HOW TO TROUBLESHOOT VW INSTRUMENTS
Vibrating Wire instruments are not only robust and reliable but very easy to identify any problems in the field through simple resistance diagnostics with readily available equipment.
Where damage to a sensor or cable is suspected this guide illustrates the way in which simple resistance checks can be taken to identify the possible cause of the problem.
Resistance checks can be made with most types of multi-meter which are readily available in the market.
RESISTANCE OF THE COILS
STEP 1
Set the range to 200Ω (or Ω if using a multi-meter which has automatic ranging).
STEP 2
Connect the VW+ (red) conductor to the red lead on the multi-meter and the VW- (black) to the black lead on the multi-meter.
The correct readings should be as follows ±10%
| Pressure Transducer | ~ 160Ω |
| Strain gauge | ~ 180Ω |
| Load Cell / Sister Bar / Miniature Strain gauge | ~ 50Ω |
IF THE VALUES ARE OUT OF THESE RANGES THEN THERE IS A FAULT IN THE CABLE OR LESS LIKELY THE COILS
RESISTANCE OF THE THERMISTOR
STEP 1
Set the range to 20kΩ (or Ω if using a multi-meter which has automatic ranging).
STEP 2
Connect the T+ (green) conductor to the red lead on the multi-meter and the T- (white) to the black lead on the multi-meter.
The readings will be dependent on the temperature as below:-
10°C ~ 5.971kΩ (5971Ω)
15°C ~ 4.714kΩ (4714Ω)
20°C ~ 3.478kΩ (3478Ω)
25°C ~ 3.000kΩ (3000Ω)
PLEASE REFER TO THE THERMISTOR LOOK UP TABLE BELOW
IF THESE VALUES DIFFER THEN THERE IS A PROBLEM WITH THE THERMISTOR OR IT’S CONNECTING CABLE
USING STEINHART & HART LOG
Thermistor Type: 3K @25°C
Resistance/ temperature equation:-
T= (1 / (A + B (LnR) + C(LnR) 3)) –273.2
Where:-
T = Temperature in degrees Centigrade
LnR= Natural log of Thermistor resistance.
A= 1.4051* 10-3
B= 2.369*10-4
C=1.019*10-7
(Applicable to the range –50 to + 150 Centigrade only )
Resistance versus temperature table
| Temperature | Resistance | Temperature | Resistance | Temperature | Resistance |
| (°C) | (kΩ) | (°C) | (kΩ) | (kΩ) | (°C) |
| -40 | 128.75 | 41 | 1.509 | 122 | 0.117 |
| -39 | 119.81 | 42 | 1.449 | 123 | 0.114 |
| -38 | 111.54 | 43 | 1.392 | 124 | 0.111 |
| -37 | 103.89 | 44 | 1.338 | 125 | 0.108 |
| -36 | 96.822 | 45 | 1.286 | 126 | 0.106 |
| -35 | 90.276 | 46 | 1.236 | 127 | 0.1032 |
| -34 | 84.213 | 47 | 1.189 | 128 | 0.1008 |
| -33 | 78.596 | 48 | 1.144 | 129 | 0.0984 |
| -32 | 73.389 | 49 | 1.100 | 130 | 0.0961 |
| -31 | 68.560 | 50 | 1.059 | 131 | 0.0939 |
| -30 | 64.079 | 51 | 1.019 | 132 | 0.0917 |
| -29 | 59.920 | 52 | 0.982 | 133 | 0.0896 |
| -28 | 56.057 | 53 | 0.946 | 134 | 0.0875 |
| -27 | 52.468 | 54 | 0.911 | 135 | 0.0856 |
| -26 | 49.132 | 55 | 0.878 | 136 | 0.0836 |
| -25 | 46.029 | 56 | 0.846 | 137 | 0.0817 |
| -24 | 43.142 | 57 | 0.816 | 138 | 0.0799 |
| -23 | 40.455 | 58 | 0.787 | 139 | 0.0781 |
| -22 | 37.953 | 59 | 0.759 | 140 | 0.0764 |
| -21 | 35.621 | 60 | 0.732 | 141 | 0.0747 |
| -20 | 33.448 | 61 | 0.707 | 142 | 0.0730 |
| -19 | 31.422 | 62 | 0.682 | 143 | 0.0714 |
| -18 | 29.531 | 63 | 0.659 | 144 | 0.0699 |
| -17 | 27.767 | 64 | 0.636 | 145 | 0.0684 |
| -16 | 26.119 | 65 | 0.614 | 146 | 0.0669 |
| -15 | 24.580 | 66 | 0.594 | 147 | 0.0655 |
| -14 | 23.142 | 67 | 0.574 | 148 | 0.0641 |
| -13 | 21.797 | 68 | 0.554 | 149 | 0.0627 |
| -12 | 20.539 | 69 | 0.536 | 150 | 0.0614 |
| -11 | 19.362 | 70 | 0.518 | 151 | 0.0601 |
| -10 | 18.260 | 71 | 0.501 | 152 | 0.0588 |
| -9 | 17.229 | 72 | 0.485 | 153 | 0.0576 |
| -8 | 16.262 | 73 | 0.469 | 154 | 0.0564 |
| -7 | 15.356 | 74 | 0.454 | 155 | 0.0552 |
| -6 | 14.506 | 75 | 0.440 | 156 | 0.0541 |
| -5 | 13.709 | 76 | 0.426 | 157 | 0.0530 |
| -4 | 12.961 | 77 | 0.412 | 158 | 0.0519 |
| -3 | 12.259 | 78 | 0.399 | 159 | 0.0509 |
| -2 | 11.599 | 79 | 0.387 | 160 | 0.0498 |
| -1 | 10.979 | 80 | 0.375 | 161 | 0.0488 |
| 0 | 10.397 | 81 | 0.363 | 162 | 0.0479 |
| 1 | 9.849 | 82 | 0.352 | 163 | 0.0469 |
| 2 | 9.334 | 83 | 0.341 | 164 | 0.0460 |
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