A strain gage amplifier powers a bridge transducer and turns the few thousandths of a volt it produces into a signal something else can read. Everything in that sentence is true of half a dozen products we make. What separates them is how much work you have to do before one of them is actually running.
This one arrives finished. The page describes it as a complete amplifier and signal conditioner with self-contained power, and that phrase is the whole proposition. It plugs into a wall socket. There is no DC supply to find, no enclosure to specify, no rail to mount it on, no connectors to solder, no jumpers to set with the lid off, no PC, no software and no cable to buy before you can begin. You connect the transducer, turn two dials and press a button.
That is a genuinely different product rather than a lesser version of the others. A board designed into a machine is the right answer when you are building a hundred machines. It is the wrong answer at a laboratory bench, on a test rig that changes every fortnight, in a calibration room, or in any situation where the person who needs a reading this afternoon is not the person who designs control panels.
The adjustments are mechanical and that suits the same audience. Balance and span are set with precision ten-turn potentiometers — ten full rotations from end to end, so a small movement of your fingers is a small change in the reading, and setting a value precisely is a matter of feel rather than of counting increments in a menu. There is no configuration to be corrupted, no settings file to lose, and nothing that stops working because a laptop was replaced. The instrument is set where it is set, and it stays there.
But the specification worth understanding is the shunt calibration, because this one is done properly.
Shunt calibration is the standard way of checking a strain gage system without applying a real load: a precision resistor is switched across one arm of the bridge, unbalancing it by a known amount, and the resulting output is compared against what it produced when everything was known to be right.
The question nobody asks is where the resistor gets connected. On most equipment it is switched in at the instrument, which is convenient and which verifies the amplifier, the gain setting and everything downstream. What it does not fully verify is the cable, because the simulated signal is generated on the instrument's side of it.
Here the calibration is floating and applied at the transducer. The unbalance is created where the sensor is, so the resulting signal travels the entire length of the cable exactly as a real measurement does — through every connection, every terminal and every foot of conductor. Anything the cable takes away from a real signal, it takes away from the check as well.
That converts the check from a statement about the electronics into a statement about the installation. A long cable, a corroding connection, a joint somebody added, moisture in a gland: all of these quietly reduce a real reading, and all of them are invisible to a shunt check performed at the instrument end. This arrangement sees them. On a sensor that is difficult to load by hand — built into a machine, on a vessel in service, at the top of a structure — that difference is the whole value of having the feature at all.
The excitation supply also uses a Wagner ground arrangement to improve common mode rejection, which is a topic in its own right and one we cover on the signal conditioning pages.
Excitation goes out, a differential amplifier takes the difference between the two signal wires, and a filter decides what gets through. The choices worth understanding on this instrument are the filter, the excitation budget and the gain range.
The filter is not a setting. It is a personality chosen at the time of order. Three versions exist — sixteen hertz, one hundred and sixty, and sixteen hundred — and they are the same instrument with a different capacitor. That is unusual and it has a consequence: unlike a jumper or a menu, this is not something you change your mind about in the field on a Tuesday. Decide before ordering what your measurement actually contains, because the standard version is the narrowest of the three.
Sixteen hertz is the right default for weighing and it is a poor default for anything else. At that setting vibration, machinery, agitation and electrical interference are all removed before they reach the output, which is why a scale reads steadily. Set against a press stroke, an actuator cycling or a fastener letting go, the same filter removes most of the event along with the noise, and the peak you record will be lower than the peak that happened — an error that only ever goes one direction. The higher two versions exist for exactly that reason.
And the response can be extended much further than any of the three. Removing a single capacitor takes the frequency response out to ten kilohertz, which turns a weighing conditioner into a dynamic one. That is a documented modification rather than a switch, so it is worth discussing with us before it is attempted — but it means the same instrument covers static weighing at one end and genuinely fast events at the other.
The roll-off is gentle: six decibels per octave, a single pole. That is softer than the two-pole arrangement used elsewhere in our range, and the practical consequence is worth knowing. A single pole attenuates what is above the corner rather than removing it, so something large sitting just above your cutoff will still be visible, only smaller. It also means the response begins falling well before the quoted frequency rather than staying flat up to it, so choose a version with a little margin above what you actually need to measure.
