How to Choose a Turbidity Sensor for Liquid Cooling: PG25, Accuracy and Installation

Choose a turbidity sensor for liquid cooling by checking the coolant formulation, performance at the normal operating turbidity, installation conditions and controller interface. For a PG25 loop operating near 0-5 NTU, a wide measurement range is only part of the specification: low-end accuracy, bubbles and mounting geometry deserve equal attention.

The RIKA SENSOR RK500-07LC uses a 90° nephelometric method and lists deionized water, PG25 and EG25 as applicable media. It offers 0-10 NTU and 0-100 NTU versions, M16 connector or direct-cable configurations, and simultaneous 4-20 mA and RS485 outputs.

Turbidity describes an optical response associated with light scattered by suspended material. In a liquid-cooling loop, a change can prompt investigation of fluid condition, filtration, recent maintenance or contamination.

The RK500-07LC measures scattered light using a 90° arrangement. Its reading provides a cleanliness trend; it does not independently identify particle composition, particle size, corrosion rate or glycol concentration.

Record a commissioning baseline after the loop is filled, mixed and operating under representative conditions. Investigate sustained changes alongside temperature, flow, maintenance records and other relevant fluid checks. Set alarms from the approved coolant and equipment requirements.

The RK500-07LC V5.2.0 specifications list deionized water, PG25 and EG25. They also identify 316L stainless steel, a sapphire measurement window and EPDM O-rings.

For PG25, record whether the nominal 25% concentration is by mass or volume. Identify the commercial coolant and its inhibitor or dye package, rather than specifying only the glycol type.

Review material compatibility and measurement performance

Material compatibility concerns how the coolant interacts with wetted components and seals. Measurement performance concerns how reliably the optical system reports turbidity in that fluid. Review both during selection.

For inhibited or coloured coolant, provide the supplier with the exact product information and operating conditions. Request evaluation using the intended formulation, particularly where small changes near the clean-fluid baseline matter.

Can concentration or colour affect readings?

The effect of coolant formulation should be assessed with the instrument's optical design and the actual fluid. A general PG25 media listing does not quantify the bias associated with every dye, inhibitor package or concentration change.

During evaluation, keep fluid identity, concentration, temperature and bubble conditions consistent. Compare repeated readings using an appropriate reference method and agreed acceptance criteria. Treat the fresh-coolant reading as a recorded baseline; follow the approved procedure for any zero adjustment.

RIKA specifies the RK500-07LC accuracy as:

±2% of reading or ±0.1 NTU, whichever is greater.

The stated resolution is 0.1 NTU. The following examples show how the accuracy formula applies:

These are calculations from the published specification, not a PG25 test dataset. Across 0-5 NTU, the ±0.1 NTU term determines the stated limit.

Figure 1. RK500-07LC stated accuracy limits at low turbidity, calculated from the published specification.

Separate accuracy, resolution, repeatability and drift

Resolution describes the measurement increment. Repeatability concerns the spread of repeated measurements under the same conditions. Drift concerns changes over time when the measured conditions remain stable.

For an application normally below 5 NTU, request repeated-reading and drift results in the intended coolant. Define the change the system needs to detect and the observation period. This gives the evaluation a useful target beyond the full-scale range.

The RK500-07LC provides both ranges. Its V5.2.0 specification lists the same resolution and accuracy expression for them.

Choose the span according to the expected clean-fluid readings and the abnormal conditions the monitoring system must capture. A 0-10 NTU range may cover a low-turbidity operating region, while 0-100 NTU provides additional measurement headroom.

When using 4-20 mA, confirm the configured full-scale value and check that the receiving input can resolve the changes of interest. Validate scaling at commissioning. For digital acquisition, confirm the reported units, data format and update behaviour.

A smaller range alone does not establish better repeatability, lower drift or better performance in PG25.

Installation can determine whether a reading represents the coolant or an avoidable optical disturbance.

Use stable flow and manage bubbles

RIKA recommends installation in a bypass pipe section for maintenance access, with stable flow and no obvious turbulence. The preferred arrangement is vertical upward flow. The horizontal positions shown in the installation drawing are also acceptable.

