Questions and answers
118 questions answered across the knowledge base, grouped by the guide they come from. Each answer stands on its own; follow the link for the full reference behind it.
Wet Well Level Control
Read the guide →- How wide should the level band be?
- Wide enough that at the lowest expected inflow the pump still runs long enough to satisfy minimum run time and stay within its starts-per-hour rating, and narrow enough that detention time at that same low flow does not turn the well septic. Calculate both, then use the widest band that satisfies the detention limit.
- What level technology is best for a wet well?
- Non-contact radar is the usual answer for raw wastewater because nothing touches the liquid and it is unaffected by vapor and temperature. Submersible pressure transducers are common and less expensive but sit in the liquid and foul. Ultrasonic drifts with temperature and struggles with vapor and foam.
- Do I still need floats with a good level transmitter?
- Yes. The floats are not there because the transmitter is unreliable. They are there because they fail independently. A single measurement with a single path to the pumps has no fallback when it reports a wrong but plausible value.
- Why does my station short cycle at night?
- Because inflow is at its lowest, so the well takes a long time to fill and the pump empties the band just as fast as always. The run is short, and if the band is narrow the pump exceeds its starts-per-hour rating. Widen the band, subject to the detention time limit.
4-20 mA Current Loops
Read the guide →- Why is 4 mA used instead of 0 mA for zero?
- So that a genuine zero measurement is distinguishable from a failure. At 4 mA the loop is proven intact and the transmitter is powered. At 0 mA something is broken. A 0-20 mA scheme cannot tell those apart.
- What is the maximum distance for a 4-20 mA signal?
- Distance is limited by total loop resistance, not by a fixed length. Calculate available voltage divided by 0.020 amps to get the maximum resistance, subtract the receiver and any devices, and the remainder is your wire budget. In practice, several thousand feet of ordinary instrument cable is routinely fine.
- Can I read a 4-20 mA signal with a voltage input?
- Yes, by placing a precision resistor across the input. 250 ohms converts 4-20 mA to 1-5 V, and 500 ohms gives 2-10 V. Use a precision resistor with a low temperature coefficient, and include the resistor in your loop resistance budget.
- Why does my analog signal jump when a VFD starts?
- Almost always induced noise coupling into the signal cable, or a ground loop that the drive is exciting. Confirm the signal is quiet with the drive stopped, verify the cable shield is grounded at exactly one end, and check that the signal cable is not sharing a tray or conduit with drive output conductors.
- What does it mean if the loop reads 21 mA or higher?
- On a NAMUR-compliant transmitter it means the device has detected an internal fault or lost its measurement and is deliberately reporting out of range. Read the transmitter diagnostics rather than assuming the process is above full scale.
- Should I use 4-20 mA or a digital protocol for new work?
- Both have a place. 4-20 mA is simple, well understood by every technician, and fails in obvious ways. Digital protocols carry more data per device and less wire. A common approach is HART transmitters on 4-20 mA loops, which gives the robustness of analog with digital diagnostics available when you need them.
Water and Wastewater Utility Threat Landscape
Read the guide →- Are small water systems really targeted?
- Most reported incidents were not targeting a specific utility. They were opportunistic access to whatever was reachable and weakly protected. Being small offers no protection when the exposure is discoverable by automated scanning.
- What is the single highest-value thing to do first?
- Eliminate direct internet exposure of control systems, and put multi-factor authentication on every remote access path. That combination addresses the initial access method in the large majority of publicly reported water sector incidents.
- Does cyber insurance cover a control system incident?
- It depends entirely on the policy, and many policies have requirements or exclusions relating to remote access controls and multi-factor authentication. Read the policy with your broker before you need it.
- Do we have to report an incident?
- Reporting obligations vary by state, by system type, and by federal rule, and they have been changing. Confirm the current requirement with your state primacy agency and have the reporting path written down before an incident, not during one.
Duplex Lift Station Controls
Read the guide →- Why are floats still used when a level transmitter is installed?
- Because they fail differently. A float is a mechanical switch on its own circuit that does not depend on the transmitter, the analog card, or the controller. When the primary measurement fails in a way that looks plausible, the float is what keeps the station from overflowing.
