Spacial arrangement of thermometers
In general, electrical thermometers work independently of their position. The preferred mounting position is „vertically suspended” that means connection head on the top and protecting tube below.
Installation of thermometers
Usual installation methods are:
- screwed thread (cylindric and conical outer thread)
- swivel nut with inner thread
- adjustable flanges and counter flanges, only tight with small gas pressures
- welded flanges
- adjustable clamped screwings
- welded into tubes/ connection pieces, walls
- bayonet nipples, not liquid or gas tight
In case that the protecting tube must be welded, the neck pipe with connection head and gauge sildes have to be
dismounted. Before mounting again, it has to be assured that the bore of the protecting tube does not show any
grease or impurities.
Ceramic protecting tubes
Ceramic protecting tubes have to be protected against mechanical stress (stroke) and temperature shocks. If they are
mounted or exchanged with running processes, they have to be slowly pushed in.
Standard values are the following:
10-20 cm/min at + 1200°C
1-2 cm/min at + 1600°C
or the protecting tubes have to be adequately preheated.
Horizontal and unsupported lengths of more than 500 mm at temperatures of more than + 1200 °C have to be avoided as well as direct contact with flames
Positioning
In order to place the cable, the connection head with cable screwing can be turned. Therefore, the screwing of the neck pipe at the connection head respectively in the neck pipe is loosened, the connection head turned into the required direction and the screwing fastened again.
Operational conditions
It is of great importance for all mentioned mounting methods, that the connections with the process are tight, fix and secure according to the accepted technical rules and the local prescriptions. Furthermore, it has to be assured that the thermometers have a sufficient „surface for heat exchange” with the medium to be measured and that faults due to heat elimination by the protecting tube are kept small. This can be achieved for technical applications, if the following immersion lengths are chosen:
| for measurements in liquids | EL + 5x protecting tube diameter |
| for measurements in gases | EL + 10x protecting tube diameter |
(EL = effective length of the probe)
The generally applied measuring probes respectively resistance thermometers have an effective length of 5 up to 30 mm depending on the design and construction type.
The temperature sensitive length of thermocouples can generally be ignored. In pipelines with small diameters the required immersion length can only be achieved, if the protecting tube is mounted in the pipe bend against flow direction. In case of a too small immersion length, big faults can occur compared with the standard tolerances.
Attention: In general the fitting length is larger than the immersion length and only the latter is of great importance.
Surrounding conditions at connection head
The ambient temperatures shall range from -25 °C up to + 80 °C. If silicone gaskets are used, temperatures up to +150°C are possible. The stress on gaskets, cable insulation, on the material of the connection heads, etc. has to be considered! If electronic transmitters are mounted in the connection head, their temperature limits have to be taken into account! The admissible temperature of the weakest component part is important! The protecting type of the connection heads has to be chosen according to the expected ambient conditions.