en
cz

Correct placement of sensors in technologies

When reviving measurement and control systems, technicians very often encounter incorrectly installed sensors or their parts. Mounting faults are already very difficult to rectify at this stage, and if they are not rectified, an incorrectly installed sensor can have a major effect on the function of the control circuit. So let's recall a few basic rules for mounting sensors: these are simple principles, but not always followed - either out of ignorance or just because someone wanted to simplify their work.

In general

Try to follow the instructions of the manufacturers (usually included with the sensor or printed on the packaging) and the designers. If the designer's requirements conflict with local conditions, contact the designer or sensor supplier. Mount the sensors so that they do not protrude into the through points, so that they do not hang (except for ball thermometers in the room) and are protected from unintentional damage. In publicly accessible areas (outdoor sensor), protect them even from intentional damage, eg by positioning. Avoid the possibility of injury.

Before each installation, consider:

  • maximum and minimum ambient temperature (eg for heat exchangers and outdoor sensors)
  • ambient humidity, the effect of splashing water (cooling towers)
  • shocks (mounting on air handling units)
  • explosion protection (project control)

Allow for an active sensor length, especially for air speed sensors. Each sensor should have an appropriately marked inspection hole, closed with a plastic stopper.Allow for an active sensor length, especially for air speed sensors. Each sensor should have an appropriately marked inspection hole, closed with a plastic stopper.

Route the cable from below to prevent water from entering the terminal cover. At the bottom, provide it with a spare loop, which on the one hand allows the sensor to be removed from the flange or well without disconnecting the cable, and on the other hand prevents condensed moisture from flowing into the terminal box. When mounting the sensor on an insulated pipe or duct, do not compress the outer insulation - use a graduated mounting flange. If the flange does not have a suitable graduation, use spacer tubes.

When concealed installation in ceilings, under plasterboard, etc., clearly mark the installation location, if possible, and draw it in the documentation. You will save later work for yourself and others. In the immediate vicinity of the sensor, place a label with a description, which should include the function of the sensor and the position according to the drawings. Do not attach the label directly to the sensor, as replacing the sensor would damage the label.

Water temperature

The temperature sensor should be immersed in the medium for the entire active length. For pipes with cold and ice water, we must prevent condensate from condensing in the sump - in the area of insulation, extend the sumps with a plastic pipe and thoroughly seal the hole in the insulation (eg with waterproof adhesive tape).

Fig. 1

The sumps should be mounted against the direction of the medium flow (Fig. 1a). An example of an unsuitable assembly where the end of the sensor with the measuring element is in the direction of flow is shown in Fig. 1b. When mounting in bends, make sure that the sump does not touch the inner wall of the pipe (Fig. 1c). If the active sensor length is greater than the pipe diameter, mount the sump at an angle or in the bypass (Fig. 1d, 1e). In the case of installation in a bypass, the bypass pipe must extend up to the main pipe (Fig. 2a). Add a control sump in critical or unclear places, later draining of the system is difficult and almost impossible in winter, for example.

Fig. 2

Please note that the sensor will need to be removed during service. Observe the distance A (Fig. 2b) to the obstacle (even the future one): this can be a switchboard, door, frequency converter on the air handling unit, other piping, etc. If the sensor does not have a sump and there is a risk of leakage special label.Please note that the sensor will need to be removed during service. Observe the distance A (Fig. 2b) to the obstacle (even the future one): this can be a switchboard, door, frequency converter on the air handling unit, other piping, etc. If the sensor does not have a sump and there is a risk of leakage special label.

After mixing two streams of water at different temperatures, keep a sufficient distance from the mixing element to the sensor (Fig. 2c) - layers may form just behind the mixing element.

For cable and contact sensors, their function does not depend on orientation, but it is necessary to ensure thorough contact between the sensor and the pipe: for contact sensors, clean the pipe contact surface with a file and fill the space between the pipe and sensor with thermally conductive paste to improve heat transfer. The paste is supplied (usually at an additional cost) by the sensor supplier.

Air temperature

Tubular sensors, although usually having a measuring element at the end, should be surrounded by air along their entire length. Where you expect to form layers (eg behind large mixing flaps, heaters, coolers or recuperators), do not use tubular sensors, but sensors of average value.

Fig. 3

For cartridge sensors with a capillary, install the terminal block higher than the capillary to prevent condensed moisture from draining into the terminal block. Mount the cartridge tilted downwards (Fig. 3a). Do not bend the capillary too sharply, the bending radius should be at least 50 mm. Always provide the passage of the capillary through the walls and walls with a passage pipe and insulation, use a rubber sealing bushing when passing the capillary through the plate of the ventilation duct. Roll up the unused section of the capillary neatly (spiral diameter approx. 10 cm, Fig. 3b).

Sensors for measuring the average value must have their entire length in the ventilation duct, distribute the active length of the sensor evenly over the entire cross-section of the pipe. For units with a water scrubber, install the sensor downstream of the drip trap (Fig. 3c). Use the supplied fasteners for installation. A separate article will be devoted to frost protection.

Room temperature

The ideal location of the room temperature sensors is 1.5 m in the area of residence of persons, at min. distance of 50 cm from the nearest next wall (Fig. 4a). Avoid mounting in sunny places! (Pay attention to the movement of the sun during the day and the low angle of illumination in the winter months.) In the case of solid walls (steel, stone, concrete), use a thermal insulation pad (Fig. 4b).

