Factors Affecting Choice in Temp Sensor

Simply select a transmitter that will match the temperature sensor to the PLC input.

What is the temperature sensor: RTD? Thermocouple? Or what?

"Simply select a transmitter that will match the temperature sensor to the PLC input.

What is the temperature sensor: RTD? Thermocouple? Or what?"

I'm not sure what you mean by transmitter?

I havent got as far as distinguishing between thermocouple or RTD, but i do understand the advantages of both in terms of what what temp range they are better suited for. How do i know if i buy any particular sensor that it will work with my plc?

thanks for the help

RTD: resistence-temperature device. Many of these have a resistance of 100Ω at 0degC, for example.

Thermocouple: a junction between two dissimilar metals that presents a voltage that varies with temperature.

Transmitter: a device that converts one of the above signals into an industry-standard protocol and that is used as the interface between one of the above devices, its cabling back to the PLC, and the PLC input. One readily-used protocol is a 4-20mA current loop signal.

Does that look familiar?

Hi, According to my knowledge ,you should consider the expected range of temperature for that system and how much accuracy you are expecting? .then you can select a temperature sensor.eg:- You want measure the temperature range 0 to 100 degree and you want want to consider 0.5 degree temperature variation. if you are taking 10vdc output for (0-1oo) degree range temperature sensor.you will have to consider 10/200 =0.05volt per per 0.5 degree in plc programming . all the best.

Here's a little blurp on temp xmtrs to help you understand a little more. After digesting this you need to tell us the type of PLC or PC you're working with as you need to match the transmitter output to something that the monitoring system can interperet.

Temperature measurement using modern scientific thermometers and temperature scales goes back at least as far as the early 18th century, when Gabriel Fahrenheit adapted a thermometer (switching to mercury) and a scale both developed by Ole Christensen Røemer. Fahrenheit's scale is still in use, alongside the Celsius scale and the Kelvin scale.

Many methods have been developed for measuring temperature. Most of these rely on measuring some physical property of a working material that varies with temperature. One of the most common devices for measuring temperature is the glass thermometer. This consists of a glass tube filled with mercury or some other liquid, which acts as the working fluid. Temperature increases cause the fluid to expand, so the temperature can be determined by measuring the volume of the fluid. Such thermometers are usually calibrated, so that one can read the temperature, simply by observing the level of the fluid in the thermometer. Another type of thermometer that is not really used much in practice, but is important from a theoretical standpoint is the gas thermometer.
Temperature transmitters, RTD, convert the RTD resistance measurement to a current signal, eliminating the problems inherent in RTD signal transmission via lead resistance. Errors in RTD circuits (especially two and three wire RTDs) are often caused by the added resistance of the leadwire between the sensor and the instrument. Transmitter input, specifications, user interfaces, features, sensor connections, and environment are all important parameters to consider when searching for temperature transmitters, RTD.

Transmitter input specifications to take into consideration when selecting temperature transmitters, RTD include reference materials, reference resistance, other inputs, and sensed temperature. Choices for reference material include platinum, nickel or nickel alloys, and copper. Platinum is the most common metal used for RTDs - for measurement integrity platinum is the element of choice. Nickel and nickel alloys are very commonly used metal. They are economical but not as accurate as platinum. Copper is occasionally used as an RTD element. Its low resistivity forces the element to be longer than a platinum element. Good linearity and economical. Upper temperature range typically less than 150 degrees Celsius. Gold and Silver are other options available for RTD probes - however their low resistivity and higher costs make them fairly rare, Tungsten has high resistivity but is usually reserved for high temperature work. When matching probes with instruments - the reference resistance of the RTD probe must be known. The most standard options available include 10 ohms, 100 ohms, 120 ohms, 200 ohms, 400 ohms, 500 ohms, and 1000 ohms. Other inputs include analog voltage, analog current, and resistance input. The temperature range to be sensed and transmitted is important to consider.

Important transmitter specifications to consider when searching for temperature transmitters, RTD, include mounting and output. Mounting styles include thermohead or thermowell mounting, DIN rail mounting, and board or cabinet mounting. Common outputs include analog current, analog voltage, and relay or switch output. User interface choices include analog front panel, digital front panel, and computer interface. Computer communications choices include serial and parallel interfaces. Common features for temperature transmitters, RTD, include intrinsically safe, digital or analog display, and waterproof or sealed. Sensor connections include terminal blocks, lead wires, screw clamps or lugs, and plug or quick connect. An important environmental parameter to consider when selecting temperature transmitters, RTD, is the operating temperature.

prob use thermister for most applications

thermocouple will have better accuracy, but would need a conversion chip to get a usable signal

Hi all your replys are very helpful!

I am using the Allen Bradely Pico GFX-70 coupled with some expansion modules.

Will any of those common outputs work with any plc, so would it come down to choosing the right transmitter?

And generally what type of sensor are the ones that are usually found mounted in homes , offices etc. They are normally housed in a plastic cover. I'v seen alot on the net but find it hard to work out wether they are thermocouple or rtd.

Also from you ppls experience which sensor type is the easiest to work with? The system i'm working on is not critical, well it is but it wouldnt hurt if the sensor was slow or a little inaccurate.

Why are you not using commercially available thermostats that have all manner of sensing, remote and local temperature sensing and indicating?? It seems that your reinventing the wheel when Honeywell for instance has created thermostats for about 100 years capable of any commercial or residential applications. Woody HVAC company owner.