherwise generated spark. Gases and vapours are divided into Classes in which the temperature of the surface must always be lower than that of the mixture. (T1 > 450°C, T2 > 300°C, T3 > 200°C, T4 > 135°C, T5 > 100°C, T6 > 85°C).
What does "Pt100" mean?
Pt stands for Platinum with a nominal resistance of 100 Ohm at 0 °C (EN 60751).
What does "negative temperature coefficient thermistor" mean?
Negative temperature coefficient thermistors conduct electricity better at higher temperatures than at lower temperatures. They are also known as NTC resistances (Negative Temperature Coefficient). Typically, NTC is used in the plastics and food and beverage industries.
What does "positive temperature coefficient thermistor" mean?
Positive temperature coefficient thermistors conduct electricity worse at higher temperatures than at lower temperatures. They are also known as PTC resistances (Positive Temperature Coefficient). Typically PTC are used in high-value temperature measuring points, e.g. in the chemical industry.
What are 2-, 3- and 4-wire circuits?
They describe the number of wires with which the measuring resistor (e.g. a PT100) is connected. While with the simplest 2-wire connection, the lead resistance can falsify the measuring result, this negative influence can be compensated within the 3- or 4-wire connection, and thus the accuracy of the measurement improved.
What are mineral-insulated (MI) cables?
Mineral-insulated cables for resistance thermometers consist of one or more copper wires that are embedded in highly-compacted magnesium oxide and sheathed in casing tube made from, for example, 1.4571 stainless steel. For thermocouples, instead of copper wires, thermocouple cables suitable for the thermocouple type are used. The most common standard sheath material for thermocouples is Inconel 2.4816.
What is the permissible minimum bending radius for an MI cable?
VDI/VDE 3511 Sheet 2 recommends a radius of curvature R of >= 5 x D (D=external diameter of the MI cable), some manufacturers of MI cable even give >= 3 x D as the minimum bending radius.
How thick is the wall thickness of an MI cable?
Most manufacturers give a minimum wall thickness which corresponds to 10% of the external diameter of the MI cable.
How large is the measuring error caused by the internal lead resistance with a PT100 built into an MI cable with Cu internal wires in a 2-wire connection?
D=3 mm : 0.28 Ohm/m = 0.7 K/m (measuring error)
D=6 mm : 0.1 Ohm/m = 0.25 K/m (measuring error)
(D=external diameter of the MI cable)
Why should PT100 measuring circuits with reduced tolerance class A or AA per DIN EN 60751 be used in at least a 3- or 4-wire connection?
The 2-wire connection is not permissible for classes A and AA per DIN EN 60751 since here the internal lead resistance of the wires is added to the measured value here. This will usually exceed the specified tolerance for the thermometer. A measurement of the cable resistance at room temperature and adjusting this in the transmitter (for example) is possible, but the temperature-dependent resistance of the inner conductor of the cable would still be added to the reading as an error. Conclusion: A 2-wire circuit is not suitable for accurate temperature measurement.
How high is the permissible vibration loading fo WIKA PT100 probes?
The standard WIKA measuring insert allows use up to 3g (amplitude). This corresponds to a loading of 6g, peak to peak, per DIN EN 60751 (58.86 m/s^2). In EN 60751 only 20-30 m/s^2 peak-to-peak is specified (1 g = 9.81 m/s^2). The vibration-resistant design is suitable for up to 20g peak-to-peak. Special designs, on request, up to 50g are possible.
(The values given above always apply to the vibration load directly at the measuring resistor)
What are the Callendar - van Dusen coefficients and how do I calculate these?
The Callendar - van Dusen coefficients are used to describe a polynomial function of the actual characteristic of a platinum measuring resistor. This can be stored in a transmitter and thus increases the accuracy of the entire measuring chain. To calculate the Callendar - van Dusen equation in the temperature range over 0 ° C, the resistance at 0 ° C and two other test temperatures are collected by comparative measurements. Hence, the a and b constants are calculated. For the negative temperature range, the inclusion of a measured value for another test temperature is needed in order to determine the d constant. One can, however, represent the characteristic curve of the platinum measuring resistor just as well mathematically using the polynomial equation per DIN EN 60751 with the constants A, B and C (see also WIKA data sheet IN 00.17, page 4) and also determine these by calculation from the measurement of 3 (or 4 at t <0 ° C) test temperatures. Similarly, one can convert the constants A, B, C into the Callendar - van Dusen constants.
