Dial thermometers

What is an electronic contact (model 830.E)?

Electronic contacts are fitted with non-contact slot sensors. They are especially suitable for oil-filled gauges and should be preferred for low voltages and small DC loads, such as for the signal input for a PLC (programmable logic controller).

What are mechatronic measuring instruments?

Where electronic components or assemblies are integrated into mechanical measuring instruments. Thus, there is a purely mechanical on-site display giving either an additional electrical output signal or offering a switch function. The advantage of instruments like this is that, should the power supply or the measuring signal be disrupted or interrupted, the measured value can nevertheless still be read reliably on site.

What does NAMUR stand for?

NAMUR is the abbreviation for "Normenausschuss Mess- und Regeltechnik" (Standards committee for measurement and control).

What function does the Hall sensor provide in intelliGAUGE/intelliTHERM instruments?

The magnetic field that affects the Hall sensor comes from a moving permanent magnet, that is arranged at a fixed distance from the Hall sensor. Thus the angle of rotation of the permanent magnet in relation to the Hall sensor can be measured. In intelliGAUGE/intelliTHERM instruments a permanent magnet is fixed on the pointer, central to the pointer shaft. When the pointer turns, the magnet turns with it. Thus the angle of the field lines, which run between the two poles of the magnet, changes relative to the Hall sensor. Since for each angle of the field lines to the Hall sensor there is a different field strength, the Hall sensor generates a Hall voltage that is proportional to the angle of rotation of the pointer and thus proportional to the pressure.

What is the Hall effect?

If a constant current is running through a semiconductor component, and this component is then placed in a magnetic field, then a voltage (Hall voltage) is generated within this semiconductor component which is proportional to the strength of the magnetic field.

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.

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.

Can mechanical dial thermometers with a CE marking be placed on the market?

With CE marking, the manufacturer, the company or person placing the product on the market or the EU authorised representative declares that the product complies with all relevant European guidelines.
Since there is no binding instruction (directive) for mechanical thermometers in accordance with DIN EN 13190, such instruments can also be placed on the market without a CE marking.

Do gas-actuated thermometers fall within the Pressure Equipment Directive and may an appropriate CE marking be applied?

 

Gas-actuated thermometers fall within the scope of the 2014/68/EU directive (previously PED 97/23/EC), without additional adapted thermowell as “pressure accessories” and also in the non-fitted state or fitted within a thermowell as “pressure vessels”. The classification is based on Article 4, Paragraph 3 of the applicable 2014/68/EU directive (Design, manufacturing and testing carried out in accordance with sound engineering practise). The products should not be marked with CE or their conformity confirmed in respect of the above-mentioned directive. A manufacturer's declaration can be requested if required.


Does the ambient temperature have any influence on the measuring tube within the case of the gas-actuated instrument?

Yes, that is why a bimetal compensation is installed between the movement and the measuring tube. This compensation is technically limited within a specific range. For WIKA gas-actuated instruments the standard ambient temperature is approx. 23 ± 10 °C. Deviating ambient temperatures are possible, but have to be specified.

How can one prevent the zero position of the thermometer which has already been set during the manufacturing process, from altering (drifting) in later use?

Such a drift can be anticipated through suitable heat treatment (ageing). The finished bimetallic springs, ready for installation, should be held at a temperature of 350°C to 400°C (if required, higher temperatures are also possible), but stabilised below their application limit and subsequently slowly cooled.

How does a bimetal thermometer work?

A strip made from permanently-laminated rolled sheets, made from metals having different coefficients of expansion ("bimetal"), will bend as a result of any temperature changes. The bending is roughly proportional to the change in temperature. For bimetallic strips, two different measurement systems have been developed: helically wound or spirally wound. Through the mechanical deformation of the bimetal spring in either of these spring forms, on any change in temperature a rotational movement occurs. If one end of the bimetal measuring system is fixed securely, the other will rotate a pointer shaft. The scale ranges are between -70 and +600 °C with an accuracy class of 1 and 2 in accordance with EN 13190.

How does a gas actuated thermometer operate?

The measuring system of the dial thermometer consists of a stem, capillary and Bourdon tube in the casing. These parts are combined to form a single unit. The entire measuring system is filled with an inert gas under pressure. A change in temperature will cause a change in internal pressure in the stem. The pressure deforms the measuring spring and the deflection is transferred to the pointer via a dial movement. Fluctuations in ambient temperature affecting the casing can be neglected, as a bimetal compensation element is fitted between the dial movement and the measuring spring. The scale ranges are between -200 and +700 °C with an accuracy class 1 in accordance with EN 13190.

How does an expansion thermometer work?

