Steps In MTP Post-Campaign Data Analysis

MJ Mahoney

2. Calibrate Platinum RTDs

The MTP uses platinum resistance temperature detectors (Pt RTDs) to measure the target and window temperatures. Unlike the thermistors used to measure other engineering temperatures in an MTP, RTDs have a linear (rather than logarithmic) response to temperature changes, and are more appropriate when large temperature changes are possible. (Thermistors are easier to implement if the temperature changes are expected to be small.) Because the target temperature must be known accurately for good gain calibration, their calibration should be checked before and after every field campaign.

RTD_calibration

Figure 1. PT-539AW Calibration Curve

Pt RTDs have a well-defined relationship between resistance and temperature, which is adequately approximated by the third-order polynomial fit shown in Figure 1 for the temperature range of -100 C to +45 C appropriate for MTP applications. A constant current source is used to pass a current through the RTD and the resulting voltage is measured and converted to counts. Thus, there is a linear relationship between counts and resistance, which when substituted into the above relationship between temperature and resistance establishes a relationship between counts and temperature.

Letting T(R) = A + B*R + C*R² + D*R³ (see Figure 1) and R(V) = k1 + k2 * V, where R is resistance and V is voltage (or counts), it is easy to show that:

T(V) = c0 + c1 * V + c2 * V² + c3 * V³

where
c0 = (A + B * k1 + C * k1 * k1 + D * k1 * k1 * k1)
c1 = (B * k2 + 2 * C * k1 * k2 + 3 * D * k1 * k1 * k2)
c2 = (C * k2* k2 + 3 * D * k1 * k2 * k2)
c3 = (D * k2 * k2 * k2)

What the calibration process checks is whether k1 or k2 have changed. This shouldn't happen, but it is important to KNOW that is hasn't happened. There are two or three connectors between the RTD voltage detector and the RTD itself, and resistance changes could potentially affect these constants. To check the calibration, precision wire resistors are inserted in place of the RTD over the relevant range of resistances, the counts noted, and a fit made for R(V). This could be done in a spreadsheet, but the entire process has been simplified on the Tools form of MTPbin.

Note that this procedure only checks the calibration of the detector circuit; it does not check whether the RTD is healthy. We recently added a second Pt RTD to the DC-8 MTP target: the original one was buried in the center of the target, and the new on was on the edge to check for temperature gradients in the target. Except during ascent and descent, gradients should not be present, and they certainly should not be present in the lab after allowing time for the target to equilibriate. After calibrating the DC-8 target circuit we discovered that the two RTDs differed by nearly 0.5 K in temperature. Clearly there was a problem. To sort this out, we have now inbedded a precision thermistor (with replacement accuracy <0.1 K) in all targets so that the integrity of the RTD can be verified. In the case of the DC-8 target we discovered that the new RTD was bad, and that the old RTD agreed with the thermistor to 10 mK. The thermistor temperature is derived by measuring it's resitance with an accurate voltmeter, and then using the Steinhart-Hart Equation (see C:\MTP\Excel\Steinhart_Hart_Equation.xls):

(1)    1/T = A + B ln(R) +C ln(R)³

where T is in K and R is in ohms, to convert the R values to T. We use a YSI 44032 thermistor which has a 0.1 K interchangeability over the range -80 C to +75 C, and a resistance of 30,000 ohms at 25 C. These thermistors have a thermometric drift of <0.01 C at temperatures below 0 C over 100 months, <0.02 C at temperatures <25 C over the same period! YSI provides a spreadsheet (see C:\MTP\Excel\44032_data.xls) with (R,T) pairs over a wide range of temperatures. By using three pairs of these temperatures over the range of temperature measurements in Equation (1) (we used -25 C, 0 C, and +25 C), and solving this set of linear equations for A, B, and C, we find:

    A= 0.000927034
    B= 0.000222241
    C= 0.000000124

Using these values, we found excellent agreement with the center RTD in the DC-8 target, and a 0.35 K bias in the edge RTD at -25 C. The bias was smaller at 20 C, and we would therefore expect it to be larger at the colder flight temperatures, in agreement with the observed bias of 0.47 K between the two RTDs.

The first step is to use a text editor to create a file containing resistance and counts measurements. It should have the following format:

9
H
591.01 F28
566.81 DF1
512.08 B2D
476.98 964
465.08 8C9
443.08 7AE
383.68 4AD
350.74 303
309.67 0EF

where the first line indicates the number of measurements (nine in this case), the second line contains an H or D (indicating whether the counts are hexidecimal or decimal numbers), and the remaining nine lines contain the resistance/counts pairs. Normally, the counts will be hexidecimal as this is what is provided on the D-line of the raw MTP output file. This file should have the following filename:

Pt_AC_xxxx_Y.txt

where AC is the two-letter aircraft designation (DC, ER, WB, etc.), xxxx is tgt1, tgt2 or win on the DC-8, or low, high or win on the other aircraft, and Y is either B or A (for measurements made Before or After a campaign). Note that the DC-8 has two target measurements (one on the center and one on the edge), while the ER-2 type instruments have only a single target RTD at the center of the target, but with low and high temperature ranges. (The cable between the ER-2 Data Unit and Sensor Unit does not have enough wires to implement the preferable DC-8 RTD configuration.)

The Pt_AC_xxxx_Y.txt file should be saved in the mission setup folder (e.g., C:\MTP\Data\DC8\PAVE\Setup\). For the PAVE campaign, the target 1 RTD calibration information made before the campaign would be saved as: C:\MTP\Data\DC8\PAVE\Setup\Pt_DC_tgt1_B.txt. Once this and the other RTD calibration files for tgt2 and win (or low, high and win) have been created, click on the Open Tools button on the bottom right side of MTPbin. Next, select the Pt Fit tab of the Tools form, and click the Import button in the Pt Fit Input frame. This will open a dialog box where the resistance/counts measurements that you just saved can be selected and imported. (Note that these measurements could have also been entered manually in the Pt Fit Input frame, but I find entering them in a text editor easier.)
Tools_Pt1

Figure 2. The Tools form Pt Fit tab after the Import button of the Pt Fit Input frame has been clicked.

Figure 2 shows the Resistance and Counts list boxes populated after the Import button was clicked. Note that although the numbers in the Pt_DC_tgt1_B.txt file are hexidecimal, the program knows this (because of the H on the second line) and converts them to decimal. Next, click the Do Fit button on the Fit frame. The fit results for R(counts) and T(counts) are shown in the Fit frame (see Figure 3).
Tools_Pt2

Figure 3. The result of depressing the Do Fit button on the Fit frame.

Finally, in the File Name frame select the Sensor Unit in the SU combo box, the RTD being calibrated, and whether it is Before or After. The Default Filename text box will reflect these choices. Then, depress the Write button in the Fit frame to write the default calibration file. The fit information will be appended to the file that you originally imported and will contain the date and time of the fit, and the R(counts) and T(counts) fit coefficients.

9
H
591.01 F28
566.81 DF1
512.08 B2D
476.98 964
465.08 8C9
443.08 7AE
383.68 4AD
350.74 303
309.67 0EF
03-30-2005 23:07:28
R(Counts) = c(0) + c(1)*Counts
c(0) = 291.218
c(1) = 0.07725
T(Counts) = c(0) + c(1)*Counts + c(1)*Counts^2 + c(1)*Counts^3
c(0) = -103.1573
c(1) = 0.037426
c(2) = 2.983E-7
c(3) = -9.070E-12

The R(counts) coefficients will tell you whether the calibration has changed, and the T(counts) coefficients are the numbers which must be entered into the appropriate "Thermistor" function in the Calibration module.

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