Martel Matters

Volume 1, Issue 2

Welcome to MARTEL MATTERS, the e-newsletter for Martel and BETA calibrators. We hope you find the content useful and (somewhat) enjoyable. If not, see the end of the newsletter for how to unsubscribe.

This newsletter contains a short "tips" article on temperature calibration, an introduction to our new 3001 High Accuracy Bench Calibrator and more.

In This Issue:
>> Calibration Tips #2: Thermocouple Calibrations
>> Martel 3001 Precision Bench Top Calibrator
>> Register your Martel product on-line
>> Here's How: Precision Temperature Measurement with the 3001
>> Isolated vs. Non-isolated Pressure Sensors
>> New Ranges & Capabilities for the BetaGauge PI Digital Test Gauge

Calibration Tips (#2 in a series)

Temperature Calibrations (Thermocouple Edition)

It's well known that temperature is the most common process variable measurement. These measurements are performed by thermocouples (T/Cs), resistance temperature detectors (RTDs), filled systems, non-contact infrared thermometry and by other means.

Of these, the most common on-line measurements are by T/Cs and RTDs, either directly connected to the process control system or data acquisition system or indirectly connected by the use of a process transmitter.

Generally speaking the T/C or RTD being a primary element cannot be calibrated (adjusted). They're either working properly or not. They can be certified, if needed. This type of certification requires a device such as a temperature bath or "dry well" temperature calibrator. But, we're not going into that procedure here.

There are a few "gotchas" in temperature calibration when we're talking about T/Cs and RTDs. Each has its own problems, This article will focus on calibration of T/C instrumentation.

Thermocouples work on the principle that (EMF) proportional to the temperature difference between the "hot" junction where the measurement is made and the "cold" junction where the measurement is observed.

That brings us to the first potential problem. You can only find out the hot junction temperature if you know the cold junction temperature. In the old days, temperature calibrators were simple milliVolt generators. The technician could look up the EMF value for a given temperature, use a local mercury thermometer to measure the cold junction temperature, perform a little math (adding or subtracting the EMF for the cold junction value) and come up with the correct simulation value. A similar process could be used to make a temperature measurement.

Today's temperature calibrators all have what's called automatic cold junction compensation. They also do all the "looking up" of EMF values, etc. It makes it easy and usually quite correct.

If the instrument being calibrated also has automatic cold junction compensation and the calibrator's automatic cold junction compensation is active, you're in business. If this is not the case, you will calibrate in an error equal to the difference between ambient temperature and 0°C. That can be quite a lot. So, it's worth checking to see what's what.

A quick tip about this. If the calibrator has been moved from a relatively warm place to a cold place or vice versa, it may require some time for its cold junction compensation to stabilize for accurate performance.

By far the most common error in calibrating thermocouple instrumentation is the failure to use thermocouple extension wire that matches the thermocouple in question when connecting the calibrator to the instrument.

Why?

Because the calibrator is measuring a cold junction at its terminals. If copper test leads are used, the actual cold junction will be where the copper leads connect to the instrument. Whatever temperature difference there is between these two points will be calibrated into the instrument as an error. If the two are really close together, it may be small, but why not do the job right? Also, many modern calibrators are designed to use miniature thermocouple plugs and jacks to connect, which makes it really easy to use the right materials.

If you think this is trivial, let me tell you that I have seen technicians connect a calibrator located in front of a control panel to an instrument located inside the panel with looooong copper leads. I'm sure that resulted in an error of 5-10°C. Considering that many processes have tolerances of 1-2°C, that's going to cost a lot of money.

So, two things to remember when calibrating T/C instrumentation: one, make sure automatic cold junction compensation is enabled on both the calibrator and the instrument, and, two, use thermocouple extension wire that matches the curve of the thermocouple to connect the calibrator to the instrument. Today's smart temperature calibrators make the rest easy.

Martel 3001 - Powerful And Accurate

There is nothing else like the 3001 in its price range. With basic DC accuracy of ±0.0025% of reading accuracy it is equal to units that cost $20,000.00 or more. At less than $5,000, the 3001 is in a league of its own!!

The 3001 is capable of simulating/sourcing and measuring thermocouple, RTD, DC Voltage and current with an easy to learn user interface. It also has pressure capability using BetaPort-P digital pressure modules. You can also use Fluke and Mensor pressure modules with this unit.

Another valuable feature of the 3001 is the isolated read back circuit with built-in 24 VDC instrument power supply.

Among other high end features is full remote control capability using RS-232 or IEEE-488 communications. It's MetCal® compatible, too!