- Should a lift station use variable frequency drives?
- Sometimes. VFDs allow level to be held steadily and reduce starts, and they are valuable where the force main or the downstream plant benefits from a smooth flow. They add cost, heat, harmonics, and a maintenance item at an unattended site. Many utilities use across-the-line starters at typical stations and reserve drives for large or hydraulically sensitive ones.
- How long can a station run without SCADA?
- Indefinitely, if it is designed correctly. The panel controls the pumps. SCADA provides visibility, alarming, and history. Losing communication means the utility is blind, which matters a great deal, but it should never mean the station stops pumping.
- What causes a high level alarm at a station that seems to be working?
- In rough order of frequency: a rag-bound pump that is running without moving water, a level measurement reading low, an inflow event exceeding station capacity, a partially closed discharge valve, and a check valve stuck shut. Trend level against pump run status and the answer usually becomes obvious.
OT Security vs IT Security
Read the guide →- Should OT and IT networks be completely separated?
- Physically air-gapped networks are rare and often less secure in practice, because data still moves by USB drive and remote support still happens, just without controls. A segmented architecture with a controlled, monitored boundary is the realistic and generally recommended approach.
- Can I run antivirus on an HMI or SCADA server?
- Sometimes, and only with the control vendor guidance on exclusions and versions. Real-time process software has been disrupted by on-access scanning. Application allowlisting is often a better fit for a machine whose software set never changes.
- How do we handle an unsupported operating system on a control workstation?
- Treat it as unfixable and wrap it: strict segmentation, no internet path, no email or browsing, removable media controls, tight firewall rules, and monitoring. Then plan its replacement with a real budget line rather than an intention.
- Who should own OT cybersecurity?
- It has to be shared, with a single accountable owner named. The failure mode is a gap where IT assumes operations handles the plant network and operations assumes IT handles security. Write down who owns the boundary.
UL 508A Industrial Control Panels
Read the guide →- Does every industrial control panel have to be UL 508A listed?
- Not universally. Requirements depend on the authority having jurisdiction and the project specification. NEC 409.110 requires the SCCR marking on industrial control panels regardless. Many municipal specifications require the listing outright, and it is common practice for public work.
- Can I build a UL 508A panel myself?
- You can build to the standard, but you cannot apply the label unless your shop holds the listing and operates under the associated program with periodic audits. A panel built to the standard without the label is not a listed panel.
- What is the default SCCR if nothing is calculated?
- Panels frequently default to 5 kA, which is very low for anything fed from a substantial service. Determine the actual available fault current at the installation and design the panel to exceed it, using the supplement SB method or a high-fault-rated combination.
- Does the listing cover the PLC program?
- No. UL 508A is entirely about construction and safety. Program correctness, sequence of operation, and control performance are separate and belong in the functional specification and the factory acceptance test.
How to Program Lead/Lag Pump Control
Read the guide →- Should alternation happen on start or on stop?
- On cycle completion, meaning when all pumps stop. Alternating at start means the roles change mid-cycle when the lag pump joins, which produces confusing behavior and uneven wear.
- What if both pumps are called and one fails?
- The remaining available pump should keep running and the failed one should drop out of the rotation. Alarm the failure and the loss of redundancy separately, because they need different responses.
- How long should minimum run time be?
- Long enough that the motor stays within its starts-per-hour rating at the worst case, which is the lowest inflow. Ninety seconds to several minutes is common for municipal lift stations. Calculate it from the wet well volume and pump capacity rather than guessing.
- Do I need lead/lag logic if the panel has a mechanical alternator?
- A mechanical or dedicated alternator will run the station, but the utility cannot see the alternation state, cannot see run hours per pump, and cannot change the strategy without a site visit. Any station on SCADA is better served with the logic in the controller.
4-20 mA Signal Is Unstable
Read the guide →- Why does my analog signal only get noisy when a pump runs?
- Either the drive output cable is coupling into the signal cable, or the motor current is exciting a ground loop. Check cable routing and separation first, then shield grounding, then add an isolator if the second ground cannot be removed.
- How much filtering is acceptable?
