A TXV sensing bulb and an external equalizer may both connect near an evaporator outlet, but they carry different information. The bulb responds to outlet temperature; the equalizer conveys outlet pressure. For a 313A study diagram, label those two inputs before trying to explain how the valve maintains superheat.
What does the sensing bulb tell the valve?
Danfoss describes three forces at a thermostatic expansion valve: bulb pressure, evaporator pressure and spring pressure. The bulb is attached at the evaporator outlet and responds to suction-line temperature. Its charge acts on the diaphragm as temperature changes; it does not measure suction pressure. The bulb placement guide covers where to mount it on a real line; this exercise asks which variable it represents.
What does an external equalizer add?
An internally equalized TXV senses evaporator pressure at the valve outlet. Danfoss says an externally equalized valve uses a line connected near the evaporator outlet, so the valve responds to pressure after a distributor and evaporator pressure drop. On a multi-circuit evaporator with meaningful pressure loss, that difference matters: pressure at the valve outlet is not necessarily the same as pressure where the bulb senses temperature. The equalizer is a pressure connection, not a second temperature sensor.
How do you read a simple TXV diagram?
Draw a distributor and evaporator between the valve and suction outlet. Put a “T” beside the bulb on the outlet line and a “P” beside the external equalizer connection. If the diagram shows a pressure drop through the distributor and coil, the external line supplies the downstream pressure that pairs with the bulb's downstream temperature when the valve responds to superheat. The spring supplies the opposing setting force. This is a reasoning sketch, not a valve-sizing or adjustment instruction; verify the equipment manufacturer's valve and piping directions before field work.
The superheat guide reviews the temperature-pressure idea. Red Seal still labels the previous standard as the current 313A exam basis on 2026-10-08. Try the free original, unofficial sample for a separate concept check; its draft questions await certified-mechanic review and do not predict a pass.
Sources
- Danfoss: How thermostatic expansion valves workThree forces and internal versus external equalization; checked 2026-10-08
- Danfoss: Thermostatic Expansion Valve TroubleshootingBulb and equalization as distinct troubleshooting inputs; checked 2026-10-08
- Red Seal: Refrigeration and Air Conditioning MechanicCurrent versus newer exam-standard status; checked 2026-10-08
Examen Studio is independent and not affiliated with the Red Seal Program or Skilled Trades Ontario. This article is a study aid; on the job, follow current codes, regulations and manufacturer instructions.