A vessel scale with four load cells suddenly develops a slowly drifting zero point after several years of operation. On dry days, the scale operates normally. After significant temperature changes, cleaning or periods of high humidity, however, the weight reading fluctuates noticeably. The load cells themselves show no visible damage. Can even a small amount of moisture inside the junction box cause such a deviation?
Yes. Load cells typically operate with very small electrical bridge signals. A sensor with, for example:
2 mV/V
at an excitation voltage of:
10 V
produces only:
20 mV
of output signal at rated load.
Even very small electrical changes within terminals, circuit boards or cables can therefore become significant relative to the useful signal.
If moisture enters a summing or junction box, the insulation resistance between electrical conductors can decrease. Together with dust, oil, salts or other deposits, conductive surface paths can form. Small unwanted currents can then flow and influence the sensitive bridge signal.
Moisture in the junction box should therefore not simply be compensated for by performing another zero adjustment. First, the cause of the moisture ingress, terminals, cable glands, insulation and the individual load cell channels must be checked.
What is the purpose of a load cell junction box?
In larger scales, several load cells are often used together.
A typical vessel, for example, may be supported on:
3 or 4 load cells
.
Each load cell provides its own electrical output signal.
A conventional analogue junction box combines these signals into one common weighing signal.
In simplified form:
Load cell 1 ─┐
Load cell 2 ─┤
Load cell 3 ─┼→ junction box → weighing electronics
Load cell 4 ─┘
Depending on the design, individual load cells can also be adjusted or trimmed inside the junction box.
The junction box is therefore an important component of the electrical measuring chain.
Why are load cell signals particularly sensitive?
A conventional strain-gauge load cell contains a Wheatstone bridge circuit.
The weighing electronics supply this bridge with a defined excitation voltage.
The applied load slightly changes the resistance of the strain gauges and thereby generates a small differential voltage.
Typical rated outputs are, for example, in the range of:
1 ... 3 mV/V
.
At:
2 mV/V × 10 V = 20 mV
even:
20 µV
already corresponds to:
0.1% of the rated signal
.
This shows why electrical interference, contact problems or unwanted leakage currents can become relevant in weighing systems.
How does moisture enter a junction box?
Moisture can enter a junction box in several different ways.
Typical causes include:
- cable glands that have not been tightened sufficiently,
- incorrect cable diameter for the installed gland,
- damaged cover gasket,
- enclosure cover not closed correctly,
- unused cable entries left unsealed,
- damaged cables,
- high-pressure cleaning,
- temperature changes and condensation.
The entry point does not necessarily have to be directly above the moisture that later becomes visible.
Water can travel along cables or enclosure surfaces and collect at the lowest point.
Why can condensation occur even inside a closed enclosure?
An enclosure that appears completely closed is not automatically free from moisture.
The enclosure contains air, and this air contains water vapour.
If the junction box cools down, the relative humidity can increase.
If the dew point is reached, water can condense on cold surfaces.
Typical situations include:
- large temperature differences between day and night,
- outdoor installations,
- cold vessel contents,
- cleaning processes,
- rapid cooling after production stops.
Repeated temperature cycles are particularly critical.
An initially very small amount of moisture can develop into a permanent problem over time.
What are leakage currents?
Conductors that are electrically insulated from one another should ideally not exchange any current across their insulation surfaces.
In practice, however, every insulation has a finite resistance.
Moisture can significantly reduce this resistance.
A particularly critical combination is:
moisture + conductive contamination
.
This can create an unwanted conductive path across the surface of a circuit board or terminal block.
Very small currents can then flow along this path between, for example:
- excitation and signal line,
- Signal+ and Signal−,
- signal and shield,
- conductor and enclosure
.
In a conventional power circuit, such currents might be insignificant.
In a load cell signal in the millivolt range, however, they can be relevant.
Why is insulation resistance important?
Load cell measurement requires the signal lines to be sufficiently insulated from:
- the enclosure,
- the shield,
- other conductors
.
If the insulation resistance decreases, an additional electrical path is created in parallel with the actual measuring circuit.
The fault does not have to be large enough to blow a fuse or cause the weighing electronics to detect a clear wire break.
