GETTING THE BEST RESULTS FROM TEMPERATURE TRANSMITTERS
      A BRIEF DESCRIPTION
      Temperature transmitters, especially head mounting types, are becoming more popular, so that it is important for plant and other instrument engineers to have a better understanding of their properties, advantages and disadvantages.
      The basic principle is that the transmitter accepts an input from a temperature sensor, usually thermocouple or resistance bulb, and converts it to an industry standard 4-20mA signal for transmission over a twisted pair of copper conductors to one or more receiving instruments.
      The most popular technique is for the transmitter to receive its power over the same pair of wires as the signal, thereby minimising the number of conductors used and reducing installation costs. A four wire system, where a separate 24V DC supply is provided is also used, but rarely for head mounting units. Mains powered units are also available, but again not in head mounting units.
      WHY DO WE USE THEM?
      Using transmitters can save cost on long cable runs by replacing expensive thermocouple compensating cable with ordinary copper wire.
      In the case of RTD transmitters cost saving is not so obvious however an improvement in accuracy can result especially if a third compensating lead can not be provided.
      The conversion of the signal into a low impedance current signal will usually give better noise immunity as current signals are inherently less susceptible to noise.
      It is often convenient to convert a signal into 4-20mA so that it can be added on to a system where 4-20mA equipment already exists, saving the cost of new instruments.
      SUPPLY VOLTAGE CONSIDERATIONS
      A 4-20mA loop is typically powered by a 24V DC power supply. A temperature transmitter is basically a current sink which pulls current from the power supply. For most transmitters a stabilised supply is not required as they are true voltage to current converters, however there is usually a small error introduced by changes in voltage.
      To function correctly a typical transmitter requires a minimum of 12 volts across its supply terminals. This means that 12 volts is left for the rest of the instrumentation. Using Ohms Law this means that all the other instruments in the same loop must have a total impedance of less than 600ohms.
      If the impedance of the instruments is greater than 600ohms (and this may be the case in Intrinsically Safe circuits where zener barriers are used), then problems arise.
      One solution is to increase the power supply voltage to 36 or even 48 volts. This normally works well, but if for some reason instruments are taken out of the loop, it could lead to voltages above 24V on the zener barriers which would blow their fuses and lead to expensive replacements.
      A better technique is to use a "floating" power supply. This type of power supply monitors the voltage across the zener barrier (or transmitter if barriers are not used) and floats its output voltage up or down to maintain a constant 24V at the transmitter. This means that, within the limits over which the power supply will operate, the transmitter will always have sufficient volts with which to operate. As the voltage at the transmitter remains constant at all currents, this technique also minimises errors in the transmitter due to voltage changes and can give improved performance even for normal loops. If even more resistance needs to be driven then a signal converter or isolator may be used to generate a second 4-20mA loop which tracks the first. The instruments are then divided between the two loops.
      AMBIENT TEMPERATURE CONSIDERATIONS
      The operating temperature range of a typical temperature transmitter is -5 to +60deg.C so it is obviously important to prevent them from operating outside this range. In the case of very high temperatures such as furnaces this is a more urgent consideration, and I have actually seen transmitters "returned for repair" where the plastic casing has melted.
      The following ideas can help.

      1. Mount the probes from the side rather than the top if possible to prevent overheating from rising hot air.

      2. Allow the head of the probe to extend some way into free air if at all possible.
      3. Fit some type of flange to act as a heat deflector.
      4. In bad cases mount the transmitter in a separate housing a little way from the probe and connect with compensating cable.
      Even if the operating temperature range is still within normal limits it may still be an advantage to look at ways to keep the transmitter at a more stable temperature. All electronic circuitry drifts with changes in temperature and there is also a small error associated with ambient compensation circuits for thermocouples. In a typical thermocouple transmitter which has been calibrated at 25deg.C a change in its operating temperature to 50deg.C can introduce an error of as much as 2deg.C from these sources . For a span of 1000deg. this is only 0.2%, however for a span of 100deg. it is a 2% error.
      CHOICE OF PROBE ELEMENT
      The choice of probe element obviously depends on several factors, not least of which is cost. The most popular types are thermocouple, resistance bulb and thermistor. As thermistors are only really popular and readily available in the U.S.A. they will be ignored in this article.

