Key Technical Specifications
| Parameter | Specification |
|---|---|
| Sensor Type | 2-electrode contacting conductivity cell |
| Cell Constant | 0.1/cm or 1.0/cm (pre-calibrated, factory-set) |
| Measurement Range | 0.4–2,000 µS/cm (0.1 constant); 4.0–5,000 µS/cm (1.0 constant) |
| Temperature Element | Built-in Pt1000 RTD for automatic temperature compensation |
| Pressure Rating | 7 bar / 100 PSIG maximum |
| Maximum Temperature | 100°C / 212°F |
| Wetted Materials | 316 Stainless Steel body and electrodes |
| Installation Connections | 3/4″ NPT fitting; 1″, 1.5″, or 2″ Tri-Clamp options |
| Integral Cable | No — requires external cable to analyzer/transmitter |
| Compatible Analyzers | Yokogawa SC402, FU20, FU24, FLXA202/203 series |
| Form Factor | Small Bore (compact insertion-style probe) |
Product Introduction
I’ve installed dozens of these on boiler blowdown lines where the water chemistry would eat a lesser sensor alive in a week. The SC42-SP34 is Yokogawa’s workhorse for low-to-mid range conductivity measurement in power generation and water treatment. It’s not fancy — just two stainless steel electrodes, a Pt1000 for temperature compensation, and a body that doesn’t corrode. That’s exactly why it works.What keeps this sensor in rotation after all these years is its simplicity and the pre-calibrated cell constant. You don’t need to field-calibrate it against a known standard every time you swap it. The 316SS construction handles typical boiler water chemistry without pitting, and the Pt1000 gives you accurate temperature-compensated readings even when the process temperature swings. One thing to note: the “-SP34” suffix denotes a specific process connection and cable configuration. Verify the connection type (NPT vs. Tri-Clamp) against your existing installation before ordering — I’ve seen guys order the wrong fitting and spend a half-day machining an adapter they didn’t need.
Quality SOP & Tech Pitfalls
The Lab Report (SOP):
Every SC42-SP34 goes through a four-step check before shipping. Visual inspection of the electrode surfaces under magnification — we’re looking for pitting, scaling, or coating buildup that would throw off the cell constant. We then run a resistance check across the two electrodes with a Fluke 115 to verify the internal connections are intact and the insulation between electrodes hasn’t degraded. The Pt1000 element gets tested at room temperature to confirm it reads within ±0.3°C of a reference thermometer. Finally, we verify the process connection threads are clean and undamaged, then seal the sensor in anti-static packaging with the cell constant value tagged on the outside.The Engineer’s Warning (Pitfalls):
Two things will give you garbage readings within hours of installation. First: cable length matters. The SC42 has no integral cable — it relies on an external cable to the analyzer. If you’re using a cable longer than what the analyzer’s cell constant calibration accounts for, the cable resistance introduces measurement error. Yokogawa specifies a maximum cable length (typically 10–30 meters depending on the analyzer model). Check the analyzer manual. Second: electrode coating. In boiler blowdown applications, iron oxide and hardness salts will plate onto the electrode surfaces over time. This acts as an insulating layer and drives your reading low. If your readings start drifting downward while the process conditions haven’t changed, pull the sensor and inspect the electrodes. A light acid wash (dilute HCl) usually clears it up. If the coating is baked on, the sensor is done — replace it.
Installation & Configuration Guide
- Pre-Installation ⚠️
- Isolate the process line and depressurize the sample point. Verify zero pressure with a gauge.
- Confirm the process temperature is below 100°C before proceeding.
- Photograph the existing sensor’s installation — note the immersion depth, flow direction, and cable routing. This prevents rework.
- Removal
- Disconnect the cable from the analyzer/transmitter end first. Label both ends.
- Use the appropriate wrench to unthread the sensor from the process fitting (3/4″ NPT) or release the Tri-Clamp ferrule.
- Pull the sensor straight out of the fitting. Inspect the fitting threads for damage.
- Installation
- Verify the cell constant printed on the new sensor’s tag matches the value programmed in your analyzer. This is critical — a mismatch between the physical cell constant and the analyzer setting will give you a proportional error across the entire range.
- Apply a thin layer of high-temperature thread sealant (Teflon tape or pipe dope rated for 100°C+) to the NPT threads. Do not get sealant on the electrode surfaces.