The excitation supply has a current budget, and it decides how many sensors you can connect. It produces eight volts and can deliver a good deal of current, but the current limit is set at the factory to a much lower figure than its maximum — a deliberate protective choice rather than a shortfall. Work with the limit rather than the maximum. A single 350 ohm bridge draws well under a third of what is available. Two of them in parallel are still comfortably inside it. Three are not. If you are planning a multi-sensor arrangement, do that arithmetic before the mechanical design is finished.
The gain range is centered on the sensors most people connect. Gain runs from sixty-seven to seven hundred and thirty-seven, and the page states the output as ten volts with a 2 mV/V transducer — which is the arithmetic working out exactly, since two millivolts per volt at eight volts of excitation gives sixteen millivolts, and sixteen millivolts at a gain of about six hundred and twenty-five is ten volts. A higher sensitivity sensor simply uses less gain. A very low sensitivity sensor is the case to check: at one millivolt per volt the amplifier runs out of gain before it runs out of output range, so full capacity lands nearer six volts than ten. That is still a perfectly usable signal, but if your recording equipment expects a full ten volt span, it is better known in advance.
Balance has a wide range, which is more useful than it sounds. Zero can be moved by up to thirty percent of output on a standard bridge, which absorbs a fixture's dead weight, a preload built in by a mounting, or a transducer whose zero has shifted with age. Use it to accommodate what you understand, and investigate what you do not — a large offset that appeared suddenly is information, not an inconvenience.
Choosing between conditioners is mostly a question of who is going to install it and what has to happen to the output. Describe the setup to our applications engineers before you settle on a part number; the component that suits often becomes obvious once the rest of it is on the table.
Ask first whether you want an instrument or a component. If a person needs a working amplifier on a bench this week, this is the answer — it is complete, it plugs in, and it needs nothing bought alongside it. If it is being designed into a product you will build repeatedly, a board is cheaper, smaller and easier to power from what the machine already has. The two are not competing on quality; they are competing on what stage of the job you are at.
Then be honest about how fast your measurement is, because this decision is permanent. Write down a number in hertz. A tank filling, a fixture holding, a slow proof load: the standard version. A press cycle, an actuator, a machine running: the middle version. Faster than that, or genuinely dynamic work: the highest version, or a conversation with us about extending the response further. Ordering the wrong one is not a jumper away from being right.
Check what the output has to reach, because this is a voltage instrument. It produces up to ten volts, with a low output impedance and modest current drive, which suits a data acquisition card, a chart recorder, a bench display or a meter sitting nearby. What it does not produce is a 4–20 mA current loop, so if the signal has to travel a long way through a plant or arrive at a controller expecting a process signal, this is not the right part and we will point you at one that is.
Count your sensors against the excitation budget before the mechanics are built. One or two standard bridges are straightforward. Three or more are not, and a low resistance bridge uses more of the budget than a high resistance one. This is a five-minute calculation that becomes an expensive problem if it is left until the fixture exists.
Consider where the instrument will physically live. It is a small enclosed box, a couple of pounds, intended for a bench, a shelf or a rack rather than for a sealed panel or a machine. It has no ingress rating and its operating temperature range is the narrowest of our conditioners, so a hot cabinet or a wash-down area is not where it belongs. A laboratory, a test cell, a calibration room or an equipment rack is.
Record the shunt calibration reading on the day everything is right. This matters more here than on most instruments, because the check covers the cable as well as the electronics, which makes it a genuinely complete health check — but only if you know what a healthy result looks like. Press the button when the system is newly commissioned and calibrated, write the number down, and keep it with the instrument. Without that baseline the button tells you very little; with it, the same button becomes a complete health check on the installation.
Send us the transducer and its sensitivity and resistance, how quickly your measurement changes, how many sensors are involved and what is going to read the output, and we will confirm which version you want before you order.
TMO-2 Strain Gage Amplifier Applications.
The Transducer Techniques TMO-2 is a complete differential amplifier and signal conditioner with self-contained power, providing excitation, balance and span adjustment on precision ten-turn potentiometers plus shunt calibration, for coupling bridge type transducers to indicating instruments.
- Laboratory and Test Benches: A complete amplifier that plugs into a wall socket, requiring no separate supply, enclosure or configuration software.