More typical are gradual effects such as:
- zero-point shift,
- slow drift,
- temperature-dependent deviation,
- fluctuating readings.
How does moisture cause signal drift?
A moisture-related fault is often not constant.
The electrical surface resistance can change due to:
- temperature,
- humidity,
- drying,
- renewed condensation,
- degree of contamination.
As a result, the unwanted electrical leakage path also changes.
The scale may therefore, for example, show an offset of:
+3 kg
in the morning and return to almost the correct value of:
0 kg
a few hours later.
This type of behaviour is typical of a fault whose cause changes with the environmental conditions.
Why is the zero point often affected first?
When a scale is unloaded or tared, the evaluation system expects a very small differential signal.
An additional electrical offset is particularly noticeable under these conditions.
The display may, for example, begin to drift slowly around:
0 kg
.
The operator may initially be tempted to zero the scale again.
However, this only compensates for the current electrical deviation.
If the moisture conditions subsequently change again, the offset changes as well.
A repeatedly drifting zero point should therefore not be permanently compensated for by performing repeated zero adjustments.
Why does the fault sometimes occur only at certain temperatures?
Temperature influences a moisture-related fault in several ways.
It changes:
- relative humidity,
- condensation conditions,
- electrical resistance,
- drying rate.
A scale may therefore be completely stable during a warm, dry afternoon.
After a cold night, however, condensation may form inside the junction box.
This time-dependent behaviour is exactly what makes troubleshooting difficult.
If the measuring point is not inspected by a service technician until several hours later, the original moisture may already have partially evaporated.
Why does contamination increase the effect of moisture?
Pure water has different electrical properties from contaminated process or cleaning water.
Industrial surfaces often contain:
- dust,
- salts,
- detergent residues,
- oil films,
- process deposits.
If moisture dissolves components of these deposits, a significantly more conductive film can form on the surface.
Even an apparently small amount of condensation can therefore have a much greater electrical effect.
For this reason, simply removing visible water droplets is often not sufficient on a contaminated circuit board.
What role do the terminals play?
The junction box contains numerous connection points between the load cell cables and the summing circuit.
Problems can arise due to:
- corrosion,
- loose screw terminals,
- contaminated contacts,
- broken strands,
- moisture between adjacent terminals.
Contact problems can create both additional resistance and unstable signal conditions.
During inspection, therefore, not only visible water but every individual connection point should be examined.
How do faults occur at cable glands?
A cable gland can only provide its intended degree of protection if:
- the cable diameter matches the seal,
- the gland is tightened correctly,
- the cable is round and undamaged,
- the gland is suitable for the environment in which it is used.
Problems can arise, for example, when several thin cables are inserted into a gland that is actually designed for one round cable.
A cable that is too thin may also fail to seal adequately even if the gland nut has been fully tightened.
Can water enter the junction box through the cable?
Yes.
Water does not necessarily have to enter directly through the cable gland.
With damaged cables or unsuitable installations, moisture can migrate along the cable construction.
Poor cable routing can also direct water straight towards the cable gland.
For outdoor installations or wet environments, the complete cable routing should therefore be considered.
A suitable drip loop before the cable enters the junction box can prevent water from running along the cable sheath directly towards the gland.
What must be considered regarding shielding and grounding?
Load cell signals are sensitive to electromagnetic interference.
Shielded cables are therefore often used.
The implementation of:
- shield connection,
- equipotential bonding,
- grounding
must comply with the intended plant concept.
Moisture can also create unintended connections between the shield, enclosure and signal conductors.
This can electrically alter an originally correct grounding and shielding concept.
When troubleshooting, unwanted connections to the enclosure should therefore also be checked.
How can one moisture-affected channel influence the entire scale?
In a conventional analogue junction box, several load cells are connected in parallel.
As a result, only one combined weighing signal is available at the output.
A faulty channel can therefore influence the overall result.
For example:
Load cell 1 correct
Load cell 2 correct
Load cell 3 moisture-related offset
Load cell 4 correct
still results in:
incorrect summed signal
At the output of the junction box, it is initially not immediately apparent which load cell or connection channel is causing the problem.
How can the affected load cell be identified?
In a multi-load-cell weighing system, the individual channels should be compared systematically wherever possible.