For the full 3001 story, please visit our web site: http://www.martelcalibrators.com/precision_calibrator.html.

Register Your Martel Product On-line

You can register your Martel BETA calibrator on-line to receive notices about updates, lock in warranty benefits and get a periodic recalibration reminder. Just go to Calibrator Registration and complete the form. PLUS, you'll receive a special thank you gift when you do.

Here's How

Here's how is a series of short articles featuring a step by step guide to using one of the powerful tools included in your Martel BETA calibrator.

3001 Precision Bench Top Laboratory Calibrator

Precision Temperature Measurement with IBP-2 Probe

One of the advanced features of the Martel 3001 Precision Bench Calibrator is the ability to use custom coefficients for RTD measurement. This allows the "matching" of a probe to the calibrators for very high accuracy temperature measurement. The IBP-2 probe is a high quality Platinum 100 (DIN) probe that comes with traceable calibration data. By entering the appropriate data from the probe, we can match it to the calibrator.

The information in this application note is taken from sections 4.3, 4.5 and 7.12 of the 3001 User Manual. With registration, this manual can be downloaded from our web site.

Entering RTD Calibration Data

NOTE: This procedure uses the Calendar-Van Dusen coefficients. To use ITS-90 coefficients, the data must be entered using the calibrator's serial port.

To select an RTD curve follow these steps:

• Press the key to select thermocouple and RTD/W mode, if not already selected. If thermocouple mode is displayed, press the key again to cycle to RTD/W mode.
• If output mode is displayed, select input mode by pressing the and keys
• Press the key to select the USR_DEF RTD type.
• Connect the IBP-2 probe to the 4 wire RTD/W input terminals of the 3001 as shown below.

To enter the coefficients for a custom RTD curve:

• Select RTD measure mode as described above.
• Press the key until the USR_DEF type is selected.
• Press the key to display the curve selection prompt "RTD CUSTOM (1-5)".
• Press the numeric key corresponding to the custom RTD curve to be entered.
• At the "SET(1)/RECALL(2)" prompt, press to select custom RTD curve data entry.
• At the "ENTER MIN TEMP" prompt, enter the minimum temperature limit for the custom RTD curve, and press the key.
• At the "ENTER MAX TEMP" prompt, enter the maximum temperature limit for the custom RTD curve, and press the key.
• At the "ENTER R0" prompt, enter the nominal resistance value (R0) for the custom RTD curve, and press the key.
• At the "ENTER COEFF A" prompt, enter the first (A) coefficient for the custom RTD curve, and press the key. To enter a coefficient that includes an exponent, enter the mantissa, press the and keys to select the EXP function, enter the exponent, and press the key.
• When prompted, enter the second (B) and third (C) coefficients in the same manner.
• To abort the curve entry without saving any changes, press the key.

Isolated vs. Non-isolated Pressure Sensors

There are 2 types of silicon strain gauge sensors used in BetaGauge pressure calibrators. Most ranges have the isolated sensors and a few have non-isolated sensors. What's the difference and why should you care?

Isolated sensors have 316 S.S. wetted parts and are compatible with a wide range of pressure media (air, water, oil, what-have-you). In non-isolated sensors, the silicon sensing element is directly exposed to the media. These are normally limited to applications where the media is a clean dry non-corrosive gas.

So, you're probably saying to yourself, why not just use isolated sensors on everything. Here are 2 very good reasons. Sensitivity and sensitivity.

Actually, the 1st sensitivity is hysteresis. Isolated sensors because they have a thicker and stiffer diaphragm have more inherent hysteresis than non-isolated sensors. The hysteresis is negligible on higher ranges, but very much a part of the accuracy specification at ranges of 15 PSI (1 Bar) and lower. Hence, isolated sensors aren't used below 5 PSI (340 mBar).

The second problem with isolated sensors is that they are more position sensitive than non-isolated sensors. Position sensitivity refers to a measurable zero shift when the calibrator is tilted from vertical to horizontal. It wouldn't be a problem is the calibrator isn't moved after being zeroed, but that can be a problem in practice.

Now you know...

New Ranges And Capabilities For BetaGauge PI Digital Test Gauge

Absolute style pressure sensing ranges are coming to the popular BetaGauge PI Digital Test Gauge. The absolute ranges are 15 PSIA, 30 PSIA, 100 PSIA and 300 PSIA. Shipments are expected to begin after January 1, 2008.

Check with your local Martel BETA distributor for more information. Find a local Martel distributor on our web site or call 800-821-0023.

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Martel Matters is an electronic publication of Martel Electronics Corporation. © 2008 All Rights Reserved.