- Calibration Rooms: Shunt calibration applied at the transducer verifies the whole measuring chain, cable included, without applying a physical load.
- Research and Development Rigs: Mechanical balance and span adjustment suits test arrangements that change frequently and are set up by hand.
- Weighing and Static Load: The standard narrow filter removes vibration and machinery noise so a slowly changing reading settles.
- Press and Machine Monitoring: Wider filter versions follow a working cycle faithfully rather than averaging the peak away.
- Dynamic Force Measurement: Frequency response can be extended for impact, drop and fast transient work.
- Data Acquisition Interfacing: A bipolar output of up to ten volts matches the input range of common acquisition hardware directly.
- Chart Recorder and Analog Instrumentation: Low output impedance drives recorders, meters and bench displays without an intermediate stage.
- Long Cable Installations: Transducer-end shunt calibration reveals losses in the cable that an instrument-end check cannot see.
- Torque and Pressure Measurement: The same amplifier conditions any full bridge transducer from 120 to 1000 ohms, not only load cells.
The TMO-2 works with our full range of load cells, torque sensors and pressure transducers, and is available with three standard filter frequencies to suit static or dynamic measurement.
Frequently Asked Questions
What does "self contained power" mean in practice?
That the instrument plugs into a wall socket and needs nothing else to run. There is no DC supply to specify, no power module to buy, no enclosure to house it in and no rail to mount it on. It draws only a few watts and has its own internal fuse. That matters because it changes who can put one into service: a complete amplifier with a line cord can be set up by whoever needs the measurement, whereas a board needs somebody to design a supply and a housing around it first. If you need a working channel this week rather than a component for a product, this is the difference that decides it.
What do the two ten-turn potentiometers do?
One sets balance and one sets span — zero and scale, the two adjustments every strain gage system needs. Ten-turn means the control rotates ten times from one end of its range to the other, so a small movement of your fingers makes a small change to the reading and a precise setting is achievable by feel. The balance range is wide, reaching about thirty percent of output on a standard bridge, which is enough to zero out a fixture's own weight or a preload built into a mounting. Because the settings are mechanical there is no configuration file to lose and nothing that stops working because a computer was replaced.
Why is the shunt calibration applied at the transducer?
So that the check covers the cable as well as the electronics. Most shunt calibration switches a resistor in at the instrument, which verifies the amplifier and everything after it but generates the test signal on the instrument's side of the wiring. Here the circuit is floating and the unbalance is created at the transducer, so the resulting signal travels the whole cable exactly as a real measurement does — through every connection and every foot of conductor. Anything that reduces a real reading reduces the check too. A long run, a corroded terminal, moisture in a gland: all invisible to an instrument-end check, all visible to this one.
What does the calibration resistor simulate?
One millivolt per volt of transducer output, which is a deliberately round figure. On a 2 mV/V load cell that is half of full scale, and on a 3 mV/V sensor a third — so you can predict what the button ought to give you before you press it. The check is single point and momentary: you press, read, and release. The most useful thing you can do with it is establish a baseline. Press it on the day the system is newly calibrated and everything is known to be right, write the reading down, and keep it with the instrument. After that the absolute number matters far less than whether it has moved.
Which filter version should I order?
Work it out from the speed of what you are measuring, because the filter is set by a component rather than a switch and cannot be changed on site. Three versions are available at the same price. The standard one is narrow and correct for weighing, load holding and slow proof testing, where its job is to remove vibration and machinery noise. The middle version suits presses, actuators and machine cycles. The widest suits faster work. Choose with some margin above your actual measurement frequency, because a single-pole filter begins rolling off below its quoted corner rather than staying flat right up to it.
Can the bandwidth be changed after purchase?
The frequency response can be extended substantially — out to ten kilohertz — by removing a single capacitor, which is a documented modification rather than a user setting. In practice that means the same instrument can serve static weighing at one extreme and genuinely dynamic measurement at the other, which is unusual for a conditioner of this kind. Because it involves opening the unit, talk to us before doing it: we can confirm what is involved for your particular unit and what it means for the calibration and the warranty. Do not assume it is reversible in an afternoon.
What sensors will the TMO-2 work with?
