Thermistors

Thermistors are temperature-sensitive resistors that provide precise temperature monitoring in municipal water and wastewater treatment processes by changing electrical resistance in response to temperature variations. They operate through semiconductor materials that exhibit predictable resistance changes, typically offering ±0.1°C accuracy across municipal operating ranges of -10°C to 85°C. Common applications include monitoring digester temperatures, chemical feed systems, and process water heating. The primary limitation is their non-linear response curve, requiring signal conditioning electronics for accurate temperature conversion in SCADA systems.

• Digestor Temperature Control (5-50 MGD plants): Thermistors monitor sludge temperature in anaerobic digestors, typically maintaining 95-98°F for mesophilic operation. They connect upstream to heat exchangers and downstream to SCADA systems, providing faster response than RTDs for critical temperature control. Selected for their high sensitivity (±0.1°F accuracy) in the narrow operating range.
• Chemical Feed System Monitoring (0.5-20 MGD): Installed in sodium hypochlorite and polymer feed tanks to prevent degradation from temperature fluctuations. Thermistors trigger cooling systems when temperatures exceed 75°F, protecting chemical integrity. Their compact size allows installation in confined chemical storage areas where space is limited.
• Effluent Temperature Compliance (All sizes): Monitor final effluent temperature for NPDES permit compliance, particularly in plants with thermal discharge limits. Connected to data loggers for continuous recording, thermistors provide the precision needed for regulatory reporting while withstanding outdoor installation conditions.

Daily Operations: Operators monitor thermistor readings through SCADA displays, checking for temperature trends that indicate process upsets. Critical applications like digestor temperature require hourly logging during startup periods. Setpoint adjustments are typically made through the control system rather than at individual instruments, with operators verifying local display readings match SCADA values.

​

Maintenance: Monthly calibration checks using portable temperature calibrators ensure ±1°F accuracy. Quarterly cleaning of probe housings prevents biofilm buildup that affects response time. Annual replacement of connecting cables in harsh environments prevents moisture intrusion. Maintenance requires basic electrical safety training and standard PPE including safety glasses and electrical gloves rated for 480V systems.

​

Troubleshooting: Common failures include moisture infiltration causing erratic readings and cable damage from rodents or maintenance activities. Warning signs include sudden temperature jumps, readings that don't respond to actual temperature changes, or loss of 4-20mA signal. Typical service life is 5-7 years in municipal applications, with earlier replacement needed in high-chlorine environments or areas with frequent lightning strikes.

• Thermistor Element: Semiconductor material (typically nickel-cobalt oxide) providing temperature-dependent resistance. Available in bead (0.4-5mm), disc, or probe configurations. Municipal applications typically use -40°F to 250°F range with 10kΩ resistance at 77°F. Selection based on required accuracy and response time.
• Protective Housing: Stainless steel 316L or PVC housing protects the element from process fluids. Threaded fittings (½" to 1" NPT) or flanged connections for larger lines. Housing material selected based on chemical compatibility and pressure ratings up to 150 PSI typical for municipal applications.
• Signal Conditioning Electronics: Converts resistance changes to standardized 4-20mA output signals. Includes linearization circuits to compensate for thermistor's non-linear response. Often integrated into transmitter housing with LCD display for local indication and HART communication capability.
• Connecting Cables: Shielded twisted-pair cables minimize electrical interference. Typically 18-22 AWG with PVC or XLPE insulation rated for wet locations. Cable length affects accuracy; municipal installations typically limit runs to 1000 feet without signal boosters.

• Temperature Range and Accuracy: Operating range: -40°F to 200°F (-40°C to 93°C) for municipal water applications; Accuracy requirements: ±0.2°F (±0.1°C) for process control, ±0.5°F (±0.3°C) for monitoring; Response time: 5-30 seconds in flowing water, 30-90 seconds in still water
• Electrical Specifications: Resistance values: 2,252Ω to 30,000Ω at 77°F (25°C), with 10kΩ most common; Excitation current: 10-100 microamps to minimize self-heating; Output signal: 4-20mA or digital protocols (Modbus, HART)
• Physical Requirements: Immersion depth: 4-6 inches minimum for accurate readings; Flow velocity tolerance: 0.1-10 fps without measurement drift; Pressure rating: 150-300 PSI for typical municipal piping systems; Housing materials: 316SS or Hastelloy C-276 for chlorinated water service
• Installation Parameters: Pipe size compatibility: 2-inch minimum diameter for insertion sensors; Mounting options: Threaded (½" NPT), flanged, or insertion style; Cable length: Standard 25-100 feet, with junction boxes for longer runs

• What temperature accuracy is required for the specific process application? Process control applications (chemical feed systems, heat exchangers) require ±0.2°F accuracy, while general monitoring can accept ±0.5°F. Higher accuracy sensors cost 2-3x more and may require temperature compensation. Decision impacts: Wrong accuracy selection leads to either over-specification costs or inadequate process control.
• Should you specify analog (4-20mA) or digital communication protocols? Plants with existing SCADA infrastructure typically use 4-20mA, while new installations benefit from digital protocols (Modbus RTU/TCP, HART). Digital provides diagnostics and eliminates signal degradation over long cable runs (>500 feet). Decision impacts: Protocol mismatch requires expensive signal converters or complete system redesign.
• What response time is acceptable for your control loop requirements? Fast response (5-15 seconds) needed for chemical dosing control, while slower response (30-60 seconds) acceptable for trending/alarming. Faster sensors require smaller thermal mass and cost 20-40% more. Decision impacts: Slow response causes control system instability; over-specifying wastes budget.
• What level of explosion-proof rating is required? Non-hazardous areas use general purpose housings, while chemical storage areas may require Class I, Division 2 ratings. Explosion-proof ratings add $200-500 per sensor. Decision impacts: Under-specification violates electrical codes; over-specification inflates project costs unnecessarily.

• Primary: Division 40 - Process Integration (40 05 23 - Process Control Instrumentation)
• Secondary considerations: Division 33 - Utilities (33 11 00

• Material/Equipment Verification: Verify wetted materials compatibility (316SS minimum for wastewater); Confirm temperature range matches process requirements (-40°F to 300°F typical)
• Installation Requirements: Thermowells required for pressurized systems >50 psi; Minimum 4-inch immersion depth for accurate readings; Conduit sealing for hazardous area classifications
• Field Challenges: Cable routing through existing structures; Interference from VFD electrical noise
• Coordination Issues: SCADA integration testing with controls contractor; Lead times: 6-8 weeks for custom configurations

• Omega Engineering - RTD-NPT series for municipal applications, widely used in chlorine contact chambers
• Watlow - Series 935 immersion sensors popular in digester temperature monitoring
• Pyromation - Model 1415 sanitary sensors common in membrane bioreactor installations
• Endress+Hauser - iTHERM TrustSens offers municipal-grade reliability for critical process monitoring applications

• RTDs (PT100/PT1000) - Better accuracy and stability, preferred for custody transfer applications, 15-20% higher cost
• Thermocouples - Faster response time, better for high-temperature applications >300°F, lower cost but require cold junction compensation
• Infrared sensors - Non-contact measurement for rotating equipment, 3-5x higher cost but eliminates maintenance access issues

Establish preferred vendor relationships early - Omega and Watlow provide excellent municipal support with 24-hour replacement guarantees. Specify redundant sensors for critical applications like chlorine residual monitoring. Buy spare sensors during initial procurement to avoid emergency pricing. Consider wireless transmitters for retrofit applications where conduit runs are prohibitive. Always verify calibration certificates match your process ranges before installation.