Depending on the design, this may include:
- electrically testing each load cell individually,
- measuring the output signal of each individual cell,
- comparing the resistance of the bridge circuits,
- checking insulation against the enclosure or shield
.
With a uniformly loaded vessel, the individual load cell values should remain within the range expected from the actual load distribution.
A significantly deviating load cell is therefore an important diagnostic indication.
Modern digital junction boxes can make this type of individual-channel diagnosis easier.
Which resistance measurements are useful?
A conventional load cell has defined resistances within its bridge circuit.
Depending on the sensor, the following can be checked:
- input resistance,
- output resistance,
- continuity of individual conductors,
- insulation against the enclosure or shield.
The measured values must be compared with the data sheet for the specific load cell.
There is no universal resistance value that applies to all load cells.
Comparing several identical load cells within the same scale can be particularly useful.
How should an insulation resistance test be performed?
Insulation resistance can provide important indications of moisture or damaged cables.
However, particular care is required.
Before an insulation test, sensitive electronic components must be disconnected in accordance with the manufacturer’s instructions.
An insulation tester can operate at significantly higher test voltages than a normal multimeter.
An unsuitable test voltage can:
- damage weighing electronics,
- destroy digital components,
- subject sensors to electrical loads beyond their permissible limits.
Never therefore perform an arbitrary high-voltage insulation test across the fully connected junction box. The test voltage and measuring method must be explicitly permissible for the load cell, cable and electronics.
Is drying the junction box sufficient?
Removing the moisture is an important first step.
However, it does not automatically eliminate the cause.
The following must also be checked:
- where the moisture entered,
- whether terminals are corroded,
- whether circuit boards contain residues,
- whether cables have been damaged,
- whether seals have aged.
A connection point that has already corroded can continue to cause an electrical fault even after it has dried completely.
The problem will also return if the actual entry point is not eliminated.
How can the junction box be permanently sealed?
The enclosure should be sealed in accordance with its intended design.
The following should be checked in particular:
- cover gasket,
- sealing surfaces,
- cable glands,
- blanking plugs,
- cable diameter,
- tightening of the glands.
Damaged seals should be replaced with the designated replacement parts.
Improvised additional holes or unapproved sealing compounds can impair the specified degree of protection of the enclosure.
If pressure equalization is required due to significant temperature fluctuations, only ventilation or pressure-equalization elements specifically intended for this purpose and suitable for the enclosure protection concept should be used.
What does the IP rating mean in practice?
A high enclosure protection rating is an important requirement for weighing technology in harsh environments.
However, it only applies to the correctly installed complete system.
This includes:
- suitable cable glands,
- a correctly closed cover,
- undamaged seals,
- properly sealed unused cable entries.
A cable gland with an unsuitable cable diameter can, for example, result in the actual sealing performance being significantly poorer than expected from the enclosure rating.
The IP rating should therefore not be regarded as a property of the empty metal enclosure, but as a property of the correctly installed complete unit.
When does the scale need to be adjusted again?
After repairs to the junction box, the scale should be checked metrologically.
This is particularly relevant if:
- terminals have been replaced,
- trim settings have been changed,
- a load cell has been replaced,
- a cable has been replaced,
- the junction box has been replaced.
A suitable test procedure may include, for example:
- zero-point check,
- load test using a reference load,
- if necessary, corner-load or load-distribution test,
- repeatability test.
Moisture-related drift, however, should not be compensated for by trimming or recalibration while the electrical cause is still present.
When is the fault actually mechanical?
Not every case of signal drift is caused by moisture.
Mechanical causes can also occur with vessel and platform scales.
Examples include:
- force shunts through pipework,
- stressed vessel connections,
- side loading of a load cell,
- contaminated or blocked weighing elements,
- foundation movement,
- thermal expansion.
After the electrical inspection, the mechanical force introduction should therefore also be examined.
A particularly useful question is whether the fault:
correlates with load, temperature, moisture or mechanical movement
.
Practical example: zero point drifts after cleaning
A production vessel is supported on four load cells.
After routine plant cleaning, the reading begins to drift by several kilograms.
A few hours later, the signal stabilizes again.
When the plant is dry, the scale operates for several days without any noticeable deviation.
Initially, a temperature-dependent drift of one load cell is suspected.