- Enough to remove measurement noise without hiding a real process change the control loop needs to see. On a wet well level a few seconds is usually harmless. On a pressure loop controlling a pump, heavy filtering will destabilize the control.
- Can a bad power supply cause an unstable analog signal?
- Yes. A loop supply with excessive ripple or one operating near its current limit puts that variation directly into the loop. Measure the supply output with a meter on AC volts to check for ripple, and confirm it is not overloaded.
- The reading is stable at the panel but jumps on the HMI. What now?
- The field is fine. Look at the controller scaling, any filtering, the update rate between controller and SCADA, and the SCADA tag configuration. A slow poll rate showing a fast-changing value can look like instability that is not there.
What Is a PLC?
Read the guide →- What does PLC stand for?
- Programmable logic controller. The name describes what it replaced: hardwired relay logic that could only be changed by rewiring, replaced by logic you can change by programming.
- What is the difference between a PLC and a microcontroller?
- A microcontroller is a chip. A PLC is a finished industrial product built around a processor, with rated I/O, a power supply, diagnostics, an environmental rating, and a standardized programming environment. You can build a controller from a microcontroller, but you would be rebuilding everything a PLC already provides.
- Do I need to know how to program a PLC to work with one?
- No, but you need to be able to read one. Technicians who can follow a rung and see which condition is holding a pump off resolve calls far faster than those who can only check voltages.
- How long does a PLC last?
- Processor and I/O hardware routinely runs fifteen to twenty-five years. The practical limits are usually spare parts availability, software that no longer runs on a supported operating system, and communication protocols the rest of the plant has moved past.
Modbus Device Intermittently Offline
Read the guide →- How do I know if termination is correct?
- Power down the bus and measure resistance across the A and B pair. Approximately 60 ohms indicates two 120 ohm terminators, which is correct for a properly terminated segment. Higher suggests one or none; much lower suggests several devices have termination enabled.
- Do I need the signal common on RS-485?
- Yes, in practice. The differential pair carries the data but every transceiver has a common-mode voltage limit. Without a shared reference, ground potential difference between devices can exceed it, and the bus works until it does not.
- Why does the bus fail only when a pump starts?
- Because the drive output cable is coupling into the communication cable, or the motor current is shifting ground potential between devices. Check routing and separation first, then the signal common, then consider an isolated repeater.
- Should I lower the baud rate?
- It can help as a diagnostic and sometimes as a workaround, because lower rates tolerate reflections and marginal cabling better. Treat a bus that only works at a reduced rate as an installation defect to be corrected, not as a solution.
The Purdue Model
Read the guide →- Is the Purdue model outdated?
- The strict hierarchy is dated; the boundaries it identifies are not. Most current guidance still starts from these levels and then adapts for cloud, remote sites, and vendor connectivity. It remains the most widely shared vocabulary for discussing industrial network architecture.
- Do I need a DMZ for a small utility?
- You need the boundary. It may be a single firewall with a small DMZ segment holding one replica server rather than a large architecture. The principle that no business host connects directly to a control device applies at every size.
- What is Level 3.5?
- A commonly used informal name for the DMZ between site operations and business networks. It is not part of the original model but it is universally understood in industrial security discussions.
- How does this relate to ISA/IEC 62443?
- IEC 62443 uses zones and conduits, which is a more flexible way of expressing the same idea: group assets with similar security requirements into zones, and control every communication path between zones. The Purdue levels are a common starting point for defining those zones.
PID Control for Pumps and Valves
Read the guide →- Do I need the derivative term?
- Usually not, in water and wastewater. Flow, pressure, and level signals carry enough noise that derivative action mostly amplifies it. Derivative earns its place on slow processes with real thermal or transport lag.
- Why does my pressure loop oscillate only at low flow?
- Process gain changes with operating point. A pump and a system curve are not linear, so tuning that suits high flow is often too aggressive at low flow. Tune at the worst-case condition, or use gain scheduling if the platform supports it.
- Can I use one set of tuning constants for both pumps?
- If the pumps and drives are identical and piped symmetrically, usually yes. Verify by running each individually and watching the response, because impellers wear at different rates and a station rarely stays symmetric.
