A two-wire transmitter needs a minimum voltage across it to operate. Everything else in the loop consumes voltage: the sense resistor in the PLC input, the cable resistance, any indicator or isolator, and the barrier if the area is hazardous. If the supply cannot cover the total, the transmitter stops regulating and the reading collapses at the top of the range, which looks like a sensor fault.

Working the numbers

The calculation is simple and worth doing on paper before ordering:

• Available supply voltage, for example 24 V
• Minus the transmitter minimum, typically 10 to 12 V
• Minus the sense resistor drop, 250 Ω at 20 mA is 5 V
• Minus the cable drop, roughly 0.02 Ω per metre per core, doubled for the return path
• Minus any indicator or isolator burden, typically 2 to 3 V

If the result is negative, the loop will misbehave at high signal. The fix is a higher supply, a smaller sense resistor, or a transmitter with a lower minimum voltage. Note it now, not after commissioning.

Where earth loops come from

Two earth connections at different potentials, joined by a signal cable, form a loop. Current flows through the shield or the signal return, and the resulting offset appears as a measurement error that changes with plant load. Common sources are a sensor earthed through its housing at one end of a building and the panel earthed at the other, or a thermocouple welded to a grounded pipe with a grounded transmitter.

Galvanic isolation breaks the loop without changing the wiring topology. That is why an isolating conditioner is often the cheapest repair for a signal that has been unreliable for months.

Noise from drives

Variable frequency drives inject common-mode noise onto cables that run in the same duct. Symptoms are a reading that jumps when a motor starts, or a value that sits a few percent high all the time. Three measures, in order of effectiveness: separate the signal cable from the drive cable, shield the signal cable and bond the shield at one end, and add an isolating conditioner at the panel.

Digital alternatives

IO-Link avoids most of these problems because the transmission is digital and the sensor reports its own diagnostics. For new machines, a master with eight ports often costs less than the analogue input card plus the isolation it would need.

Checklist

• Calculate the loop budget with the sense resistor and cable included
• Isolate any signal that leaves the panel or crosses between buildings
• Bond shields at one end only, and keep them continuous through junctions
• Separate signal and drive cable by 200 mm in the duct
• Verify the loop at two points, not just at zero

The post 4-20 mA Loop Budgets, Isolation and Noise: A Practical Guide appeared first on DuoMy Sensing.

A sensor that survives ten years in a dry panel fails in eighteen months in a washdown area. The difference is almost never the sensing element. It is the jacket, the gland and the connector.

Jacket material

If the cable sees coolant mist, specify PUR and stop there. It is the single change that removes most premature failures on machine tools.

Drag chain and continuous flexing

A standard cable in a drag chain fails at the conductor strands, not the jacket, and it fails after a predictable number of cycles. Use a cable with a published cycle rating, respect the minimum bend radius in the moving direction, and mount it so the cable moves in one plane only.

For a rotating joint, a drag-chain cable is the wrong part. Torsion-rated cable has a different conductor lay and will survive rotation that would destroy a flex-rated cable.

Shielding

Shield any cable that runs parallel to a drive output or through a welding cell. Bond the shield at one end, at the panel, and keep it continuous through junction boxes. A shield bonded at both ends creates a circulating current that is worse than no shield at all.

Connectors

The IP rating of a connector only applies when it is mated and correctly tightened. An IP69K sensor with an IP67 cordset is an IP67 installation, and a hand-tightened gland that is a quarter turn loose is not IP67 at all.

• Use overmoulded cordsets for production machines; field-attachable plugs are for repair
• Match the cable diameter to the seal insert, every time
• Count pins for the next upgrade: a 5-pin cordset allows IO-Link without re-cabling
• For tool changers, specify high mating-cycle connectors and keep spares

Washdown specifics

In food and beverage plants, the cable is cleaned as often as the machine. Specify IP69K on the sensor, the cordset and the junction box as a set, mount connectors so they do not collect water, and use stainless junction boxes where the housing will be hit by a pressure jet.

The post Sensor Cable Selection: Washdown, Drag Chain and Welding Cells appeared first on DuoMy Sensing.

Radar for foam, vapour, dust, condensation and long range. Ultrasonic for clean water in a simple tank where cost matters. Everything else is a detail.

