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Westach General Information - Instrument Calibration Checking

Checking Operation / Accuracy of Westach Instruments

CALIBRATION ACCURACY CHECK FOR CONTINUED AIRWORTHINESS

TSO'D AIRCRAFT INSTRUMENTS

(FORM 200-1 10/98) (MAY BE USED FOR OUR NON-TSO'D INSTRUMENTS AS WELL).

Every instrument that Westach ship has it's calibration checked twice against N.B.S. / N.I.S.T. traceable standards. The following information is a basic description of the procedures that Westach use and is presented here for those who wish to check the accuracy themselves. Each of the product groups described below consist of an indicator (gauge) and a temperature of pressure sender or transducer. Under normal circumstances this system should give many years of accurate reliable service. Westach will try to provide here, simple inspection procedures to insure airworthiness. There are no user accessable calibration adjustments, however calibration may be checked for accuracy. For the sake of simplification, all Westberg Mfg. Inc. Instruments that hold approval to use the TSO, can be separated into three main groups for testing purposes:

(1) THERMOCOUPLE INSTRUMENTS: Includes: K28 Exhaust Gas Temperature (EGT), K29 Cylinder Head Temp (CHT), K31 Dual Exhaust Gas Temp. / Cylinder Head Temp. (EGT/CHT), K32 Dual Exhaust Gas Temp. (EGT/EGT), K33 Dual Cylinder Head Temp (CHT/CHT), and many NON-TSO'd models.

Operation: The system operates as follows: The thermocouple sender probe (CHT or EGT), is a device which generates a small voltage when the probe end is heated, which is proportional to the difference in temperature between the hot (probe) end and the cold (connector) end. The indicator gauge is powered directly by the voltage generated by the sender.

Needed equipment: Calibrated millivoltage source with output adjustable from 0 to 40 millivolts, a 2.5 ohm resistor (EGT calibration), a 1.5 ohm resistor (CHT calibration), accurate digital (or analog) multimeter.

Procedure: Connect patch cable to gauge as detailed in the instruction sheet that is packed with it. Do not connect the thermocouple probe for this test. Connect a resistor of 2.5 ohms for EGT, or 1.5 ohms for CHT, in series with one of the patch cable's leads (this is to simulate thermocouple resistance). Connect the other end of the resistor and the remaining patch cable lead to the millivoltage source. Use the appropriate chart (CHT or EGT) for the millivoltage settings needed to simulate thermocouple temperature. Check accuracy at five check points across the dial. If there is no reading, reverse polarity of input millivoltage. Accuracy should be within 2% of full scale at any given setting. Checking thermocouple probes (713 series EGT or CHT probes): Due to the unchanging nature of thermocouple characteristics, all that should ever be needed to check thermocouple probes, is a simple go/no go test; With the thermocouple disconnected from the gauge, connect an ohmmeter to the output pins of the thermocouple (the probe does not need to be heated for this test). The ohmmeter should be set to it's lowest resistance range. The ohmmeter should read about 1.5 ohms for a CHT probe or about 2.5 ohms for an EGT probe. If the ohmmeter shows an open circuit (infinite resistance), the probe is bad.

(2) THERMISTOR INSTRUMENTS: Includes: K30 Carburetor Temp., K34 Air Temp., K35 Oil Temp., K36 Water temp, and many NON-TSO'd models.

Operation: The system operates as follows: The 399 series temperature probe is a device which functions as a variable resistor, whose resistance changes in proportion to temperature. The indicator gauge contains a transient protected, temperature compensated integrated circuit voltage regulator, which powers an electronic resistance bridge circuit, that measures the probe's electrical resistance and displays that reading as temperature.

Needed Equipment: Source of D.C. power (12 to 24 volts) to power the gauge, an accurate resistance substitution box.

Procedure: Connect the gauge to the power and sender patch cables as detailed in the instruction sheet that comes with it. Do not connect the temperature sender for this test. Connect the resistance substitution box in place of the sender. With the instrument powered, use the chart for the resistance settings needed to simulate sender temperature. Check accuracy at five points across the dial. Accuracy should be within 2% of full scale.