What minimum insertion lengths are recommended, as a rough guide, for multipart thermowells (from tubing) in order to minimise the heat dissipation error?
for gaseous media: 15...20 x thermowell tip diameter
for liquid media: 5...10 x thermowell tip diameter
for solid media: 3...5 x thermowell tip diameter
(these standard values are only valid for static mediaThe gap between the thermowell and measuring insert should be < 0.5mm)
What is green rot?
Thermocouples are subject to ageing and change their temperature/thermal voltage characteristic.
In Type K thermocouples high temperatures can result in substantial changes to the thermal voltage due to chrome depletion in the NiCr leg, leading to a lower thermal voltage.
This effect is accelerated if there is a shortage of oxygen, since a complete oxide layer, which would protect it from further oxidation, cannot be formed on the surface of the thermocouple. Chromium is oxidised, while nickel isn't. This results in the so-called "green rot", destroying the thermocouple.When NiCr-Ni thermocouples that have been operating above 700 °C are cooled quickly, this cooling causes certain states in the crystalline structure (short-range order) to freeze, which in Type K thermocouples can result in a change of the thermal voltage of up to 0.8 mV (K effect).
What is thermal voltage (or the Seebeck effect)?
The effect, named after Thomas Johann Seebeck, describes the fact that an electric voltage exists when two different metallic conductors are connected at two different points, if there is a temperature difference between the connected and the open end of the "thermocouple".
Can I replace U and L thermocouples per DIN 43710 with Type T and J thermocouples per DIN IEC 60584?
No. Type T and J themocouples have a different thermal voltage characteristic, which would lead to a measuring error. Type U and Type L thermocouples should only be delivered as replacement parts in old installations. With the construction of new plant, these are no longer permitted.
What measures should be taken with the installation of high-temperature probes with ceramic thermowells?
Installation and removal should be carried out slowly and gradually, in order to prevent the ceramic thermowell being destroyed by internal thermal stresses. This must be either preheated or slowly inserted, e.g. 1 ... 2 cm / min for temperatures to 1600 ° C and 10 .. 20 cm / min at 1200 ° C.
What effect does poor insulation resistance have?
In accordance with DIN EN 60751 Section 6.3.1 the insulation resistance between each measuring circuit and the sheath, at a minimum test voltage of DC 100 V, must not be less than 100 MΩ. Should the insulation resistance be too low, a measuring error occurs that causes the display of too low a temperature. In relation to a resistance thermometer (with sheathed cable) this results, with an insulation resistance of 100 kΩ, in a display error up to 0.25 Ω and at 25 kΩ up to 1 Ω. On all WIKA resistance thermometers, an insulation test with DC 500 V and an insulation resistance of > 1000 MΩ is carried out. i.e. We test to a factor of 50 better than specified by the standard.
What are the response times of the various measuring inserts?
The measurement of the response time is carried out in flowing water in accordance with DIN EN 60751 Section 4.3.3 and VDI / VDE 3522.
| v=0,4m/s |
T start = approx. 20°C |
T end = approx. 30°C
|
| 6mm 1xTyp K, isolated: |
T50= 4,0sec. |
T90=11,0sec. |
| 6mm 1xTyp K, non-isolated: |
T50= 1,0sec. |
T90=2,7sec. |
| 6mm 1xPt100, thin-film sensor: |
T50= 8,5sec. |
T90=20,5sec. |
| 6mm 1xPt100, ceramic sensor: |
T50= 7,0sec. |
T90=19,0sec. |
Can electrical thermometers be calibrated?
It is not possible to calibrate resistance thermometers (e.g. measuring inserts).
Since electrical thermometers are normally connected to a measuring instrument or evaluation unit, it is only possible to calibrate the entire measuring chain.
Measuring inserts can, however, be subjected to a design test with a design test certificate.
Application area: e.g. resistance thermometers for mineral oil meters.
How is the accuracy class calculated?
Per DIN EN 60751 Point 5.1.3 Table 3 in °C
| Klasse AA |
± |
(0,1+0,0017 * t) |
| Klasse A |
± |
(0,15+0,002 * t) |
| Klasse B |
± |
(0,3+0,005 * t) |
| Klasse C |
± |
(0,6+0,01 * t) |
Why has there been, for some time, a separation between the accuracy classes for "wire-wound resistance" and "film resistance" Pt100 measuring resistors?