      A little bit of theory here might help.

      The output of a thermocouple is proportional to the DIFFERENCE in temperature between the hot and cold junctions. The hot junction is usually the one measuring the process and the cold junction is at the head itself. If the temperature of the head changes then the output from the thermocouple will change even if the temperature of the process has not changed. To compensate for this most temperature transmitters incorporate a cold junction compensating circuit (CJ). The non linearity of the thermocouple characteristic means that this circuit is usually an approximation and has an error usually of the order of 1deg. for an ambient change of 25deg.. This is particularly significant if the process temperature is close to ambient but becomes less noticeable at very high or very low temperatures.

      The CJ sensing device must be mounted as close as possible to the cold junction, however in practice it is embedded in a resin and there is usually a small gap between the two. This means that there will be a slight delay in compensation as the ambient temperature changes. This can usually be demonstrated by blowing warm air across the thermocouple terminals.

      As mentioned above this effect can be minimised by keeping the transmitter at as constant a temperature as possible (which is a good idea anyway) but basically what it boils down to is that a thermocouple is not a good idea for monitoring temperatures close to ambient if accuracy is required.

      A resistance sensor, such as a Pt100, has a resistance which depends on the ABSOLUTE temperature and no cold junction compensation, with its possible error, is required.

      As a rule of thumb a Pt100 will always give better results than a thermocouple (and with the correct construction a Pt100 probe can cover the range from -200deg.C to +600deg.C) and is the preferred device for process temperatures within about 200deg.C of ambient.

      The disadvantages of a resistance bulb are usually physical strength and, that because it is physically larger than a thermocouple junction, it tends to have a slower response time.

      Pt100 elements used to be made by winding Platinum wire on a ceramic former. These days they are made by deposition of a film onto a ceramic substrate. The latest Pt100 elements are smaller than 3mm square by 1mm thick. They are available in various grades, 0.1% and 0.2% accuracy being probably the most common.
       

      CHOOSING A THERMOCOUPLE

      Most articles about thermocouples give a lot of detail about which type of thermocouple should be used or not used in different atmospheres. In practice, as most thermocouples are in sheaths and can be sealed from the environment, this may not be as important as it sounds. If in doubt get advice from your local supplier.

      The following information is a guide for use with temperature transmitters and does not take these effects into account.

      Bear in mind that thermocouple errors, usually because of variations in the composition of the alloys used, tend to be larger than any error in the electronics.

      Type J - Iron/Constantan

      Large output - Approx. 52microV/deg.C.
      Useful range +20 to +700deg.C continuous.
      -180 to +750deg.C intermittent.

      Reasonably easy to linearise over any part of range. Best choice for lower spans as larger output implies lower drift error on instrument.

      Type K - Chromel(NiCr)/Alumel(NiAl)

      Medium output - Approx. 40microV/deg.C
      Useful range 0 to +1100deg.C continuous.
      -180 to +1350deg.C intermittent.

      Very easy to linearise up to about 500deg.C and over about 800deg.C but the characteristic does a bit of a dogleg between these two regions. This is not normally a problem, however recalibrating an instrument from say 0-600deg.C to 0-700deg.C could give linearity errors.

      Type T - Copper/Constantan

      Medium Output - Approx 38microV/deg.C

      Useful range -185 to +300deg.C continuous.
      -250 to +400deg.C intermittent.

      This one is the favourite for temperatures below zero. The linearity is bad compared to type J and the linearity errors will be generally higher. Best avoided if possible.

      Type R - Platinum/Platinum 13% Rhodium

      Low Output - Approx 10microV/deg.C
      Useful range 0 to 1600deg.C continuous.
      -50 to 1700deg.C intermittent.

      Not linear. Output very low at low temperatures.
      Best calibrated with live zero eg. 400deg.C to 1400deg.C.

      Type S - Platinum/Platinum 10% Rhodium

      Similar to type R but slightly lower output. The same comments apply.

