Tag Archive for Two Wire Transmitter

Thermocouple Two Wire Transmitters



Thermocouples are temperature measuring sensors which are made of 2 dissimilar metals. The metal is typically in the form of wire and the end of one wire is welded to the end of the 2nd wire. If the 2 wires are insulated from each other and run closely parallel to each other, a thermocouple is formed.

A voltmeter placed across the open ends of the parallel wires will indicate a voltage if the welded ends are at a different temperature . If the ends are at the same temperature, zero volts will be observed.

If the open end of the wires are held at a constant temperature, the voltage read by the meter is proportional to the temperature difference between the ends of the thermocouple.

The sensitivity of the thermocouple is determined by the two metals used in its construction.

Since the beginning of thermocouple technology, different metals have been used to make thermocouples for use in different applications and environments.

Thermocouples have their leads color coded for identification. Some have magnetic leads so they can be identified with a magnet. (see chart at end)


Thermocouples create very low signals. The J thermocouple creates about 52 microvolts per degree centigrade difference in temperature. The B thermocouple creates about 4.8 microvolts.


2 wire transmitters for thermocouples have 2 difficult requirements to meet. The signal levels are very low and temperature stability of the circuits is a challenge.

The 2nd requirement is the 2 leads of the thermocouple connected to the transmitter terminals must be held at the same temperature. Any difference in temperature at the transmitter end changes the signal by this amount.

The transmitter terminals are referred to as the cold junction compensation connection.

A typical method of controlling the transmitter end temperature is to connect the 2 wires to 2 large masses of metal which are insulated from each other. They are thermally bonded together so they maintain the same temperatures for the thermocouple leads. A small temperature sensor is embedded in the metal mass and the temperature of the mass is measured accurately. This temperature is added to the signal from the thermocouple.

Thermocouple voltage charts are referred to the temperature of the junction end with the cold junction end being at 0°C.


No thermocouple is linear over a wide temperature range. When the temperature must be known accurately, linearity and stability is required.

Many applications, such as alarms, only require accuracy in repeatability.

Accurate linearity requires a microprocessor based transmitter. With this design, a lookup table of temperature vs signal level can be used for accuracy. A single transmitter can carry lookup tables for many thermocouple types. With this asset, a single transmitter can serve in many applications.

The microprocessor based transmitter can also reduce drift by checking the gain and offset of the signal amplifiers against a precision reference voltage and zero offset (ground). It performs a math equation every sample to remove amplifier gain drift and offset drift.

This capability of auto zero makes the transmitter as accurate and stable as the reference voltage used for comparison.


Due to the low levels of signals thermocouples create, the system of thermocouple and 2 Wire transmitter requires care in wiring to prevent noise pickup.

Many thermocouple probes have the thermocouple wire welded to the end of the metal sheath which houses the thermocouple wire. Connecting the wire to the metal tube allows a faster response to temperature changes.

If the metal sheath is grounded to the earth through the pipe or other metal object it is mounted to, it makes a safer installation. Lightning in an area can create powerful magnetic fields which can create sparks from ungrounded metal object to grounded objects.

Welding the thermocouple wire to a grounded sheath prevents sparks from leaving the thermocouple wire to a grounded object. Such a spark could be devastating in the wrong environment.

The grounded thermocouple wire creates an opportunity for a ground loop if an electrical path exists from the thermocouple to another ground connection in the system.

Due to the common practice of grounded thermocouples, it is wise to use isolated 2 wire transmitters to prevent ground loops.

Thermocouple wires should not be run in the same cable trough or conduit as AC power wiring.

Capacitive or magnetic coupling will induce an AC current in the thermocouple wire and the signal will have AC “noise” imposed on the DC signal.


Thermocouple probe with head to hold 2 Wire Transmitter. Probe mounts on pipe fitting with probe protruding into pipe or tank.

Thermocouple Probe with Explosion Head

Thermocouple probe for mounting on pipe fitting with probe located at point of measurement. Connector on wire to plug into measuring instrument.

Thermocouple Probe Pipe Fitting Mount

Ceramic beaded thermocouple for high temperature measurement. Leads connect to extension wire or instrument.

Ceramic Beaded Thermocouple Probe



Common Thermocouple Chart

Common Thermocouple Chart


© Joe E. Wilkerson 2012

RTD Two Wire Transmitters and how they work

RTD 2 Wire Transmitters and how they work

An RTD (Resistance Temperature Detector) is a metal which changes its electrical resistance when its temperature changes.

The most common metal used for precision resistance measurements is platinum. Platinum is chosen because it is a noble metal which is stable, corrosion resistant, does not oxidize readily, and is easily workable into thin wire.

Copper is also used as an RTD. Because of its low resistance value, RTD’s of copper are low in ohms value and requires accurate measurements of the resistance when used as an RTD.

Because copper wire changes resistance with temperature and it is used to connect the RTD to the Transmitter, lead length between the RTD and the 2W Transmitter must be taken into consideration.


Three versions of RTD’s compensate for lead length between the RTD and the Transmitter.

The signal from the RTD is typically developed by having a constant current flow through the RTD and the voltage across the RTD is measured to determine the resistance of the RTD.

Resistance = Voltage/Current.

RTD Wiring Diagram

2 WIRE – Current flows through the RTD. The signal is the voltage drop across the RTD. The Cu leads must be short enough so their resistance change does not create a serious error in the measurement.

3 WIRE – Same as the 2 Wire except a Cu lead is brought from the RTD bottom to the TX. It allows the voltage dropped by the Common Cu lead to be measured. This voltage is doubled and subtracted from the signal voltage. This negates the voltage drop in the Common and Current Cu leads.

4 WIRE – The signal Cu leads go directly to the RTD. They allow the voltage across the RTD to be measured without the Current and Common Cu leads having any effect. This is the desired RTD setup for the most accurate measurement, when Cu leads could be a problem due to their length.

The transmitter has to be well designed so it does not have its current source, which excites the RTD, drift with ambient temperature changes. The circuits which process the signals must also be stable.

RTD’s are not perfectly linear with resistance vs temperature. Platinum RTD’s can be compensated. If the Transmitter is an analog design, linearization is accomplished by feeding a small amount of the RTD signal back to the current source which excites the RTD. This feedback increases the current a little for every positive change in temperature. With proper compensation very high accuracy can be obtained.

Microprocessor based Transmitters can manage Cu leads in a similar fashion to the analog version, but the add/subtract math is done by the microprocessor.

Linearization can be done more accurately with a microprocessor utilizing a lookup table of values.

©  Joe E. Wilkerson  2012




Get every new post on this blog delivered to your Inbox.

Join other followers: