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GENERAL QUESTIONS

What is a transmitter?

A transmitter is an electronic device that is generally mounted in the field in close proximity to a sensor. The sensor (also known as a transducer) measures a physical variable such as temperature or pressure and outputs a very low level electronic signal. The basic function of the transmitter is to provide the correct electrical power to turn on (or excite) the sensor then to read the low level sensor signal, amplify it to a higher level electrical signal and send that signal a long distance to a control or read-out device.

Since low-level electrical signals do not transmit long distances with great accuracy, installing a transmitter generally gives a tremendous improvement in the accuracy of the information delivered to a larger control system.

What is a two-wire transmitter?

A two-wire transmitter is a transmitter that needs only two wires to be run from the control system out to the transmitter. These two wires carry both the power, as well as the signal information. The output signals from two-wire transmitters can be analog, typically 4-20mA. They can also be digital such as Fieldbus, or a combination of analog and digital, such as HART. Generally two-wire transmitters run on lower power levels than three or four-wire transmitters. They have the added advantage of needing only two wires run the long distances out to the field, which results in lower installation costs.

What is a thermocouple?

A thermocouple is an electrical circuit that can be used to measure temperature. It is called a thermo-"couple" because two wires of different metallurgy are connected in two places to make two electrical junctions. One of the two junctions is generally held at a reference, or cold temperature and one at a hotter or measurement temperature. The current generated in the electrical circuit between the two junctions is proportional to the difference in temperature between the two junctions. In real life, the second, or cold-junction, of a thermocouple circuit is generally included as part of the measuring device and the sensor that is referred to as a thermocouple is just a probe with wires made up of different materials. This leads to some confusion because people tend to think of a thermocouple as a single measurement point sensor.

The relationship between the thermocouple output and the temperature is quite non linear. Different metallurgies produce different outputs and the do have a tendency to drift over time, depending on the size of the thermocouple wire, the temperature to which they are exposed and the chemistry at the measurement junction. The different metallurgies and different lineararities result in different thermocou0le designations such as "J", "K,", "N", "L", etc.

What is an RTD?

An RTD is a Resistance Temperature Device. This is a temperature sensor that has different properties than a thermocouple. Generally an RTD is made of a winding of a length of wire of an inert metal. The resistance to the flow of electricity of any conductor decreases with a decrease in temperature, reaching a superconductivity state at a temperature near absolute zero. The resistance increases as the temperature increases.

A special class of RTD's are made from platinum and are called PRT's or Platinum Resistance Thermometers. This is the dominate form of the RTD sensor and as a result in normal usage, the term PRT and RTD are often used interchangeably.

How should I select a temperature sensor?

Many articles and books have been written on selecting a temperature sensor. In recent years, the selection process has become simpler with the steady improvement in the measurement capabilities of the platinum RTD (or PRT). For almost all industrial temperature measurements under 1,000ºF a four-wire RTD gives the best combination of cost, accuracy and stability and should be considered for virtually all measurements under the 1,000ºF level. You can get more information on this subject in our technical articles.

Why should I use a transmitter?

If you are making a temperature measurement, it is not mandatory that you use a transmitter. Many temperature measurements are "direct" wired. Direct wiring a temperature sensor to a control device will generally give you an indication of the temperature. However, the transmitter will save you money on your field wiring and give you a more precise, more stable measurement.

With a two-wire transmitter, you can run just two field wires the long distance to the transmitter and then run four wires the short distance to the RTD sensor. The two field wires can be inexpensive, small diameter copper wires. With a direct wired sensor, your best measurement will be made running four wires to an RTD sensor. If you are using a thermocouple, you will to run much more expensive thermocouple extension grade wires. Frequently, the transmitter will save more in wiring costs alone than the cost of the transmitter. In addition, the transmitter will give you a high level electrical signal to run the long distance to your control system. Reading a high level signal is much easier than reading a direct sensor input. The control system card will be much less expensive and will give better measurement accuracy. Input cards for thermocouples or RTD's are always more expensive than input cards for high level electrical signals such as 4-20 milliamps.

What is a "Smart" transmitter?

A "Smart" transmitter is a transmitter that uses a microprocessor as the heart of the electronics. In addition, a "Smart" transmitter will output some type of remote digital communications allowing you to read and set-up the device from a remote position.

What is HART communications?

"HART" stands for Highway Addressable Remote Transducer. This is a low-speed, digital communications protocol. It runs simultaneously with, and on the same two wires as the conventional 4-20mA analog signal. HART is particularly useful when small amounts of data need to be transferred. Using HART, you can setup a transmitter and inquire about its health and status from time-to-time. HART communications is generally not useful when large amounts of data need to be transferred quickly, i.e. many times per second. For this type of communications, a higher speed digital protocol, such as Fieldbus, is required.

What is Fieldbus?

Fieldbus is a general term for a digital only, high speed communications protocol. The key attribute to Fieldbus communications is higher speed communications with the possibility of addressing multiple transmitters all on the same field wiring. The Foundation Fieldbus is a specific digital protocol that is often shortened to just be called Fieldbus. Other digital only communications such as Profibus are also Fieldbus protocols.

IMPROVING YOUR TEMPERATURE MEASUREMENT

Can better temperature measurements help improve my process?

