Why You Should Never Splice the Temperature Sensor Wire

A grower lost an entire greenhouse crop a few years ago. The cause traced back to a single splice in a temperature sensor wire.

Someone had cut the sensor wire to reroute it, twisted the ends together, and wrapped the connection with electrical tape. It worked fine for a few months—until water worked its way into the splice. The copper corroded, the controller lost accurate temperature readings, and by the time the problem was noticed, the crop was gone.

This was not a freak accident. It is the predictable result of splicing a sensor wire in a greenhouse environment.

What Our Temperature Sensor Actually Is

The temperature sensors we ship with our AegisTEC, AegisTEC+, and VCU2-24 controllers (and have used since 2010) are thermistors pre-wired on 100-foot or 150-foot cables.

The controller does not read temperature directly. It sends a small current through the two wires, measures the resistance of the thermistor at the far end, and converts that resistance value into a temperature reading.

This detail is critical: The entire circuit—from the controller all the way to the sensor tip—is part of the measurement. Any added resistance in the loop creates an inaccurate temperature reading.

Why a Splice Is a Problem

A fresh splice might add only a tiny amount of resistance—often too little to notice at first. The real trouble begins once the connection sits in a greenhouse for weeks or months.

Greenhouses are among the harshest environments for electrical connections: high humidity, frequent irrigation overspray, dripping condensation, fertilizer salts in the air, and wide day-to-night temperature swings that draw moisture into even tiny gaps. Water eventually finds its way into almost any field-made splice.

Once moisture reaches the copper, three things typically happen—all of which distort the sensor reading:

• Oxidation increases resistance. The controller reads a falsely low temperature. Vents may stay closed when they should open, or heaters may run when they should stay off.
• Resistance becomes unstable. As moisture levels fluctuate, the reading drifts unpredictably. The controller appears “flaky,” but the real culprit is the splice.
• Complete failure. Corrosion eventually severs the wire, creating an open circuit. Most controllers interpret this as either a freezing or overheating condition and react accordingly—often with disastrous results.

In the case that inspired this article, the sensor went fully open. The controller followed its programmed response to a missing sensor, and the crop did not survive.

What to Do Instead

Do not splice the sensor wire.

We offer sensors in 100-foot and 150-foot lengths for good reason. If the cable is longer than you need, simply coil the extra length neatly and secure it out of the way. Extra wire causes no issues. A splice in the middle does.

• Need a longer run than 150 feet? Call us first. We can recommend the best layout options for your specific setup and help you avoid problems down the road.
• Wire gets damaged (by rodents, equipment, boots, etc.)? Replace the entire sensor. The cost is minimal compared to the risk of losing a greenhouse full of crops.
• Emergency field repair needed (e.g., to get through a weekend)? Use a high-quality waterproof connection such as a gel-filled splice, adhesive-lined heat-shrink tubing, or a direct-burial-rated sealed butt connector. Treat it strictly as a temporary fix and replace the sensor as soon as possible.

The Bottom Line

A temperature sensor is one of the least expensive components in a greenhouse, yet it controls every other expensive system—vents, heaters, fans, curtains, and more. When the sensor reports inaccurate data, the entire environmental control system makes wrong decisions, and the crop pays the price.

Treat the sensor wire with the respect it deserves. No splices. No shortcuts. No “creative” routing held together with electrical tape. If the wire is damaged or compromised, replace the sensor.

If you have questions about your installation, or if you currently have a splice in a sensor wire and want advice on how to handle it, please call us. We would much rather help you prevent a problem than help you recover from one.