Case 1: Custom Calibrated Thermocouple Wire

The ASTM establishes acceptable error bands for performance of standard thermocouple wire, but these error bands can allow wire errors of four to ten degrees near the high temperature end of the thermocouple operating range. To reduce this uncertainty, some customers will calibrate individual spools of thermocouple wire and generate calibration curves for a particular spool. The E1413 can perform EU conversion to match these curves, using the DIAGnostic language tree of the SCPI command language . The program "eu_build.c" (available from HP) gives an example of custom EU conversion table generation and download, but does not consider the special case of temperature measurement.

When thermocouples are measured, there is an additional step performed in EU conversion. This step is the "reference junction compensation" of the raw voltage value, which adjusts the ADC reading to compensate for the temperature of the junction between the thermocouple wire and copper wire. (This point is usually called the Uniform Temperature Reference, or UTR panel, also called an Isothermal Panel. The terminal block on the E1413 can serve as such a panel if thermocouple wire comes directly to it, and its temperature is measured using the built-in 5000 ohm thermistor.)

In the E1413, reference junction compensation depends upon library tables which contain the voltage for various thermocouple types in a format usable by the E1413 EU conversion algorithm. These tables cover a temperature range of +/- 127 degrees Celsius and are not downloadable. There is little reason to download these tables since wire errors in this temperature range are very small (defined as zero at zero degrees C). The tables support thermocouple types E, J, K, N, R, S, and T. Only these thermocouple types can be EU converted, even when using downloaded custom thermocouple EU conversion because of the need for reference junction compensation. The custom thermocouple capability is aimed at calibrated wire of one of these seven thermocouple types.

When setting up a scan list which includes a channel measuring custom thermocouples, the E1413 must be told that the channel requires reference junction compensation, and which of the compensation tables should be used for that channel. This can be done using the SCPI command SENS:FUNC:CUST:TCtype,range,list. In this case, the type parameter selects the compensation voltage table inside the E1413 to be used for reference junction compensation. It must be one of E, J, K, N, R, S, T. The use of the "TC" tells the E1413 that channel is measuring a thermocouple, and thus turns on the compensation function.

Summary of custom thermocouple wire steps:

1. Generate the table of EU coefficients for the channel. The eu_build C program provides an example of coefficient generation.
2. Download them using the DLAG:CUST:PIECewise SCPI command. The eu_build C program provides an example of coefficient download.
3. Assign the channels using the custom conversion with the SENS:FUNC:CUST:TC command. This turns on reference junction compensation, using the table appropriate for the thermocouple type in the command.
4. Don't forget to designate the channel which is measuring the tempearture of the reference junction. This is done using the SENS:REF command, and is the same with custom thermocouples as it is with library thermocouples. Remember that reference temperature must have been measured before the thermocouple channels are measured so reference temperature will be correct when the thermocouple is EU converted. This only requires that the reference temperature channel must precede the thermocouple channel in the scan list.

Case 2: Custom Calibrated RTDs for Measurement of Reference Junction Temperature

To measure the temperature of the reference junction itself requires a transducer capable of absolute temperature measurement. The E1413 supports three such transducers, the 5000 ohm thermistor, and two 100 ohm RTDs (type "385" and type "392"). When excited with the built-in 122 microamp current source, the E1413 library EU conversions generate temperature output, and simultaneously update the reference temperature register in the EU converter. The equations used for this conversion are based on nominal performance of the transducers, and every individual transducer will depart from the nominal behavior in some small way. To reduce the errors associated with manufacturing variations in the transducers, some customers will calibrate or purchase individually calibrated transducers, and will want to generate EU conversion tables for the particular transducer.

When custom transducers are used for normal temperature measurement, the eu_build C program can be used in its present form to generate and download EU tables to the E1413. However, if the custom transducer is to be used to measure reference junction temperature, more restricted conditions apply to the generation of these tables. Also, a special SCPI command is used to designate these channels as reference channels. To understand how this works, we need to examine how reference temperature is used in the E1413.