The measured value registration is made through the liquid-filled measuring system which consists of a temperature probe, capillary and Bourdon tube. All three components combine to form a closed tube system. The internal pressure in the system changes with the adjacent temperature. This causes the pointer axis connected to the spring to turn and the temperature value to be displayed on the scale. The capillary, with lengths between 500 and 10,000 mm also enables measurements to be made on remote measuring points. The scale ranges are between -40 and +400 °C with accuracy classes 1 and 2 in accordance with EN 13190.

How long does it take for an expansion thermometer to display the true temperature of the medium?

Rule of thumb: after 90 sec adjustment time, approx. 99% of the measured value has been achieved.

Is an initial verification possible for mechanical thermometers?

Since 01 January 2015, the new Measurement and Verification Act has been in force. This no longer permits any initial verification for dial thermometers. If an initial verification is required to place the measuring instrument on the market for the first time, this requirement will be met through a conformity assessment procedure. The conformity assessment of a measuring instrument basically replaces the former initial verification.

In place of the appropriate office of weights and measures, a conformity assessment body, selected by the manufacturer, (which can still be an office of weights and measures) confirms the conformity of a measuring instrument with the valid legal requirements through issuing a certificate of conformity. On the basis of this, the manufacturer of the measuring instrument prepares a written declaration of conformity for the measuring instrument. Once the validity expires - provided the conditions are met - as is normal, a (re)verification is required so that the measuring instrument can once more be used in commercial and/or official operation.

Both the conformity certificate and also the verification certificate carry the same weight - in accordance with the guidelines of the New Approach 2004/22/EC and 2009/23/EC and also in accordance with the Measurement and Verification Act (MessEG) in conjunction with the Measurement and Verification Ordinance (MessEV) supported by it.


To which standard are gas actuated thermometers and bimetal thermometers manufactured?

Gas actuated thermometers and bimetal thermometers are manufactured to EN 13190. If electrical connectors are built in, DIN 16196 applies.

What does one mean by the active length of a thermometer?

The active length of a thermometer is the length over which the thermometer effectively averages the temperature, through in-and outflows of the heat.

What influence does the ambient temperature have on the measuring result?

This depends on the following parameters:
1. Ratio of the volume of the probe (tube) to the measuring line and Bourdon tube (rule-of-thumb: 99:1)
2. Length of the measuring line (capillary) - the longer it is, the more the influence
3. Magnitude of the ambient temperature (value), high temperatures cause an offset in the display, low temperatures cause a reduction
Possibilities for compensation:
1. via a bimetal compensation spring (opposed to the pointer direction)
2. via error adjustment (only possible if the ambient temperature is known and constant)

What is the maximum length of a capillary for gas actuated thermometers?

Theoretically, the capillary of a gas-actuated thermometer can be up to 100 m long. However, a large probe volume is needed so that the gas-actuated thermometer will operate within class 1. With the expansion thermometer, the maximum length is limited to 15 metres, otherwise the required filling volume would be too great.

What mechanical influences, other than the actuation of switch contacts, can cause measuring errors in bimetal thermometers?

In helical bimetal spring designs, a stroke movement of the pointer may occur, which may lead to the pointer touching on the dial or the window.  With the assistance of modern design and manufacturing techniques, such errors are avoided nowadays.

When does one use a gas actuated thermometer with capillary?

Gas-actuated or expansion thermometers with capillaries are used in locations which are not easily accessible and where long distances have to be bridged. As a protective coating for capillaries, a flexible spiral protective sleeve or PVC coating is available.

Which gas is used as the fill fluid for gas actuated thermometers?

Helium.

Why can't the shaft of the bimetal thermometers be manufactured longer than 1m?

Since the weight of the pointer shaft would be greater than the turning bimetal coil (i.e. the pointer would no longer be able to move).

Why is liquid damping an advantage with high vibration?

Since the pointer is damped and the temperature can be read better.

Why is liquid damping an advantage with negative ambient temperatures?

Since with unfilled instruments, there can be a possible build-up of condensing water, the window can become steamed up. This is not possible in filled instruments.

Why is liquid damping in bimetal thermometers only possible up to medium temperatures of 250°C?

Since the filling liquid (silicone oil) in the complete thermometer (i.e. it is also present in the stem) this means the filling liquid is heated to the temperature of the medium.  This can lead to fire in the silicone oil.

Why is liquid damping in gas actuated thermometers then possible with medium temperatures over 250 °C?

Since only the housing is filled, and thus the medium temperature cannot be transferred to the fill fluid, all temperature ranges from -200 °C ... 700 °C are possible.

Why is the bending on temperature rises not linear over the entire temperature range?

