Fann 35 and RheoSense, Spot Measurements and Automated Fluid Monitoring
Updated: Oct 7
Better fluid insight starts with understanding the measurements.
Welcome to the first article in IMS’ eight-part series, Drilling Fluid Monitoring: From Measurements to Decisions. Over the coming weeks, we’ll explore spot checks, automated monitoring, particle handling and what the measurements mean for drilling operations.
We begin with a comparison of Fann 35 and RheoSense.
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Drilling fluid properties can change between scheduled mud checks. For operators, understanding those changes means considering how measurements are made, how frequently they are available and how they represent the fluid being circulated.
The Fann 35 provides spot measurements of a collected sample. Intelligent Mud Solutions’ RheoSense provides automated measurements of a flowing sidestream, with measurement points updated every 5 seconds during operation. Complete rheology profiles and gel results follow their respective measurement cycles. Both provide rheological information through different measurement methods and operational workflows. [1, 2]
Comparing the same features
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Comparison | Fann 35, standard R1/B1 configuration | IMS RheoSense |
Rheology measurement method | Measures torque on a bob inside a rotating sleeve. | Derives rheological properties from measured flow rate and pressure drop across pipe sections. |
Fluid supplied for measurement | A sample is collected and placed in the instrument cup. | Fluid is supplied automatically from the connected mud system. |
Measuring geometry | Concentric sleeve and bob, with a radial gap of approximately 1.17 mm (0.046 in). | Open pipe bore within a closed fluid loop. |
Measurement programme | Six or twelve rotational speeds, depending on model. | Measurements at 16 flow velocities within the operating programme. |
Particle handling | Suitability depends on particle size relative to the measuring geometry. Wider rotor and bob combinations are available for coarser materials. | IMS specifies particle handling up to 6 mm (approximately 0.236 in), subject to suitability for the particular fluid and installation. |
Data availability during operation | Readings are available during each spot test. Sampling and testing frequency follow the operating programme. | Measurement points update every 5 seconds. Complete rheology profiles and gel results follow their measurement cycles. |
Reported properties | Rheological readings and gel strength. Density and other fluid properties require separate instruments. | Rheology and gel strength, alongside density, temperature and pressure, depending on configuration. |
Operator workflow | Collect and prepare samples, operate the instrument and record results. | Inspect and maintain the system, review automated records and perform reference checks. |
Comparison basis: Fann manufacturer documentation [1], IMS product and technical information [2], and research on measuring fluids containing coarse lost circulation materials [4]. Alternative Fann geometries are available.
The five-second interval describes measurement point updates. Complete rheology profiles and gel results follow their respective measurement durations.
Understanding sample volume, measuring geometry and throughput
A standard Fann 35 test starts with 350 mL (11.8 US fl oz) of fluid in the sample cup. The rheological measurement takes place in the annular space between the sleeve and bob. [1]
Geometry note: Using the published R1/B1 dimensions, the cylindrical annular volume is approximately 5 mL (0.17 US fl oz). This calculation represents the fluid volume in the cylindrical gap around the bob at any one time, excluding fluid above and below it. It does not define the total volume of distinct fluid contributing over an entire test. [1]
For the operating programme described here, RheoSense’s system throughput is approximately 1.2 m³ per hour, equivalent to 1,200 litres, 317 US gallons or 7.5 oilfield barrels per hour. This is the cumulative volume passing through the system over one hour. Throughput depends on the operating programme. [2]
RheoSense can connect to any part of the active mud system using a 3″ supply line and a 1″ return line. It automatically draws fluid through a closed loop with an open pipe bore, measuring flow rate and pressure drop at 16 flow velocities to determine rheological properties. The resulting data provide an ongoing record of fluid conditions at the selected sampling point. [2]
Converted figures are rounded. One oilfield barrel equals 42 US gallons.
Visibility between scheduled mud checks
In a February 2026 interview, Newpark described historical practice as two to three mud checks daily, corresponding to intervals of approximately eight to twelve hours. The interview highlighted its cooperation with Intelligent Mud Solutions to provide greater visibility into fluid conditions between scheduled checks. [3]
We value our cooperation with Newpark, combining their drilling fluid expertise with IMS’ automated fluid monitoring technology. We look forward to supporting oilfield operations alongside the Newpark team, helping operators and drilling crews access timely fluid information and make informed operational decisions.
The actual testing programme depends on the well, operating conditions and properties being measured. Additional spot checks may be required, and the Fann 35 can be used more frequently.
RheoSense complements this workflow by providing automated measurements and an ongoing record of the connected fluid stream between scheduled checks.
Why particle handling matters
The Fann R1/B1 radial gap of 1.17 mm (0.046 in) describes instrument geometry. It should not be interpreted as a maximum particle diameter that can be measured reliably.
Research on drilling fluids containing coarse lost circulation materials identifies limitations of the standard geometry and demonstrates the use of wider rotor and bob combinations for these applications. [4]
IMS specifies particle handling up to 6 mm (approximately 0.236 in), subject to the fluid and installation. This is an IMS supplied capability specification; suitability for a particular mud or lost circulation material programme requires application specific assessment. [2]
For either approach, assessment of a mud or lost circulation material programme should consider particle size, shape, concentration, carrier fluid and the sampling arrangement.
What this means for operators
With measurement points updated every five seconds, RheoSense builds an ongoing record of the connected fluid stream. Changes in that record can prompt investigation, support treatment decisions and improve information available during operational handovers. [2]
Spot measurements remain useful for reference checks and investigation. A meaningful comparison requires aligned sampling locations, timing, temperature and fluid conditioning, together with acceptance criteria appropriate for the actual mud system. Gel strength comparisons also require matched rest periods and an agreed test procedure.
Discuss your next drilling campaign with IMS
Evaluating automated monitoring for your operation? Share your fluid system, particle or lost circulation material requirements, sampling location and data requirements with IMS to discuss an evaluation scope and quotation.
References
1. Fann (2016). Model 35 Viscometer Instruction Manual, Revision P.
2. IMS. RheoSuite product information and technical information supplied by IMS for this article, 4 October 2026, including the 16 flow velocities, stated particle capability and confirmed operating throughput. Manufacturer supplied information.
3. Business Focus Energy (2026). Newpark, Fluid Potential.
4. Kulkarni et al. (2016). Predicting Rheology of Drilling Fluids Containing Large Sized LCMs, AADE-16-FTCE-67.
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