      Type B - Platinum 6% Rhodium/Platinum 30% Rhodium
      Very low output - Approx 6microV/deg.C
      Useful range +100 to 1600deg.C continuous.
      +50 to 1750deg.C intermittent.

      Practically no output below 400deg.C. Very difficult to linearise, in fact just about impossible below 500deg.C. Recommended for high temperature work only. Pick a range with a large zero offset for best accuracy eg. 700 - 1700deg.C.

      Type N - Nicrosil/Nisil

      Medium output - Approx. 38microV/deg.C
      Useful range 0 to 1100deg.C continuous.
      -270 to +1350deg.C intermittent.

      This is a fairly new type with characteristics similar to the type K. Both alloys contain silicon which improves long term stability. Possibly the way to go in future.
       

      TO ISOLATE OR NOT TO ISOLATE

      The majority of head mounting temperature transmitters are non-isolated, which means that there is an electrical connection between the sensing element and the 4-20mA loop.

      In most cases the sensing element is contained within a sheath and is surrounded with an insulator such as aluminium oxide. In such cases the element can not come into contact with anything which may be connected to the loop and isolation is therefore unnecessary. Exposed sensing elements also normally cause no problem if used on individual loops.

      The major problem occurs when several transmitters are used with one receiving instrument such as a PLC. If grounded tip thermocouples, or other exposed sensors are used with a single power supply then isolation is probably a must, however it could be less expensive to look at three alternatives to buying an isolated transmitter.

      1. Replace the sensors with insulated types.

      2. Use individual power supplies for each loop.

      3. Use a separate isolating module mounted in the control panel.

      WARNING. Some ceramic type thermocouple sheaths become electrically conducting at high temperatures, giving intermittent ground loop problems on multiple installations.
       

      RADIO FREQUENCY INTERFERENCE

      Any instrument measuring PT100 or thermocouple signals is prone to receive interference from radio signals because the probe leads make nice antennae and they are designed to amplify low level signals. Careful design can minimise these effects however the use of walkie talkies or similar devices in close proximity to temperature transmitters is definitely to be avoided.

      A well designed temperature transmitter will often show little effect with 2 watt Walkie talkies and cellular telephones at distances of 500mm but 1 metre is the recommended closest distance.

      Note that IEC803 and IEC1000 specifications refer to a maximum field strength of 10V/m which is roughly equivalent to 2 watts at 1 metre. Normal RF testing is performed in a linear field however transceivers closer than about 1 metre are likely to generate a field with a large gradient and this is what causes the problems.

      Screening in metal control panels can often be a solution to RF pickup in instrumentation. Ferrite beads on the thermocouple leads and a small non-inductive capacitor (100pF is often a good starting point) across the instrument terminals can give dramatic improvements at UHF.
       

      LONG TERM CONSIDERATIONS

      Electronic components tend to change their values gradually as they age. In the case of temperature transmitters it means that for best performance they should be re-calibrated from time to time.

      Most manufacturers attempt to minimise this effect by accelerated ageing during which process the units are heated and cooled repeatedly over a period of 24 to 48 hours. This is a big help but does not totally eliminate the problem.

      Generally speaking the drift in calibration with ageing is at its worst when new and decreases as the instrument ages. Obviously user requirements for accuracy vary and it is difficult to be specific, but it is good policy to re-calibrate a unit after it has been in use for perhaps three months, again about six months later and thereafter about once a year. If the transmitter is subject to wide extremes of ambient temperature, more frequent re-calibration may be necessary.

      When calibrating a temperature transmitter the usual considerations should be taken into account, such as correct polarities for compensating cables for thermocouples and corrections for lead resistances for Pt100. Note that for optimum overall tracking accuracy the calibration may be slightly offset at zero and span.

      If the process which is being monitored only covers a relatively narrow temperature range then the calibration can be optimised over this part of the range of the instrument rather than trying to obtain a good accuracy over the whole range.

      I hope this information will enable users of temperature transmitters to make a more educated choice and to anticipate where problems could arise.

      F.R.Philpott B.Sc. (Hons) (Lond.)
      IMC. Quanta Instruments CC
      First Printed: 16th. October 1991
      This Revision: 25th. May 1997
      fred@iqinstruments.com

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