Since the specific thermal expansion coefficient of the bimetallic components is temperature dependent.

Why must the clearance between the thermowell stem wall and the bimetal coil be as low as possible?

Since the heat transfer from the wetted thermowell stem to the bimetallic spring is improved.

What are mechatronic measuring instruments?

Where electronic components or assemblies are integrated into mechanical measuring instruments. Thus, there is a purely mechanical on-site display giving either an additional electrical output signal or offering a switch function. The advantage of instruments like this is that, should the power supply or the measuring signal be disrupted or interrupted, the measured value can nevertheless still be read reliably on site.

What does the abbreviation TGT stand for?

Model TGT (Temperature Gauge Transmitter) instruments are mechatronic temperature measuring instruments which display the temperature without needing external power, and simultaneously generate an electrical output signal.

What function does the Hall sensor provide in intelliTHERM instruments?

The magnetic field that affects the Hall sensor comes from a moving permanent magnet, that is arranged at a fixed distance from the Hall sensor. Thus the angle of rotation of the permanent magnet in relation to the Hall sensor can be measured. In intelliTHERM instruments, a permanent magnet is fixed on the pointer, central to the pointer shaft. When the pointer turns, the magnet turns with it. Thus the angle of the field lines, which run between the two poles of the magnet, changes relative to the Hall sensor. Since for each angle of the field lines to the Hall sensor there is a different field strength, the Hall sensor generates a Hall voltage that is proportional to the angle of rotation of the pointer and thus proportional to the temperature.

What is the Hall effect?

If a constant current is running through a semiconductor component, and this component is then placed in a magnetic field, then a voltage (Hall voltage) is generated within this semiconductor component which is proportional to the strength of the magnetic field.

Are there any GOST certificates for thermowells?

No. GOST certificates only exist for measuring instruments and a thermowell is only considered a component part of a thermometer.

Do thermowells need to be CE marked?

Thermowells must not be CE marked, in principle. An exception as a result of its special design is the model TW61 thermowell with DN>25, suitable for orbital welding. This must be CE marked in accordance with the Pressure Equipment Directive (2014/68/EU).

How high is the permissible pressure loading for thermowells?

In the Appendix to DIN 43772 are loading diagrams, from which, depending on temperature and medium, can be taken, the maximum allowable pressure load for the different geometries. If the conduit geometry does not correspond to DIN 43772, individual calculations can be performed in accordance with ASME PTC 19.3 or Dittrich / Klotter, which as static results include the max. pressure loading.

What are suitable materials for thermowells for negative temperatures?

The first choice for high-temperature applications should always be stainless steel, such as 1.4404 or 316L. (Approval per AD2000 W10 down to -270 °C) Carbon steels should be considered carefully in detail, through the effect of the drop-off effect.

What are the factors influencing the response times of thermowells?

Put simply, one can say that the more stable a thermowell is constructed, the slower it reacts to temperature changes. In order to optimise the response time, there are thin wall thicknesses and low air space between sensor and the bore's interior walls. Further optimisations in design are pocket-drilled bottoms and effective insertion lengths of more than 100 mm.

What is the difference between one-piece and fabricated thermowells?

Fabricated thermowells are manufactured from tubes which are sealed by a solid welded tip (for example) at the process. Single piece thermowells are manufactured from a complete element of bar stock (round or hexagonal).

What is the maximum insertion length for a thermowell?

For multi-piece thermowells, the maximum length is limited by the manufactured lengths of the tubes, which is about 5-6 meters.  One piece thermowells are made of solid material and limited by the production length of the drill hole, which, for each product is between 1,000 mm and 2,000 mm. . Longer one-piece thermowells have to be made by welding individual elements together.

What is the maximum permissible temperature for thermowells?

The maximum temperature depends on the materials used and the standards which must be met.  So, for example, a standard stainless steel can be used in air up to about +900 °C, the maximum operating temperature is approximately +600 °C and an approval can be made up to +450 °C.

What is the minimum insertion length of a thermowell?

The insertion length of a thermowell will be specified through the thermometer  used. In general one can assume a length of  60-100 mm for mechanical thermometers from a minimum total length. Electrical thermometers need an insertion length of at least 35 - 50 mm. Each individual case should be checked, though.

What tests and inspections are stipulated for thermowells?

In accordance with DIN 43772 Point 4.6, all tests and certifications should be agreed between the manufacturer and operator.

When are one-piece or fabricated thermowells typically used?

Fabricated thermowells are generally recommended for low to medium process loads. One-piece  thermowells are suited to the highest process loads, depending on their design. Thus internationally or in the petrochemical industry, one-piece thermocouples are now used almost exclusively.