"Better" temperature measurements will improve many, but not all processes. Generally "Better" means more stable temperature measurements with less drift. For temperature measurements that rely on indication only, or on single loop feedback control, measurement repeatability is the important variable. Less drift means better repeatability. Measurements can and do drift several degrees depending on the ambient temperature change and the long term drift of the sensor and measurement electronics. As a rule of thumb, for single loop control or indication only, if a 15ºF shift in the temperature measurement would have no effect on your process, then you probably have better things to do with your time than to tighten up this temperature measurement point. However, if you answer the question that says "Yes, an error of 15ºF would affect the results that I am trying to achieve", then you should take a second and look at your temperature measurements, because the chances are that you will have an error approaching this magnitude somewhere.

If you are using your temperature measurements as in input to a computer calculated formula, or in a control situation where one measurement is being compared to another one, then you will definitely see large process improvements in making tighter temperature measurements. You should be seriously looking at "Smart" Temperature Transmitters. The time you spent improving these measurements will pay big dividends.

How can I achieve accurate, repeatable temperature measurements?

This answer is simple, Install the right temperature sensors and transmitters. Look seriously at 4-wire RTD's for the sensor and a quality "Smart" transmitter to read it.

Are my temperature measurements drifting?

Unfortunately, the answer to this question most likely is "YES". The biggest culprits in drifting measurements are mechanical temperature sensors, thermocouples and analog electronic measurement devices.

There is no way to guarantee that you do not have any measurement drift. Simply putting two, or even three, elements in the same sensor sheath is no guarantee. In fact, with multiple sensors in the same sensor sheath, whatever external factor has caused one element to drift will most likely cause all the elements to drift! There are measurement practices you can follow to reduce drift, such as using a 4-wire RTD as noted below. You can use multiple sensor elements, preferably in different sensor sheaths, placed in close proximity to each other. You can remove the sensor element and check it against a standard. All of these methods provide varying degrees of information about sensor drift. What you choose is dependent upon the demands of your situation.

Why should I use a 4-wire RTD?

Quite simply, a 4-wire RTD will significantly reduce the amount of sensor drift. Most RTD's are made from platinum and are known as PRT's or Platinum Resistance Thermometers. The platinum in the element is extremely inert and impervious to chemical attack. Numerous laboratory tests have substantiated the stability of these sensors and the National Institute of Standards and Technology (NIST) uses them exclusively for intermediate temperature measurement points between their fixed points.

The true 4-wire measurement technique is also proven to eliminate measurement error and drift. Measurement error in a 3-wire RTD measurement can easily exceed 20°F. The true 4-wire measurement technique reduces this to an error of less than ±0.1°F.

How can I achieve the best accuracy possible?

If you are looking for the best temperature measurement accuracy possible, you should be looking at the Accutech AI-2000UP temperature transmitter. This device uses a 4-wire RTD exclusively as an input sensor. Individually calibrated RTD's can significantly cut your error. It is possible to get a total absolute accuracy error to less than ±0.1°F with a measurement system like this.

How can I get measurement instrumentation good enough for on-line enthalpy balances?

For on-line enthalpy balances, small measurement errors can add up. Because measurements are also compared to each other, you need absolute accuracy and extreme stability in all of the measurements. Generally, if the measurement errors are less than ±0.5°F, and they are stable and accurate on an absolute scale, this is good enough. This level of precision can be achieved with Accutech's AI-1500 or AI-2000 products, if coupled to a precision sensor. If you need better accuracy, just contact Accutech directly.

Can I get NIST traceable measurements?

Absolutely, Transmitters with NIST traceability are available. Entire measurement systems, consisting of the sensor and transmitter can also be certified through factory calibration.

MOUNTING YOUR TRANSMITTER

Can I head-mount the transmitter?

Yes. The compact design of the Accutech AI-1000 and AI-1500 easily accommodates head mounting. You can use the WP head for weatherproof mounting, or the XP-Head for explosion-proof requirements. These heads are even small enough to mount directly to the end of a thermowell.

What other kinds of mountings are available?

Accutech offers more mounting flexibility than any other transmitter. We make accessories for pipe mounting, DIN-rail mounting, panel mounting, explosion-proof, and more. If you don't see the mounting accessory that you need, just give us a call or email.

COMMUNICATING TO AND PROGRAMMING YOUR TRANSMITTER

What is a "Programmable" Transmitter?

"Programmable" is the name given to a microprocessor-based transmitter that supports multiple types of inputs and configurations, but does not support remote communications. This type of product generally gives you most of the advantages of the "SMART" transmitters without the expense of the remote communications.

What is a "Smart" Transmitter?

This is a microprocessor-based transmitter that supports remote communication, such as HART or Fieldbus. There are generally other, more advanced, features that are also supported by "SMART" products, such as improved diagnostics, sensor input flexibility, etc.

How can I program my transmitter without a handheld terminal?

Hand-held's aren't for everyone. While they definitely fulfill a function for field use, they are expensive and often complicated. Using a plug-in display can give you almost all of the setup functionality of the handheld terminal. All the Accutech products also support a PC program setup The Windows based setup tools associated with the 1000 and 1500 series products give you much more capability than handheld terminals in a much simpler, more intuitive format.