Thermocouples or RTDs

Many people often ask about the difference between thermocouples and RTDs and the applications of these two devices.   So here’s what you need to know:

The thermocouple is made of two dissimilar metals joined together at two points.  The “hot junction” is in the process, and the “cold junction” is at the controller.  In theory an EMF (Electromotive Force) i.e. a millivolt current, is generated at each junction that relates to the temperature at each junction. To measure a single temperature, one of the junctions (normally the cold junction) is maintained at a known reference temperature, and the other junction (hot junction) is at the process to be sensed.  By incorporating an artificial cold junction compensator (a thermally sensitive device such as a thermistor or diode) the controller subtracts the temperature at the cold junction from the thermocouple’s signal to remove the cold junction’s error, thus giving a true temperature at the hot junction.  This is known as cold junction compensation.

Unlike the thermocouple, the RTD (Resistance Temperature Detector) is usually made with a platinum, nickel, or copper wire that is wound around a ceramic or glass core, or it can be made by plating a thin film element and sealing this element within a ceramic or glass capsule.  Since the RTD works on a change in resistance, the lead wires (from the RTD to the controller) have resistance and add an error to the signal. If the leads are long enough, then the error may be large enough to have to be corrected. 

Industrial applications use a 3 wire RTD. Two wires connect on both sides of the resistor. This measures the temperature at the resistor and the error of the lead wires. One of those two wires has another wire with it. When those two wires are measured at the controller, they give the resistance of only those two wires. This measurement is then subtracted from the resistor’s two wires to remove the lead wire resistance from it. Now the controller is only reading the temperature at the resistor.

Application Differences:

Based on the thermocouple type (which dissimilar metals are used in manufacturing) the thermocouple has a wide temperature range:  -328 to 4800 degrees F. Thermocouples have a fast response time, low initial cost, and durability for rugged industrial type applications.

RTDs have a temperature range of -328 to 1202 degrees F.  Because of the lower temperature range, the RTD is more accurate than the thermocouple, has more stable outputs over time, and is easier to calibrate.

Explanation of these controls can get quite technical, and I would like to thank David Cates of our staff for his help in keeping terms and statements in a this article simple.

Let Stromquist and Company help you with your needs for thermocouples or RTD’s by calling us at 1-800-241-9471 or contacting one of our many CGNA members.

New Delphi Video Presentation

Honeywell has added a new Delphi video presentation to its Environmental and Combustion Controls website.

Go to: and look to the left of the page under the “What’s New” section, then click on the Delphi Combustion Efficiency Panel Video Presentation (in red) and follow instructions to the video.

Other Delphi and related Information:

Stromquist Radio Interviews Brian Turner from Control Co

Eric interviews the innovative Brian Turner from Control Co. Eric and Brian discuss everything from “open systems” and how a building owner can make sure they get a great integration job to the beautiful graphical interface that Control Co developed called prophetsuite. Check out this interview for the latest trends in building automation controls.

Video thumbnail. Click to play

Steam Safety Relief (Pop) Valves

Boiler contractors see these valves all the time when working on equipment.  Generally the steam relief valve is often little understood, often incorrectly installed, and usually neglected. A little refresher on these valves might be in order.

How Relief Valves Work

As the pressure of the steam within a boiler approaches the set pressure of the valve, the steam pressure on the underside of the actuating disc approaches the pressure of a spring applied to the outer side of the disc. When equilibrium is passed, the disc starts to lift off its seat. The moment this happens, steam is suddenly released all around the disc to what is called the “huddling chamber.” This chamber increases the area of the disc that sees steam pressure, thus increasing force. This increased area under steam pressure makes the pressure much more unbalanced in the direction of the valve discharge opening and therefore pops the valve into a wide open position. When the valve opens with a “pop” the valve seat is preserved from wiredraw caused by slow opening.

Closure of the valve occurs only after the boiler pressure is dropped several pounds below the set point. The reduction of the area of the disc seeing steam causes the disc to firmly close against the valve seat.

Relief Valve Installation

Proper installation of a steam relief valve seems somewhat simple and is, as long as two areas of concern are followed.

The first area of concern is valve distortion. Valve distortion occurs when the valve is improperly wrenched in, using the valve body instead of supplied wrench flats. Distortion also occurs when the discharge side of the safety relief valve is made to bear the weight of the discharge piping. To prevent this distortion use a short nipple from the valve to an independently supported bell reducer or drip pan elbow. These valves are precision devices and any distortion will affect accuracy and calibration.

