The thermistor on this Xenta shows the temperature jumping when the OP power wires are connected

Issue

There is no explicit documentation which explains methods of wiring an STR sensor to an Xenta 100, 280 or 300 other than which wires connect to which terminals. If OP power wires are in the same bundled cable being used for the sensor and override button and potentiometer, it can cause temperature swings in the readings.

Product Line

TAC Vista

Environment

  • Xenta Operator's Panel (OP)
  • Xenta Programmable controllers
  • Xenta 281, 282, 283, 301, 302, 401

Cause

The problem is that when you connect either one of those wires for OP power to the controller, you have created a capacitor between the signal wire and the OP power. This can cause temperature swings of typically 0.5°F and up to 2.5 or 3°F.

Resolution

Ideally you'll separate that pair of wires from the others by at least a few inches from the bundle. Even if they are in the same conduit, the reading will still be better with OP power on a separate pair from the bundled cable. Twisted temp sensor wire is also preferable for minimizing this effect.

What about the sensors that are already connected this way? You can eliminate most of the variation by installing a 33 microFarad capacitor on the thermistor input terminals of the Xenta in parallel with the sensor. This should be a 10V or higher, low leakage cap (e.g. tantalum) and you should be careful to observe the polarity.