- What is the difference between proportional band and gain?
- They are reciprocals scaled by 100: proportional band equals 100 divided by gain. A gain of 2 is a 50% proportional band. Increasing gain and decreasing proportional band mean the same thing, which is why copying a number between platforms without checking the convention can invert your intent.
- Should the PID run in the PLC or in the drive?
- In the PLC for anything the plant depends on. The controller has the full process picture, the interlocks, and the historian. A drive-resident PID is convenient for a standalone booster but leaves the control strategy invisible to SCADA and difficult to change.
Lead/Lag Pump Control
Read the guide →- How often should pumps alternate?
- For a duplex station, every cycle is the usual default and it produces the most even wear. Where cycles vary greatly in length, alternating on accumulated run hours balances wear better.
- What triggers the lag pump?
- Most commonly a higher wet well level, meaning the lead pump is not keeping up. It should also start on lead pump failure, and it may start on a high-level condition regardless of the normal sequence.
- Should alternation survive a power failure?
- Yes. Store the alternation state in retentive memory. Otherwise every power blip resets the station to pump 1 as lead, and over years that pump accumulates significantly more wear.
- How many starts per hour is too many?
- Check the motor nameplate and manufacturer data. For the submersible motors typical at municipal lift stations, six to ten starts per hour is a common limit. If your station exceeds that, widen the level band rather than accepting the cycling.
- Should lead/lag logic live in the PLC or in a pump controller?
- In the PLC for any station on SCADA. A dedicated pump controller is fine for a standalone station, but it puts the alternation state, the run hours, and the failure logic somewhere the utility cannot see or change.
Ethernet Device Drops Offline
Read the guide →- What causes late collisions on a full-duplex link?
- A duplex mismatch. One end is running half duplex, almost always because it is auto-negotiating against a hard-set partner. Set both ends the same way.
- How do I find a duplicate IP address?
- Look for ARP conflict messages in device or switch logs. The simplest field test is to disconnect the suspect device and ping its address. If something answers, another device is using it.
- Should I hard-set speed and duplex on industrial devices?
- Only if both ends are hard-set identically and it is documented. Auto-negotiation on both ends is reliable on modern equipment and avoids the mismatch that causes most of these problems.
- Why does my network fail when I add more devices?
- On an EtherNet/IP network, usually multicast flooding without IGMP snooping. It can also be controller connection limits, or a genuinely saturated uplink. Check switch port utilization and multicast rates.
ISA-18.2 Alarm Management
Read the guide →- What is the difference between an alarm and an event?
- An alarm requires a timely operator response. An event is recorded but requires nothing. A pump starting is an event. A pump failing to start when commanded is an alarm.
- How many alarms should a system have?
- The standard sets rates, not counts. A large plant may have thousands of configured alarms and still be well within rate targets, because most are rarely active. A small plant with forty alarms can be unusable if six of them chatter constantly.
- Is ISA-18.2 mandatory?
- It is a consensus standard, not a regulation, and it is not generally mandated for municipal water and wastewater. It is widely treated as good practice, and it is often referenced in specifications. Where a regulator or insurer asks how alarms are managed, this is the framework they expect to hear about.
- Where do I start on an existing system nobody has ever rationalized?
- Measure for thirty days. Rank alarms by frequency. Take the top ten and address each one by fixing the underlying cause, adding a delay or deadband, changing the setpoint, or reclassifying it as an event. That work alone typically removes most of the volume, and it builds the case for funding a full rationalization.
How to Size a Control Panel Power Supply
Read the guide →- How much headroom should a control power supply have?
- At least 25% above the calculated steady-state load, and more where spare I/O capacity exists. The step to the next catalog size is usually inexpensive relative to the cost of diagnosing an intermittent brownout later.
- What are the symptoms of an undersized supply?
- Intermittent controller resets, communication modules dropping out, analog readings shifting when relays energize, and faults that cluster around specific sequence steps. All of these look like other problems, which is why they take so long to find.
- Should the PLC and field devices share a supply?