The E1413 uses a hybrid table lookup/interpolation technique to convert voltage readings into Engineering Units (EUs) at full measurement speed. The technique uses the familiar Y = MX + B equation to generate EU, and the "voltage space" of the 16-bit ADC is divided into 128 different segments, each having its own M and B coefficients. To maximize resolution and accuracy, the technique also includes an exponent factor which is used to scale the "M" and "B" coefficients to appropriate size for the particular voltage segment. This factor, which we will call "EX" is selected to maximize either M or B to fill their available number space (16 or 32 bits, respectively).

When the HP E1413 is measuring thermocouples, the temperature of the reference junction panel (where the thermocouples connect to copper wire) must be known to correctly calculate thermocouple temperature. This reference temperature is stored in a special format in a register of the E1413 DSP chip, and is used in calculation of all thermocouple channels measured later. The reference junction temperature may be measured in real time, by designating a channel (or channels) as reference channels. When a reference channel is measured, its temperature is not only recorded in the FIFO and/or CVT, its temperature is also recorded in the reference temperature register of the DSP chip.

To allow the EU conversion to execute quickly, this reference temperature is stored in a special format, and is not in the full IEEE-754 Standard 32-bit floating point format. This integer number is extracted from the EU conversion process at an intermediate stage of conversion. To assure that this intermediate integer value is in the correct scale, the EX or exponent coefficient must have a fixed value of 8B (hexadecimal). The eu_build.c program selects the value for EX based on the size of M and B, and so must be modified to force EX to the fixed value and allow M and B to scale appropriately.

Generating EU Coefficients for Reference Measurement

The C program for generation of Custom EU conversion tables eu_buil.c may be used for generation of reference junction measurement coefficients with minor modification. The modification is to add one line as follows:

 if (exponent2 < exponent) exponent = exponent2;

 exponent - 18; Xfactor = (int32) exponent;

The modification will force scaling of the M and B coefficients to the fixed scale factor, and allow extraction of reference temperature. When coefficients for a reference channel are downloaded, use the DIAG:CUST:PIECewise command as above. Use the following process to generate EU coefficient for Custom Reference Transducers.

1. Generate the table of EU coefflcients for the channel. The eu_build.c program provides an example of coefficient generation, but use the above code modification to lock the EX coefficient at the required fixed value.
2. Download the coefficients using the DIAG:CUST:PIECewise SCPI command. The eu_build.c program provides an example of coefficient download.
3. Assign the channels using the custom reference with the SENS:FUNC:CU command. This causes the E1413 to update the reference temperature register whenever this channel is measured.

Checking the EU Coefficients

To check the correctness ofthe table, it must be downloaded into the E1413A/B with the DIAG:CUST:PIECewise SCPI command, and designated as "reference channel" type with the SENS:FUNC:CUST:REF SCPI command before measurement. This second command sets a flag on the channel so that the E1413 will recognize the channel as reference and extract the temperature and store it at the appropriate stage of EU conversion.

The second step is to initiate and trigger the E1413 on a scanlist containing the reference channel, then, using a diagnostic command, dump the value of the internal temperature register into the FIFO for examination. This confirms that (a) the conversion coefficients are correct, (b) the fixed Exponent value is correctly scaling the intermediate value, and (c) the reference flag is being set and the intermediate value is being extracted and stored correctly. Because the result of the diagnostic command is put into the FIFO, it is a good idea to clear the measurements out of the FIFO before the diagnostic command is issued. Use the following SCPI sequence to perform the check:

DIAG:CUST:PIECewise ,,(~100) - Download the new table

SENS:FUNC:CUST:REF ~range~ 100) - designate channel 100 as reference

ROUT:SEQ:DEF LISTl,(~100,100) - set up scan list sequence

INIT;TRIG;DATA:FIFO? - trigger a scan and unload the data

DIAG:CUST:REF:TEMP - dump reference temp register to FIFO

DATA:FIFO? - read the diagnostic data value