Replacing Omega HX94 RH/Temp Sensors with a Second Source

Second-Source Options for the HX94 Series

Before anything else, confirm what the project actually needs from the sensor: the feature set (RH plus temperature in one head), the mounting arrangement, and the electrical connection. Those three items, not the brand, are the fixed constraints. The HX94 Series is a relative humidity and temperature transmitter sold under the Omega label, and the practical question is which supplier can meet the same envelope with acceptable commercial behavior.

Two routes exist. The first is a direct cross-reference to an established humidity instrumentation vendor. The second is identifying the contract manufacturer behind the Omega label by opening a unit. Both are legitimate; they trade time against certainty differently.

Selection Criteria That Decide This Case

Rank the candidates on the criteria that will break a substitution in the field, in this order:

1. Output signal and scaling. An RH/T transmitter feeds an analog input card or a controller. If the replacement uses a different output type or a different engineering-unit span, every downstream scaling block changes. Match this first.
2. Supply voltage and wiring scheme. Loop-powered two-wire, three-wire, and four-wire devices are not interchangeable at the terminal strip. A mismatch here means panel rework, not just a probe swap.
3. Mounting geometry. Probe diameter, insertion length, thread or flange, and enclosure style dictate whether the existing penetration and bracket are reused.
4. Connection hardware. Connector type or cable gland size determines whether existing field cables plug in.
5. Range and accuracy. Check that the replacement covers the process RH and temperature span with equal or better stated accuracy across that span, not just at one calibration point.
6. Response time and sensor protection. Filter cap material and sensing element type set the response and the resistance to condensation or contamination.
7. Commercial terms. Lead time, minimum order quantity, and the vendor's willingness to publish a full datasheet without a sales call.

Do not move on to price until items 1 through 4 are satisfied. A cheaper head that forces a new AI channel type or a new duct cut is not cheaper.

Recommended Path: Spec Capture, Then Vendor Cross-Reference

Use the cross-reference route with Vaisala or Rotronic as the primary candidates, Michell as the process-grade fallback, and Dwyer as the budget option where the application is HVAC-class. Hold the teardown in reserve as an identification tool rather than the main plan, because it only pays off if the board carries a legible manufacturer mark and that manufacturer sells direct.

The reason to start with spec capture rather than catalog browsing: the failure mode on RH/T substitutions is almost never the RH sensing element. It is the output scaling, the supply topology, or a probe that is 20 mm too long for the existing fitting. Capturing those values from the installed unit turns the vendor search into a filter exercise.

Spec Capture Procedure on the Installed Unit

1. Photograph the nameplate and record the full model string including any dash-number options. The option suffixes encode output type, probe style, and range on this class of device.
2. At the controller, open the analog input configuration and record the signal type, the raw range, and the engineering-unit span assigned to both the RH channel and the temperature channel. This is the ground truth for output scaling; the nameplate alone will not tell you which span the integrator configured.
3. At the terminal strip, count conductors and identify the supply source. Measure the supply voltage at the transmitter terminals with the loop live. Note whether the transmitter is powered from the AI card loop supply or from a separate panel supply.
4. Measure the probe: outside diameter, insertion length from the mounting face, thread size or flange pattern, and enclosure dimensions. Photograph the mounting hardware in place.
5. Record the field connection: connector family and pin count, or cable gland thread and cable diameter.
6. Pull the process conditions from the historian or trend: minimum and maximum RH and temperature seen over a representative period. The replacement must cover this envelope with margin.
7. Note the filter cap type and whether condensation or particulate has been an issue. This drives the sensor protection choice on the replacement.

With this sheet in hand, filter each candidate vendor's catalog on output type, supply scheme, and probe geometry first. Only the survivors get a range and accuracy comparison.

Teardown for OEM Identification

If the cross-reference produces no acceptable mechanical match, or if the quantity is large enough that a direct OEM relationship would pay back, open one unit. Proceed only after the spec capture above is complete, because a damaged unit that has not been documented is a lost reference.

1. Use a spare or an end-of-life unit, not the one in service.
2. Open the enclosure without cutting; most heads in this class use a threaded cap or screws. Photograph each stage so reassembly is possible.
3. Inspect the circuit board for a silkscreened manufacturer name, a board part number, and an agency mark. Photograph both sides under good light.
4. Identify the sensing element package if visible; the element manufacturer is a separate lead from the assembly manufacturer.
5. Search the board part number and any manufacturer name against distributor and manufacturer catalogs. If the OEM sells the same assembly under its own label, request a datasheet and compare it line by line against the spec sheet.
6. Reassemble, then bench-test the unit against a reference before returning it to spares. Any drift after reassembly disqualifies it.

A board with no markings, or a marking that resolves to a contract assembler who does not sell direct, ends this route. Return to the cross-reference candidates.

Installation Verification of the Replacement

1. Bench-power the replacement at the measured supply voltage and confirm the output type and zero/span behavior against the AI card configuration before it goes near the field wiring.
2. Install one unit at the original mounting point using the existing penetration and hardware. Confirm the insertion depth matches the recorded value.
3. Land the field wiring and confirm supply voltage at the transmitter terminals under load.
4. At the controller, verify the RH and temperature values read within the replacement's stated accuracy of a portable reference placed at the probe. Check at the ambient condition, then at a second point if the process can be driven there.
5. Trend both channels alongside the neighboring sensor or the reference for a representative period covering the recorded process envelope. Compare response to a step change against the original unit's trend.
6. Only after the trend agrees across the envelope, release the part number for the remaining positions and update the spares list.

Frequently Asked Questions