- Preferably not. Separating them means a shorted field wire does not take down the processor, and it makes fault isolation far quicker. Where a single supply is used, distribute through individually protected branches.
- Do I need to derate for altitude?
- Above roughly 2000 metres, yes, because reduced air density lowers convective cooling. Check the datasheet. For most water and wastewater work in low-lying areas this is not a factor, but it matters for mountain installations.
How to Scale a 4-20 mA Input in a PLC
Read the guide →- Should I use the built-in scaling instruction or write the math?
- Either works. The built-in instruction is fine if you understand its parameters, which differ between platforms. Writing the arithmetic explicitly makes the constants visible to whoever opens the program next, which has real maintenance value.
- What raw value corresponds to 4 mA on my card?
- Read the module manual and then verify by injection. There is no universal answer, and assuming one is the most common cause of a scaling error.
- Do I need to clamp the output?
- Yes. Without clamping, a transmitter at 3.9 mA produces a slightly negative level and a fault at 21.5 mA produces a level above the top of the well. Downstream logic will act on both.
Ladder Logic Fundamentals
Read the guide →- Is ladder logic still used?
- Yes, heavily. It remains the dominant language in water, wastewater, and most discrete manufacturing, largely because the people who maintain these systems can read it online while the process is running.
- What is the difference between a normally-open contact in the field and in the program?
- A field contact is a physical device state. A program instruction examines a memory bit. If a normally-closed field contact is wired to an input, the input bit is on when the device is at rest. Whether you use examine-if-closed or examine-if-open in the program depends on what you want to detect, and getting it backwards is a very common commissioning error.
- Why does my output flicker?
- Usually because the same output address is written by more than one rung, or because the condition driving it is oscillating around a threshold with no deadband. Search the whole program for duplicate destructive writes to that address first.
- What is a one-shot and when do I need one?
- A one-shot is true for a single scan when its condition transitions from false to true. You need one any time an action should happen once per event rather than continuously: incrementing a counter, sending a message, or capturing a value at the moment something happened.
Ground Loops in Instrumentation
Read the guide →- Should a cable shield be grounded at one end or both?
- One end, for analog instrumentation signals, normally the panel end. Grounding both ends creates exactly the path this article is about. High-frequency network cabling follows different rules; do not apply this guidance to Ethernet.
- Which end should the shield be grounded at?
- The end where the signal reference is established, which for a loop-powered transmitter is the panel supplying the loop. Be consistent across the site and document it, because mixed practice is worse than either convention.
- Can a ground loop damage equipment?
- Usually it degrades a reading rather than causing damage. During a fault or a lightning event, however, the same path can carry very large current and destroy analog inputs. Surge protection and isolation on circuits leaving a building are worthwhile for this reason.
- How do I tell a ground loop from induced VFD noise?
- They often occur together and both correlate with drive operation. Induced noise couples into the cable and responds to routing, distance, and shielding. A ground loop responds to how many points the circuit is earthed at. Lift one shield end: if the reading settles, it was a loop.
How to Test a 4-20 mA Loop
Read the guide →- Can I measure loop current without breaking the loop?
- Yes, in two ways: measure voltage across the known sense resistor and calculate, or use a true milliamp clamp meter. Transmitter test jacks also insert a meter without opening the circuit.
- Why does my meter read zero when the loop is clearly working?
- Check the meter fuse on the milliamp range first. A blown fuse reads zero and looks exactly like an open loop. Also confirm the red lead is in the milliamp jack, not the volts jack.
- What if the reading is correct at the panel but the HMI is wrong?
- The field and the input card are fine. The problem is scaling in the controller or configuration in the SCADA system. Check the card range setting first, then the scaling constants, then the SCADA tag.
- How do I test a loop-powered transmitter on the bench?
- Supply 24 VDC through a 250 ohm resistor in series with the transmitter, then measure voltage across the resistor to read current. That is a complete loop simulator built from two parts.
What Is SCADA?
Read the guide →- Does SCADA control the plant?
- Not directly. Controllers execute the control logic. SCADA supervises: it displays, records, alarms, and lets an operator adjust setpoints or issue commands that the controller then acts on.
- What happens if the SCADA server fails?