However, inspection of the junction box reveals traces of moisture in the lower part of the enclosure.
During an earlier maintenance operation, one cable gland had been replaced with a version whose sealing range did not optimally match the existing cable diameter.
During cleaning, a small amount of moisture enters the enclosure.
On the slightly contaminated terminals, this moisture reduces the insulation resistance.
After drying, the effect decreases again.
The cable gland is replaced with a suitable version, the junction box is properly cleaned and dried, and the electrical system is subsequently checked.
A zero-point and load test of the complete scale is then carried out.
The fault does not occur during subsequent cleaning cycles.
The suspected load cell fault was therefore actually an environment-dependent insulation fault in the common junction box.
Systematically diagnosing signal drift
- Document the time and environmental conditions when the fault occurs.
- Check whether the drift is associated with cleaning, rain or humidity.
- Inspect the outside of the junction box for traces of water.
- Open the enclosure in accordance with the applicable safety requirements.
- Check for condensation, corrosion and deposits.
- Inspect the cover gasket.
- Check cable glands and blanking plugs.
- Compare the cable diameter with the sealing range of the gland.
- Check terminals for corrosion and loose connections.
- Compare the individual load cell channels.
- Check bridge resistances according to the manufacturer’s specifications.
- Check insulation against the shield or enclosure.
- Safely disconnect the electronics before performing a high-voltage insulation test.
- After eliminating the electrical fault, check the mechanical force introduction.
- Finally, check the zero point and reference load.
Correctly planning and installing a junction box
- Determine the environmental conditions of the scale.
- Take cleaning, rain, dust and temperature changes into account.
- Select a suitable enclosure material.
- Define the required degree of protection.
- Protect the junction box from direct water jets wherever possible.
- Select cable glands that match the actual cable diameter.
- Seal unused openings correctly.
- Route cables so that water does not run directly towards the gland.
- Provide suitable drip loops.
- Define the shielding and grounding concept.
- Route load cell cables separately from interfering power cables.
- Ensure adequate strain relief.
- Keep cover sealing surfaces clean.
- After installation, check sealing and electrical signals.
- Document the zero point and load distribution.
Common mistakes
- Simply zeroing out moisture-related drift: The electrical cause remains and changes again as the moisture conditions change.
- Suspecting only the load cells: The common junction box can affect all sensors simultaneously.
- Looking only for visible water: Even thin moisture films on contaminated surfaces can alter insulation resistance.
- Equating a high IP rating with absolute watertightness: The intended protection depends on correct installation of all seals and cable glands.
- Using any available cable gland: The sealing range must match the actual cable diameter.
- Leaving an unused entry unsealed: A single faulty opening can compromise the entire protection concept.
- Only superficially drying wet terminals: Corrosion or conductive residues may remain.
- Readjusting trim potentiometers before identifying the cause: A temporary moisture-related fault is merely incorporated into the adjustment.
- Performing a high-voltage insulation test with electronics still connected: An unsuitable test voltage can damage sensors and weighing electronics.
- Failing to check shield and enclosure: Moisture can create unintended electrical connections to ground or shield.
- Testing only in dry conditions: An intermittent moisture-related fault may then have disappeared completely.
- Forgetting mechanical checks after electrical inspection: Force shunts and stressed pipe connections can produce similar symptoms.
Suitable junction boxes for load cells
WIKA B6578 junction box
A specific junction box for conventional analogue weighing systems is the WIKA B6578.
It is designed for the parallel connection of up to:
4 load cells or force transducers
.
The passive input signals are in the:
mV/V range
.
This makes the junction box suitable for vessel scales, platforms, force measurement systems and other multi-load-cell applications.
Adjustment or trimming of individual sensors is also possible.
Especially with analogue mV/V signals, clean terminals, correctly installed cable glands and a dry environment inside the enclosure are essential.
Further information can be found for the WIKA B6578 junction box for load cells.
Siemens SIWAREX DB with individual-channel diagnostics
An alternative architecture is provided by the Siemens SIWAREX DB.
The digital junction box allows up to four analogue load cells to be connected.
Unlike a purely analogue summing box, additional information is available for the individual load cells.
Depending on the system configuration, this can be used to diagnose, among other things:
- wire break,
- impedance changes,
- overload or underload,
- current load on each individual load cell
.