How the two technologies fail

Ultrasonic depends on sound travelling through air and reflecting from a surface. Anything that changes the air or absorbs the sound changes the measurement: temperature gradients, steam, foam, dust, wind and condensation on the transducer face. The failure is rarely a clean fault. It is a plausible reading that drifts.

Radar depends on a radio reflection. Vapour, dust and most foam are largely transparent at 80 GHz, and the narrow beam means the reflection comes from the surface directly below the antenna rather than from a wall or an agitator blade. The failure mode is a false echo from a fixed obstruction, which is visible during commissioning as a constant distance.

Surface conditions

Vessel geometry

Radar needs a clear cone. A nozzle that protrudes more than its own diameter, a heating coil close to the beam, or a manway directly beside the antenna will all produce a reflection. The 4 degree beam of an 80 GHz unit makes this manageable in most tanks, but check the beam diameter at maximum range against the internals before ordering.

Ultrasonic needs a clear acoustic path too, but the wider beam means it is more tolerant of a slightly off-centre mounting and less tolerant of internals.

Cost and maintenance

Ultrasonic is roughly half the price of radar at the same range. That difference disappears the first time a unit needs a site visit, which is why water utilities with remote pump stations increasingly specify radar for anything that is not a clean tank.

What we recommend in practice

• Wet wells and pump stations: 80 GHz radar plus an independent low-level switch for protection
• Clean water reservoirs and cooling tower basins: ultrasonic, unless the site is remote
• Fermentation, flotation and chemical reactors: radar, always
• Silos and hoppers: radar with dust suppression, or guided radar for low-dielectric powders
• Sealed metal vessels: guided-wave radar, since neither non-contact technology can see through the wall

Whatever the technology, keep the protection function separate from the control function. One instrument doing both jobs is how tanks overflow.

The post Radar vs Ultrasonic Level Measurement: How to Choose appeared first on DuoMy Sensing.

A pressure transmitter is usually the most accurate instrument on the skid and still reports the wrong number, because the impulse line between the process and the diaphragm does more to the measurement than the sensor itself. The four failure modes below account for most of the field errors we are asked to investigate.

1. Tap location

Tap the pipe on the side, not the top and not the bottom. A top tap collects gas in a liquid line; a bottom tap collects sediment in a gas line. Both create a pocket that changes the effective head on the diaphragm and shifts the zero slowly enough that nobody notices.

For steam, use a pigtail siphon so the impulse line stays filled with condensate and the transmitter never sees live steam temperature. For dirty liquids, fit a diaphragm seal rather than a small bore impulse line that will block.

2. Impulse line slope

Slope the line continuously back to the process: down towards the pipe for liquid service, up for gas service. A line that rises and falls creates a trap, and a trap produces a reading that depends on how much liquid happens to be sitting in it.

Keep liquid lines short and equal in length on a differential transmitter. Unequal lengths produce a differential head that looks exactly like a real measurement.

3. Mounting orientation

Mount the transmitter so that the process connection is at the bottom for gas and at the top for liquid, and so that any condensate drains away from the electronics. An IP68 housing still fills with water if it is mounted with the connector pointing up and the seal is disturbed during maintenance.

Use a three-valve manifold on differential transmitters. It allows the transmitter to be zeroed in service without depressurising the line, which converts a two-hour job into a two-minute one.

4. Zeroing

Zero a gauge transmitter with the process connection vented to atmosphere, not with the line isolated and still full of liquid. Zero a differential transmitter with the high and low sides equalised through the manifold. Record the zero before and after adjustment: a drift of more than 0.5% of span between services is an early indication that the diaphragm or the fill fluid is degrading.

Commissioning checklist

• Tap on the side of the pipe, with a siphon for steam and a seal for dirty or viscous media
• Impulse line sloping continuously, no traps, equal length on differential units
• Transmitter mounted so condensate drains away from the cable entry
• Three-valve manifold fitted on differential units
• Zero recorded before and after adjustment, with the value written into the loop sheet
• Loop current verified at two points against a reference, not just at zero

If a transmitter still reads wrong after these checks, the problem is usually the reference type rather than the instrument: a gauge transmitter on a vacuum line, or a differential transmitter with one port blocked.

The post Pressure Transmitter Installation: Impulse Lines, Mounting and Zeroing appeared first on DuoMy Sensing.