- In a correctly designed system, the process keeps running on controller logic while you lose visibility, alarming, and history. If equipment stops when SCADA stops, control logic that belongs in the controller is living in the wrong place.
- How often should SCADA poll a remote site?
- Fast enough that an operator can act on a change, slow enough that the communication path stays healthy. For a lift station on licensed radio, every fifteen to sixty seconds for analog values with immediate reporting of alarms and state changes is typical.
- What is the difference between SCADA and a historian?
- SCADA maintains current values and operator interaction. A historian stores values over time and answers questions about the past. They are frequently sold together but they solve different problems, and history requirements should be specified separately.
Magnetic Flowmeters
Read the guide →- Can a magnetic flowmeter measure a partially full pipe?
- A conventional one cannot; it will read low and give no obvious indication. Purpose-built partially-filled meters exist with additional electrodes for level, but for a standard mag meter, a full pipe is a requirement.
- Do magnetic flowmeters work on drinking water?
- Yes. Potable water is conductive enough. Verify the liner and electrode materials are approved for potable contact for your jurisdiction.
- Why does my mag meter read flow when the pump is off?
- Either there is genuine flow, such as backflow through a check valve, or the measurement has no proper ground reference. Close an isolation valve to establish true zero flow and observe. If it still reads, look at grounding and electrode condition.
- How often should a mag meter be calibrated?
- The electromagnetic measurement itself is stable and does not drift the way a mechanical meter does. Most utilities verify annually and clean electrodes as needed. Where the meter is used for billing or regulatory reporting, follow the requirement that applies to you.
The PLC Scan Cycle
Read the guide →- What is a typical PLC scan time?
- For a small to medium water or wastewater application, 5 to 20 milliseconds is typical. Large plant controllers with heavy communications may run 30 to 80 milliseconds. What matters is not the number itself but whether it is stable and comfortably under the watchdog setting.
- Why did my output not turn on immediately when the input closed?
- Because outputs are written at the end of the scan and inputs are read at the beginning. Depending on where in the scan the input actually changed, the response takes between one and two full scans plus input filter time and output device switching time.
- Does putting logic in a subroutine make the scan faster?
- Only if the subroutine is conditionally called and is often not called. Moving code into a routine that runs unconditionally every scan changes nothing except organization.
- What causes a watchdog fault?
- A scan that did not complete within the configured watchdog time. Look for loops without a bounded exit, recursive routine calls, a message instruction blocking on an unresponsive device, or an unexpectedly large array operation.
Modbus TCP
Read the guide →- What port does Modbus TCP use?
- TCP port 502 by default. Some devices allow it to be changed, and some vendors additionally offer Modbus over a secure transport on a different port.
- Do I need a CRC in Modbus TCP?
- No. TCP provides error detection and retransmission, so the Modbus CRC is omitted. Sending a request with an RTU CRC appended to a TCP connection produces an error, which is a common mistake when adapting serial code.
- What unit identifier should I use?
- For a native Ethernet device, try 1 or 255; many ignore the field. For a gateway fronting serial devices, use the serial slave address of the target device. This is the correct answer far more often than people expect.
- Can Modbus TCP and Modbus RTU coexist?
- Yes, with a gateway. It presents a TCP server on the Ethernet side and acts as the single serial master on the RS-485 side, using the unit identifier to select the slave. This is extremely common in retrofits.
High Performance HMI Design
Read the guide →- Is ISA-101 required?
- No. It is a consensus standard describing a lifecycle for HMI design. It is often cited in specifications and is treated as good practice, but it is not a regulation for municipal water and wastewater.
- Will operators accept grey screens?
- Usually yes, once they have worked a shift on them, and the objection almost always comes before the trial rather than after. Involving operators in the design and converting one area first makes the difference between adoption and resistance.
- Should running pumps be green?
- Under this approach, no. Green fill on every running pump spends your most attention-grabbing resource on the most common and least interesting state. Use a subtle outline or fill difference for running, and keep saturated color for conditions that need action.
- How many displays should a system have?
- As few as will cover the plant with a clear hierarchy. Large numbers of near-duplicate displays are a symptom of missing reusable templates, and they multiply the cost of every future change.