The instructions identify these locations to avoid:

• High-point gas traps.
• Low-point sediment zones.
• Vertical downward flow.
• Positions downstream of pressure-reducing sections where sudden pressure changes may cause degassing.

Keep bubble generation away from the measurement window. Air bubbles scatter light and can affect turbidity measurements.

Allow clearance from stainless steel pipe walls

For stainless steel pipes, RIKA specifies a probe-to-wall distance of at least 30 mm to reduce backscattering distortion.

The instructions also favour a chuck installation with the measuring holes aligned parallel to flow. Use the configuration-specific drawing to establish the probe position and orientation.

Reserve space for the M16 cable

For an M16 connection, allow at least 70 mm of cable-outlet clearance to avoid excessive bending. Include this space when checking access inside a compact CDU or manifold enclosure.

The specification does not provide a numerical flow-velocity requirement. Supply the proposed pipe dimensions and flow conditions for application review.

Figure 2. RK500-07LC installation positions and clearance requirements, adapted from the V5.2.0 datasheet.

The RK500-07LC supports 4-20 mA and RS485 at the same time, with a 7-30 VDC supply. This permits analog and digital acquisition where the receiving equipment supports the selected configuration.

Before approving the electrical design, request:

• M16 pin assignments and mating-connector details.
• Supply wiring and analog-loop requirements.
• The 4-20 mA transfer relationship for the ordered range.
• The RS485 protocol, communication settings and data/register documentation.
• Confirmation of Modbus RTU support where required.
• Configuration and calibration access requirements.

RS485 identifies the electrical interface; the protocol and available functions require their own documentation.

Distinguish electrical and process connections

M16 is the electrical connector choice. The listed process connections are G3/4, NPT3/4 and 50.5 chuck, with different sealing arrangements. Confirm the process fitting, insertion geometry and seal specification separately from the cable connector.

The default cable length is 5 m, with other lengths available by customization.

Check the operating envelope

The V5.2.0 sheet lists:

Confirm the permitted continuous pressure-temperature combination and seal conditions for the application. The pressure-resistance figure alone does not define a complete pressure-temperature operating curve.

Agree on calibration and servicing arrangements before installation. Request the approved standards, preparation steps, adjustment procedure and required software or controller.

Confirm whether calibration can be performed through the available interface or requires additional equipment. Simultaneous analog and RS485 outputs do not, by themselves, define the calibration workflow.

Establish a repeatable baseline

After installation, record the coolant product, concentration basis, temperature, range and stable operating readings. Include results from any agreed reference checks.

If readings change, inspect flow conditions, bubbles, the optical window and recent fluid additions before assigning a cause. Preserve comparable conditions when checking performance after cleaning.

Match cleaning to the assembly and coolant

Use a cleaning method approved for the sapphire window, 316L body, EPDM seals and actual coolant. Establish the inspection interval from site experience and verification results.

Request any replacement-seal, consumable and service-life information needed for maintenance planning. Include application-specific repeatability and drift results in the acceptance and maintenance plan.

Before ordering, confirm:

1. Coolant identity: Commercial product, PG or EG type, inhibitor/dye package and concentration basis.
2. Measurement requirement: Normal turbidity, required detectable change and expected abnormal values.
3. Range: 0-10 NTU or 0-100 NTU, with matching controller scaling.
4. Operating conditions: Temperature, pressure, pipe dimensions, flow and potential gas release.
5. Process connection: G3/4, NPT3/4 or 50.5 chuck, with the correct sealing and insertion arrangement.
6. Electrical configuration: M16 or direct cable, supply, simultaneous outputs and cable length.
7. Installation provisions: Representative flow, bubble control, stainless steel wall clearance and cable-outlet space.
8. Validation and service: Application test criteria, communication documents, calibration method and maintenance arrangements.

Product specifications cited in this article are from RIKA SENSOR's RK500-07LC V5.2.0 datasheet, revised 10 September 2026.