Checking thermistor type temperature sender (399 series); With the sender disconnected from the gauge, connect an ohmmeter to the sender's output leads. Use the thermocouple chart to check the sender output resistance at known temperatures.

(3) PRESSURE INSTRUMENTS: Includes: K37 Fuel Pressure, K38 Pressure, K39 Oil Pressure, and many NON-TSO'd models (which use 'MM' or 'KV' type transducers).

Operation: The system operates as follows: The pressure transducer is an electronic device which outputs a voltage that is proportional to pressure. In the case of 'MM' transducers, that voltage is a millivoltage, which is then amplified by a circuit in the indicator unit. In the case of the 'KV' transducers, the voltage output is great enough to power the indicator meter directly. The indicator gauge contains a transient protected, temperature compensated integrated circuit voltage regulator, which powers a meter circuit, that measures the probe's electrical output and displays that reading as pressure.

Needed Equipment: Source of D.C. power (12 to 24 volts) to power the gauge, an accurate metered source of air pressure with a range as required by the instrument to be tested.

Procedure: Connect the system, including pressure sender as detailed in the instruction sheet that comes with it. With the system powered and the pressure sender (transducer) connected to an accurate metered, adjustable source of air pressure, check accuracy at five points across the dial. Accuracy should be within 2% of full scale.

(4) Voltmeters / Ammeters: Voltmeters may be checked using a variable DC millivolts applied.

CALIBRATION ACCURACY CHECK CHART FOR TSO'D INSTRUMENTS:

399 SERIES TEMP SENDER

 

DEG. F.

OHMS

DEG. C.

 

DEG. F.

OHMS

DEG. C.

 

-40.......
-35.......
-30.......
-25.......
-20.......
-15.......
-10.......
-5.........
0..........
5..........
10........
15........
20........
30........
32........
35........
40........
45........
50........
55........
60
........
65........
100940....
83500.......
70000.......
58900.......
49500.......
41800.......
35500.......
30000.......
25500.......
21885.......
18700.......
16200.......
13800.......
10370.......
9800.........
8980.........
7840.........
6840.........
5970.........
5230.........
4600
.........
4040.........
-40
-37.2
-34.4
-31.7
-28.9
-26.1
-23.3
-20.6
-17.8
-15.0
-12.2
-9.4
-6.7
-1.1
0.0
1.7
4.4
7.2
10.0
12.8
15.6

18.3
70........
75........
80........
85........
90........
95........
100......
110......
120......
130......
140......
150......
160......
170......
180......
190......
200......
210......
220......
250......
260......
3570.....
3150.......
2780.......
2475.......
2190.......
1960.......
1750.......
1400.......
1130.......
915.........
745.........
615.........
507.........
422.........
348.........
296.........
251.........
210.........
178.........
113.........
96...........
21.1
23.9
26.7
29.4
32.2
35.0
37.8
43.3
48.9
54.4
60.0
65.6
71.1
76.7
82.2
87.8
93.3
98.9
104.4
121.1
126.7

 

EGT (EXHAUST GAS TEMP)

MILLIVOLT OUTPUT FOR 'K' TYPE WIRE WITH COLD JUNCTION AT 75 DEGREES F. (CROMEL / ALUMEL THERMOCOUPLE)

  DEG. F.

MILLIVOLT

         
  200........
400........
600........
800........
1000......
1200......
1400......
1600......

1800......
2.87
7.36
11.91
16.58
21.31
26.03
30.70
35.24
39.67
         
 

CHT (CYLINDER HEAD TEMP)

MILLIVOLT OUTPUT FOR 'J' TYPE WIRE WITH COLD JUNCTION AT 75 DEGREES F (IRON / CONSTANTAN THERMOCOUPLE)

  DEG. F. MILLIVOLT          
  100.......
200.......
300.......
400.......
500.......
600.......
700.......
 
.72
3.69
6.72
9.81
12.90
15.96
19.04