The second area of concern is discharge piping. For a safety valve to do its job it must be sized properly to adequately relieve all the steam the boiler is capable of producing while operating at its maximum. All piping to or from a safety relief valve must be at least as large as the valve’s connections. Also, the restrictive effect of elbows and the friction losses in pipe must be taken into account. For this reason, piping runs should be as short as possible and pipe sizes should be generous.

If you need help in replacing or sizing a steam relief valve please contact Stromquist and Company at 1-800-241-9471. All others can order this product from one of our affiliates at CGNA.

Understanding: On/Off, Floating, Modulating/Proportional Control

First, to understand these types of control you must have the elements of control. The elements of control are the sensor (senses the medium being controlled), the controller (device either preset or programmed to react to the sensor), and the final controlled device such as a damper or a control valve (receives input signals from controller to affect change in controlled medium). These elements are considered the control loop.

On/Off control is the basic type of control in a control loop.  With On/Off control, the sensor senses the controlled medium and sends a signal back to the controller, which processes the signal. For ease of understanding, our example will be a heating application. The set point (the desired control point) in this case is 68 degrees with a temperature differential of 2 degrees for the controller. When the sensor’s signal to the controller reports a temperature of less than the controller’s set point, the controller sends a signal to the final control device (hot water valve) to position to fully open until set point is achieved. When the controller receives a signal from the sensor that the set point has been achieved, the controller then sends a signal to the valve to position to fully closed.  The problem with On/Off control is over-shoot temperature of the desired system set point because of reaction time between sensor, controller, and final control device. Review: With On/Off, the controller asks “Is there an error?” The controller compares the actual value of the controlled medium to the set point through the sensor. As the controlled medium deviates from set point, the controller’s output cycles the final controlled device on, and when the set point is reached the controller’s output cycles the final control device off.

Floating control is a variation of On/Off control that requires a fast responding sensor and a slow-moving actuator connected to the final controlled device (valve or damper). Using the same example as the On/Off example above, when the sensed temperature drops below the set point of 68 degrees by the controlled medium’s sensor, the controller sends a signal to activate the actuator on the final control device. The actuator starts to slowly drive open the hot water valve, increasing the heat in the controlled medium. When set point is reached the actuator stops opening the final control device (hot water valve) and tries to hold at set point. If set point starts to be over-shot, the controller sends a signal to the actuator to start to drive close the valve. Review: Set point control is achieved when the sensor signal (from the controlled medium) starts to deviate from the controller set point. The controller sends a signal to the actuator of the final control device (valve or damper) to slowly drive open. As the set point is approached the controller sends a signal to the actuator, then the actuator stops and tries to maintain set point.  If set point is passed the controller sends signal to the actuator to drive the final control device to a closed position.

Modulating/Proportional represents the higher end of control positioning. In modulating/proportional control the output varies continuously and is not limited to being fully open or fully closed. Proportional means that the size of the output is related to the size of the error detected by the controller. The key phrase for modulating/proportional control is “Continuous Control Action.” The sensor, controller, and final control device act as one unit to maintain constant precise control over the controlled medium. Continuing with the previous example, when a modulating system senses a deviation from the set point of 68 degrees, the controller calculates the amount of the error (1 degree less than set point) and sends a signal to the actuator, which will drive open the final control device (valve or damper) by a certain percentage of the controlled medium’s set point deviation (1/2 degree) to maintain set point without over-shoot. The controller calculates how much the final control device needs to open without over-shoot and will start reversing the actuator to close the final control device to a percentage of the closed position to maintain set point.
Popular modulating control signals include 4-20 ma and 0-10 volts. If you were to look into a control panel like a Hoffman Enclosure you might see controls like a Honeywell UDC3200 that could be taking a 4-20 ma signal from a device like a Hawkeye 908 current transmitter and based on the control input signal from the Hawkeye 908 ( which would most likely be a 4-20 ma signal) the UDC 3200 controller would respond with a 4-20 ma output signal to a device like a Honeywell Variable Frequency Drive which would control either a fan or a pump. This is an example of how a proportional signal like a 4-20ma signal is used in modern HVAC controls.
If you are in Georgia or Florida,the control pros at Stromquist & Company can answer your control questions.