- Thread the sensor into the process fitting hand-tight, then tighten with a wrench — do not overtighten. For Tri-Clamp connections, ensure the gasket is seated and the clamp is fully closed.
- Power-On & Testing
- Reconnect the cable to the analyzer. Verify the wiring matches the analyzer’s terminal diagram (typically two conductors for the electrodes and three for the Pt1000).
- Power up the analyzer and allow it to stabilize for 5–10 minutes.
- Enter the cell constant value from the sensor tag into the analyzer’s configuration menu.
- Verify the reading against a grab sample tested on a portable conductivity meter. If the readings are within 5% of each other, the installation is good.
Compatible Replacement Models
- ✅ Drop-in Replacement: SC42-SP34 (identical model), SC42 small bore variants with matching cell constant (0.1/cm or 1.0/cm) and matching process connection (3/4″ NPT or Tri-Clamp size). Verify the cell constant tag before installing — no analyzer reconfiguration needed if it matches.
- ⚠️ Software Compatible: SC4A(J) series — similar 2-electrode design but with different temperature limits (up to 110°C) and an integral cable option. Requires analyzer reconfiguration for the different cell constant and cable length. Budget 30 minutes for setup.
- ❌ Hardware Mod Required: SX42 large bore series — different physical dimensions, higher pressure/temperature ratings (up to 40 bar / 250°C), and requires a different process fitting (1″ NPT or 2″ ANSI flange). Only consider this if your process conditions exceed the SC42’s 7 bar / 100°C limits.
Frequently Asked Questions (FAQ)
Can I use the SC42-SP34 in ultrapure water applications?
Not recommended. The SC42 small bore is designed for low-to-mid conductivity ranges (0.4–5,000 µS/cm). For ultrapure water (below 1 µS/cm or above 1 MΩ·cm), you need a sensor with a 0.01/cm cell constant — look at the SC42 large bore or SC4A(J) series instead. Using a 0.1 or 1.0 constant cell in UPW will give you noisy, unstable readings at the low end.How often should I recalibrate this sensor?
The cell constant is factory-set and doesn’t drift under normal conditions. You don’t “recalibrate” the sensor itself — you verify the analyzer’s reading against a known standard solution or a grab sample. In boiler blowdown service, I’d check it monthly. In cleaner water service, quarterly is fine. If the reading starts drifting, inspect the electrodes for coating first before assuming a calibration issue.What cable should I use to connect this to the analyzer?
Use Yokogawa’s recommended shielded cable (part number varies by analyzer model). The cable needs two conductors for the electrode signal and three conductors for the Pt1000 RTD. Keep the cable away from power cables and VFD output leads to avoid noise pickup. If you’re running longer than 10 meters, check your analyzer’s manual for the maximum supported cable length — beyond that, you’ll need a 4–20 mA transmitter mounted near the sensor.Will this sensor work with a non-Yokogawa analyzer?
Yes, as long as the analyzer supports 2-electrode contacting conductivity sensors with a Pt1000 temperature element and allows you to enter a custom cell constant. Most modern conductivity analyzers do. You’ll need to wire it according to the analyzer’s terminal diagram — the electrode pair goes to the conductivity input terminals, and the Pt1000 goes to the RTD input terminals.What’s the difference between the 0.1/cm and 1.0/cm cell constant versions?
The cell constant determines the measurement range. A 0.1/cm cell is for lower conductivity ranges (0.4–2,000 µS/cm) — think boiler feedwater, condensate, or RO permeate. A 1.0/cm cell is for higher ranges (4.0–5,000 µS/cm) — think boiler blowdown or cooling water. Using the wrong cell constant for your application will either give you poor resolution at the low end (1.0 constant in low-conductivity water) or saturate the reading at the high end (0.1 constant in high-conductivity water).Can I clean this sensor with an acid wash?
Yes, but be careful. A dilute HCl solution (5–10%) will remove iron oxide and hardness scale from the electrode surfaces. Soak the sensor for 10–15 minutes, then rinse thoroughly with deionized water. Do not use abrasive cleaners or wire brushes — you’ll scratch the electrode surfaces and change the cell constant. If acid washing doesn’t restore the reading, the sensor is done. Replace it.