Fig. 4

Do not mount the sensor in the room on an external wall, do not place it in alcoves or niches to prevent air circulation. Beware of curtains, draperies, etc. Avoid mounting near radiators, lamps and refrigerators! Do not mount sensors on the wall through which the chimney passes.Do not mount the sensor in the room on an external wall, do not place it in alcoves or niches to prevent air circulation. Beware of curtains, draperies, etc. Avoid mounting near radiators, lamps and refrigerators! Do not mount sensors on the wall through which the chimney passes.

Always properly seal the installation pipes or cable ducts in the plasterboard with silicone sealant! Cold air from the ceilings can cool the sensor (Fig. 4c). This phenomenon occurs especially in hotel rooms and offices (long corridors), where it can lead to the erroneous conclusion that a whole series of sensors measures incorrectly.

Outside temperature

The installation location should be determined in the project with respect to the sides of the world. As a change of location in this case means an intervention in the façade, consult the measurement and control designer about any changes. Do not expose the sensor to direct sunlight, do not mount it on parts of facades with a large heat capacity or where the wall surface can be heated by sunlight.

Do not mount sensors on the wall through which the chimney passes. Avoid mounting under roof overhangs - although the sensor is shielded here, a pocket of hot air can be kept under the overhang (Fig. 4d). Under no circumstances place the sensors above the windows! Also pay attention to the exhaust air from the ventilation system, especially on roofs, where the rotating wind can carry the exhaust air to unexpected places and thus significantly affect the measurement of the outdoor temperature.

Try to protect the sensor from overcoating with facade paint. When designing the sensor, the material and color of the housing and thus its thermal properties are usually taken into account. Mount the sensor so that it is accessible to service personnel (accessible via a ladder or from assembly and technological routes).

Humidity

Humidity measurement is affected by the air flow speed, the air speed at the measuring point should not exceed 10 m / s. This problem can be solved, for example, by shielding the sensor with a perforated plate. Do not place the sensor where air does not flow. Air supersaturation occurs in these zones (Fig. 4e).

When mounted in vacuum ducts, air can flow to the sensor from the outside through its cover or mounting hole, which affects the measurement. Check the tightness of these places.

When humidifying the air with a humidifier, the air needs a certain path in order to be able to receive the required humidity - the measuring section behind the humidifier. This distance between the humidifier and the humidity sensor depends on the change in humidity and the air flow rate in the duct. The distance is determined by the designer; for example, for Dx = 4.5 g / kg and the air velocity in the duct of 3.5 m / s is 6.4 m.

Similar rules apply to room sensors as to temperature measurement.

Pressure

Some pressure sensors are only intended for mounting in the specified position. Read the installation instructions! If possible, equip the water manometer leads with T-pieces for control measurements. To prevent one-sided loading of the sensor during handling, it is recommended to provide the inlet with a lockable bypass (Fig. 5a).

Fig. 5

If there is a risk of condensation, run the inlets at a gradient of 1:30 and provide them with drain cocks at the lowest point (Fig. 5b). Protect the pipes from frost! Do not loop it. Do not run test lines in which air flows outdoors or in cold rooms or ducts. There is a risk of condensate freezing.If there is a risk of condensation, run the inlets at a gradient of 1:30 and provide them with drain cocks at the lowest point (Fig. 5b). Protect the pipes from frost! Do not loop it. Do not run test lines in which air flows outdoors or in cold rooms or ducts. There is a risk of condensate freezing.

The point where the measuring probe is connected must be at a point in the pipe where there is no turbulence. Before this point there should be a protective length of at least 6 pipe diameters (or diagonals in the case of pipes with a quadrangular cross-section), after this point the distance is at least three times the diameter or diagonal (marked Dg in Fig. 5c).

Static pressures are measured using socket probes, plastic bushings are used only for the passage of the tube through the channel wall. Orient the probe parallel to the flow, regardless of whether it is oriented in the direction of flow (Fig. 5d) or against it.

For hoses up to one meter in length, an inner diameter of 4 mm is sufficient. For larger distances between the sampling point and the pressure sensor, use a hose with a larger diameter, eg for 7 m diameter 15 mm, for 10 m diameter 20 mm. The pressure point must not be affected by pipe irregularities (Fig. 5e).

Fig. 6

When measuring the pressure in liquids, a hole with a maximum diameter of 5 mm, free of chips, should serve as a measuring point (Fig. 6a, b). The inside of the hole should be smooth. For sensors connected via a tube (especially differential pressure sensors) on the tube, create a damping loop (approx. 2 threads with a diameter of 15 cm) to prevent vibration transmission. Orient the loop horizontally to prevent condensation from accumulating in the threads.

For liquids, place the sensor lower than the measuring point; for gases and vapors, the sensor should be higher than the measuring point (any condensate flows back into the pipe and not to the sensor). The welds should not be on the top of the pipe, where air bubbles may be present, or on the bottom of the pipe, where dirt may settle. The optimal place is the lower part of the side (Fig. 6c, d). For condensing gases, on the other hand, the weld should be on top to prevent condensate from accumulating (Fig. 6e).

Other quantities

Install wind sensors on facades exposed to the main wind directions or on the roof. Respect accessibility for inspection and service. Do not place the sensors under roof overhangs and in alcoves.

For sun sensors, pay attention to the possibility of shading by surrounding objects or trees (when installed in winter, the trees are leafless).

In conclusion

These rules may in some cases act unnecessarily strict, sometimes it is not possible to meet some of them due to local conditions. The basic guide for the correct installation of not only sensors remains common sense, technical feeling and respect for the work of other suppliers.