A sound approach to connecting thermocouple probes in machinery
Careful decisions must be made on how to connect the remotely located (machine) thermocouple sensor to the host instrumentation in order to optimize accuracy and minimize cost. Remember that the resultant temperature measurement is only as accurate as the sensor and its interface.
Since a thermocouple produces only a small thermo-electric e.m.f (typically 40µV/°C), great care is needed when terminating the device. At first sight, such termination would seem to be very straightforward, after all there are only 2 colour coded tails to consider; however, care is needed to eliminate or minimize spurious thermal emfs and Ohmic effects which would otherwise result in incorrect readings.
The length of cable used between the sensor and the instrument is an important consideration because the line resistance has to be taken into account (although the instrument input is effectively potentiometric, the input usually incorporates “upscale burnout” circuitry). Most instruments can accept a maximum of 100 Ohm loop cable resistance without accuracy being compromised; this, for example corresponds to only 22m of type K, 7/0.2mm core extension cable.
Extension cable uses true thermocouple wire and is designated X (e.g. KX for type K); compensating cable has a C designation (e.g. KC for Vx, type K) and consists of Vx and U types. Extension cable has a temperature vs e.m.f relationship to an appropriate standard over the complete temperature range. It can, therefore, be used for producing a thermocouple junction and for joining thermocouples to their measuring instruments. It is limited in temperature only by the rating of its insulation.
Compensating cable is of different composition to extension cable (it uses alternative, cheaper alloys) but has a similar temperature vs e.m.f. relationship over a limited range. It can only be used in limited ambient temperature, generally not higher than 80°C. Also, compensating cable is less accurate than extension cable in terms of thermal emf tolerance values (typically double).
Screened or Unscreened?
With long cable runs, the cable may need to be screened and earthed at one end (at the instrument) to minimize noise pick-up (interference) on the measuring circuit. The most commonly used methods of connecting the thermocouple to the instrument are:
i) Direct connection using either compensating or extension cable
ii) Direct connection using cable but via compensating plugs and sockets to facilitate sensor replacement or extended wiring
iii) A temperature transmitter, usually located in the probe terminal head
Direct Connection is made using an appropriate type of cable; this is indicated by colour coding according to IEC 584-3 on the insulation. The type of insulation specified would ideally be suited to the working environment. Correct polarity and mechanically sound connection is vital.
Direct connection using connectors to facilitate interchangeability of probes or the addition of extended cabling requires the use of special plugs and sockets which utilize thermocouple alloys rather than alternative metals. Such connectors are colour coded in a similar way to cable insulations to indicate the relevant thermocouple type and should comply with IEC & CENELEC specifications. Connectors are available as “standard” size (round pins) or miniature size (flat pins), which are polarised to ensure correct orientation.
The temperature transmitter converts the small thermocouple signal into an amplified current for onward transmission to the associated instrumentation. Although the use of such a transmitter will be rather more expensive than using short cable runs, it will be cost effective compared with long cable runs, especially if the required cable is of an expensive type (e.g. 23/0.2mm PTFE insulated & screened). The scaling (range) can be determined by the transmitter thus permitting the use of process instrumentation with a standard 4-20mA, 2 wire, interface.
The advantages of using a transmitter over direct connection are:
a) Low cost copper wire is used between the device and the instrumentation (2 wire interface)
b) There is no practical limit to the length of such wiring (within reason)
c) The transmitted signal is much less vulnerable to “noise” pick-up
d) Scaling (temperature range) can be set-up on the transmitter (e.g. 4-20mA for 0 to 200°C)
e) Linearising of the thermocouple characteristic can be incorporated (sometimes optionally)
f) The 24V d.c. excitation is applied via the 4-20mA loop, (additional wires are not required)
Input to output isolation is only incorporated in the more expensive transmitters, it is therefore essential to use electrically insulated sensors with unisolated devices.