Modbus RTU
Read the guide →- What is the difference between Modbus RTU and Modbus ASCII?
- RTU sends binary data with a CRC-16 and relies on silent intervals for framing. ASCII sends hexadecimal characters with a start and end delimiter and an LRC check. ASCII is more tolerant of latency but roughly half as efficient. RTU is far more common.
- How many devices can be on one RS-485 segment?
- The protocol addresses up to 247 slaves. The electrical limit is typically 32 standard unit loads on a segment, though many modern transceivers present a fraction of a unit load and allow more. Practically, keep segments small; a bus with forty devices is slow and hard to diagnose.
- Why does my device respond intermittently?
- In order of likelihood: missing or incorrect termination, missing bias, no signal common, cable routed near drive output conductors, a timeout shorter than the device response time, and polling faster than the device can answer.
- Can I have two masters on a Modbus RTU bus?
- Not on the same segment. Modbus RTU is single-master by design. Two masters will transmit over each other. If two systems need the data, use a gateway or a data concentrator that presents itself as a slave to both.
Analog Scaling: Raw Counts to Engineering Units
Read the guide →- Why does my level read correctly at zero but wrong at full?
- That is a span error, not an offset error. The two ranges disagree about what 20 mA means. Compare the transmitter calibrated upper range value against the EU_max constant in the program, and check the analog card is configured for 4-20 mA rather than 0-20 mA.
- What raw count corresponds to 4 mA?
- It depends entirely on the module and its configuration. Do not assume. Read the manual, then verify by injecting 4 mA with a calibrator and observing the raw value online.
- Should scaling be done in the PLC or the SCADA system?
- In the PLC. The controller makes the control decisions, so it needs the correct engineering value regardless of whether SCADA is running. Scaling in SCADA leads to two different numbers for the same measurement, which is a very hard problem to unwind later.
- How do I handle a transmitter with a suppressed zero?
- A transmitter ranged 2 to 27 feet simply means EU_min is 2 and EU_max is 27. The formula handles it without modification. The mistake is leaving EU_min at zero, which produces an error of exactly the suppression amount at every point.
Radar Level Measurement
Read the guide →- Does foam affect radar level?
- Light foam is usually transparent to radar and it measures the liquid beneath. Dense, wet foam can absorb enough energy to weaken the return. Guided wave radar handles heavy foam better than non-contact.
- Why does my radar level freeze at one value?
- Almost always a false echo. The instrument has locked onto a fixed reflection from a ladder, a pipe, a bracket, or a build-up on the wall. Run a false echo suppression scan with the vessel empty, and check the mounting position against obstructions.
- Can radar measure through a plastic tank wall?
- Sometimes, for some frequencies and materials, but it is not a general-purpose solution. It is sensitive to wall thickness and material, and it degrades unpredictably. Mount through a proper nozzle where the application matters.
- What is the minimum measurable level?
- Set by the near-field blanking distance, which excludes reflections from the antenna and nozzle. It is typically a few inches to a foot depending on the instrument. Account for it when you set the low level pump-off point.
Interlocks and Permissives
Read the guide →- Should interlocks be in the PLC or hardwired?
- Both, for different purposes. Anything protecting people must be hardwired or in a rated safety system. Process interlocks belong in the PLC where they can be documented, alarmed, and historized. Critical equipment protection is often done in both, with the hardwired circuit as the enforcement and the PLC providing indication.
- What is a first-out indication?
- A record of which condition tripped first when several trip nearly simultaneously. Without it, a trip that cascades leaves you looking at five alarms with no way to tell which was the cause and which were the consequences.
- Can a permissive be bypassed?
- Sometimes it must be, for maintenance or commissioning. If you build a bypass, it should require a deliberate action, be visible on the HMI while active, be alarmed, be logged, and ideally time out on its own. A silent permanent bypass is how interlocks quietly stop existing.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.
Content on this site is general technical reference information. It is not engineering advice for a specific installation. Always follow the applicable codes, standards, manufacturer documentation, and your own site safety procedures. Verify against the equipment in front of you before you act.