In the past, no distinction had been made between the two basic types of measuring resistor and their temperature limits. Practice, however, showed that film resistors (thin-film/chipset resistors) have a (not insignificant) deviation form the characteristic. This behaviour has been accommodated in DIN EN 60751:2009-5 through the splitting of the temperature ranges within the individual accuracy classes.
What does the designation "1/3 DIN" mean with resistance thermometers?
IMPORTANT: The terms 1/3 DIN, and also 1/5 DIN and 1/10 DIN, have NOT been STANDARDISED!
By May 2009, with the introduction of the new DIN EN 60751, there was no standardised accuracy class better than Class A. Some manufacturers of resistance thermometers (including WIKA) have used these terms in order to supply customers with thermometers with a higher accuracy than Class A.What initially presented itself as a useful addition to traditional standard designation has proved to be, on closer inspection, woefully inadequate.
The typical question "1/3 DIN from what?" can be answered by the phrase "from Class B". Unfortunately defining "1/3 DIN B" makes the situation even less clear.
There are actually two ways of looking at this additional definition "from Class B".
1.) One fixes the increased accuracy to a specific temperature point: 1/3 DIN B at 0 °C.
2.) One defines a range in which this accuracy is valid: 1/3 DIN B 0 ... 50 °C.
The representation described in 2) carries an additional uncertainty. If one uses a Class B measuring resistance, so its characteristic curve has a defined pitch. In the example of 0 .. 50 ° C, a Class A measuring resistor would already deliver, at about 20 °C, a better result than 1/3 DIN B. Result: one must use a Class A measuring resistor here.. All of this "nebulosity" has ultimately led to the introduction of a new accuracy class. Since May 2009 the Class AA has been included in DIN EN 60751, which - now that it is standardised - makes the 1/3 DIN description superfluous.
Temperature transmitters
What happens with a high ambient temperature > 85 °C?
High TC error and damage to the electronic components.
What is the difference between analogue and digital transmitters?
At WIKA, analogue transmitters have analogue electronics fitted but no processors.
Digital transmitters have processors. The output signal can, however, be a 4 ... 20 mA analogue signal for both types.
What is galvanic isolation?
An isolation of the signal between the signal side and the output side (e.g. through coils).
What should be considered when measuring temperature with thermocouples?
For temperature measurement using thermocouples, a reference value is always needed and/or a compensation must be made. Thus, there is a sensor built into the digital transmitter (Pt100) which measures the terminal temperature (ambient temperature). (cold junction, CJC, cold junction compensation).
Thermocouple connection is always 2-wire. (Caution: do not swap over the plus and minus!)
No sensor short circuit monitoring is available (only sensor break monitoring)
Transmitters with galvanic isolation are strongly recommended (WIKA Models T12, T32, T53).
What should be considered when measuring level with transmitters (potentiometers)?
Pure percent measurement (ratio measurement)
T32: amplifier adjusted to input 10 kΩ to 100 kΩ
Smaller values are measurable, but with less accuracy.
What is "signalling"?
The signalling shows possible error conditions (e.g. sensor burn-out).
What are the signalling values in accordance with NAMUR?
NAMUR Downscale: 3.5 mA
NAMUR Upscale: 21.5 mA
What do "output limits" mean?
Up to these signal limits, a measuring signal is generated.
What is the value of the current at the output limits in accordance with NAMUR?
Lower output limit in accordance with NAMUR is 3.8 mA (other value possible)
Upper output limit in accordance with NAMUR is 20.5 mA (other value possible)
What does "SIL" mean?
SIL (Safety Integrity Level) refers to the standards IEC61508 / IEC61511 and describes instruments, which are used for safety-critical applications. In the event of an emergency, they must react safely and place the control circuit into a safe state.
On these grounds, all components in the control circuit (sensor, logic, actuator) must have a SIL classification.
The control circuit (safety loop), through special parameters (SFF, PFD, FIT), can have its Failure Probability evaluated.
What are the special features of the T32 SIL transmitter?
It has had a FMEDA analysis carried out on it. The T32 has been developed in accordance with IEC 61508 and has been manufactured accordingly. The T32 operates exclusively with active write protection.
What are Profibus / FOUNDATION Fieldbus?
Profibus / FOUNDATION fieldbus are digital output signal protocols.