This makes it easier, particularly in multi-load-cell systems, to locate an abnormal sensor or connection channel.
Further information can be found for the Siemens SIWAREX DB junction box.
Further load cells, mounting units, junction boxes and weighing electronics can be found under force, weighing and displacement measurement technology at ICS Schneider.
Conclusion
Moisture inside a load cell junction box can significantly affect an industrial scale even though all load cells appear mechanically intact.
The reason lies in the very small electrical signals produced by a strain-gauge load cell. Even small unwanted electrical leakage paths can be significant relative to a millivolt-level signal.
Moisture combined with dust, salts or process deposits is particularly critical. Conductive surface films can form on terminals and circuit boards, and their resistance can also change with temperature and humidity.
The typical result is not a clear total failure, but rather a fault that is difficult to reproduce:
- drifting zero point,
- slow signal drift,
- unstable display,
- temperature- or moisture-dependent deviations.
Performing another zero or corner-load adjustment does not eliminate this cause.
During diagnosis, the junction box, cable glands, terminals, individual load cell channels, bridge resistances and insulation conditions should therefore be checked systematically.
After repair, the complete scale must then be checked using the zero point and a suitable reference load.
For permanently stable weighing measurement, therefore: keep the junction box dry and clean, select cable glands to match the cable, inspect seals and blanking plugs, do not compensate for moisture by recalibration, and in the event of drift always check the individual load cell channels as well as the complete mechanical and electrical measuring chain.
FAQ: Moisture in a load cell junction box
Can moisture in the junction box change the weight reading?
Yes. Moisture can change the insulation resistance between conductors and thereby create unwanted electrical leakage paths. With the very small mV/V signals of load cells, this can result in relevant measurement deviations.
What are typical symptoms of a moisture-related fault?
Typical symptoms include a drifting zero point, slow drift, fluctuating readings and faults that depend on temperature, humidity, rain or cleaning cycles.
Why is this problem particularly critical with load cells?
Load cells normally produce only very small differential voltages in the millivolt range. Small additional voltages or leakage currents can therefore already be relevant compared with the actual useful signal.
What are leakage currents?
Leakage currents are unwanted electrical currents flowing across surfaces or insulation paths. Moisture and conductive contamination can significantly reduce the resistance of these paths.
Can one moisture-affected load cell connection influence the entire scale?
Yes. With conventional analogue summing, each connected load cell influences the common output signal. A faulty channel can therefore change the entire weight indication.
Can I simply eliminate the fault by zeroing the scale?
No. This only compensates for the current deviation. If the moisture conditions change again, the zero point can start drifting again.
How can I identify which load cell is affected?
The individual sensors should be compared systematically. This can include checking the output signal, bridge resistances and insulation behaviour of the individual channels.
Can I connect an insulation tester directly to the weighing electronics?
Not without explicit approval. Insulation testers can generate high test voltages and may damage connected sensors or electronic components. The measuring method must comply with the respective manufacturer’s specifications.
Why does condensation occur in a closed junction box?
The enclosure contains air with a certain moisture content. If the enclosure cools significantly, the dew point can be reached and water vapour can condense on cold surfaces.
Why is a high IP rating alone not sufficient?
The intended degree of protection is only achieved if the cover, seals, cable glands and blanking plugs are correctly installed and undamaged.
Why must the cable gland match the cable diameter?
The seal of a cable gland only operates correctly within its specified clamping range. A cable that is too thin or otherwise unsuitable may therefore not be adequately sealed even when the gland has been tightened.
Should the scale be checked again after a repair?
Yes. After work on the junction box or load cell cables, at least the zero point and a suitable reference load should be checked. After changes to the trim adjustment or replacement of components, a more complete recalibration may be required.
Which junction box is suitable for up to four analogue load cells?
One specific example is the WIKA B6578. It is used for the parallel connection of up to four load cells or force transducers with mV/V output and also allows individual sensors to be trimmed.
Which alternative provides individual-channel diagnostics?
The Siemens SIWAREX DB can acquire up to four analogue load cells and provide additional diagnostic information for the individual sensors. This makes it easier to identify an abnormal channel in multi-load-cell systems.