Any full bridge transducer between 120 and 1000 ohms with a sensitivity between roughly one and ten millivolts per volt, which covers load cells, torque sensors and pressure transducers alike. Excitation is a constant eight volts. One point to check at the low end of the sensitivity range: with eight volts of excitation and the gain available, a one millivolt per volt transducer reaches around six volts of output at full capacity rather than the full ten. That is still an entirely usable signal, but if your recording equipment is scaled for a ten volt span it is worth knowing before you wire anything.
What output does it give?
A bipolar voltage output up to ten volts, with low output impedance and a modest current drive capability. Ten volts is a convenient number because it matches the input range of most data acquisition hardware directly, and bipolar means tension and compression, or clockwise and counterclockwise, arrive as positive and negative without anything extra. What it does not offer is a 4–20 mA current loop. If your signal has to cross a plant, share a duct with power wiring or arrive at a controller expecting a process signal, that is a different product in our range and we would rather tell you so than sell you this one.
How stable is it over a working day?
Balance holds to about 0.2 percent and gain to 0.01 percent over eight hours, which is a helpfully honest way of specifying an instrument used for a shift at a time. Temperature is the larger effect at 0.02 percent of full scale per degree Celsius, so the practical advice is to let the instrument reach the temperature of the room before taking data you intend to keep, and to re-zero once it has. If a measurement has to be trusted across a wide temperature swing rather than across a working day, tell us the conditions, because there are conditioners in the range specified considerably tighter for drift.
How big is it and where should it sit?
A small enclosed box of roughly three by four by six inches, weighing under two pounds, with a line cord. It is designed for a bench, a shelf, a rack or a test cell — somewhere dry, reasonably temperate and reachable, because you will want to reach the adjustment potentiometers and the calibration button. It carries no ingress rating and its operating temperature range is the narrowest of our conditioners, so a sealed cabinet running hot, a wash-down area or an outdoor location are all the wrong home for it. Those situations want a different product and we can suggest one.
Questions From The Field
The reading is right at the amplifier and wrong at my recorder.
Look at what the recorder presents to the output. This is a voltage source with a low but real output impedance and a limited current capability, so a receiving device with a low input impedance draws current and pulls the voltage down — producing a reading that is consistently low by a fixed proportion, which looks like a scaling error rather than a loading problem. Check the input impedance of whatever you have connected, and check whether more than one device is sharing the output. Also check the ground arrangement: measuring a voltage against a different ground than the one it was produced against is the other common cause of a stubborn discrepancy.
Can I run two load cells from one TMO-2?
Two standard 350 ohm bridges in parallel are within the excitation supply's current limit, so electrically yes. Three are not, and a lower resistance bridge uses more of the budget than a higher one, so run the arithmetic for your actual sensors rather than assuming. Be clear about what you get, though: paralleling cells gives you their average, not their individual outputs, and it assumes they are matched. If you need a true sum of several cells with each one still visible, that is a different instrument in our range and worth asking about before you build the fixture.
I need a 4–20 mA output. Can I get one from this?
Not from the instrument itself — it is a voltage output device. You have two routes. A separate voltage-to-current converter after it will do the job and is sometimes the right answer where a TMO-2 is already installed. But if you are specifying from scratch and the destination is a PLC or a controller expecting a process signal, it is cleaner to use a conditioner or transmitter that produces the current loop natively, and there are several in our range that do. Tell us where the signal has to arrive and we will point you at the shortest path.
My zero drifts through the day.
Temperature first. This instrument's zero moves with temperature at a rate that is real and specified, so a room that warms through the day, sunlight falling on the bench, or an instrument switched on cold and used immediately will all show it. Let it stabilize before taking data and re-zero when it has. If the drift persists in a stable room, the next suspects are mechanical rather than electrical: a fixture relaxing, a transducer creeping under a sustained load, a cable pulling as something settles. Use the shunt calibration to separate the two — if the shunt reading stays put while zero moves, the electronics are behaving and something mechanical is changing.
The shunt check reads differently than it did at installation.
Take it seriously, because on this instrument that check covers the cable as well as the electronics, so a change points at a real problem somewhere in the chain. Inspect the cable and every connection between the amplifier and the transducer — a corroding terminal, a strained conductor, moisture in a gland or a joint someone added will all reduce the reading. Confirm the span potentiometer has not been moved. Then check the transducer itself: bridge resistance measured with the sensor disconnected, and insulation between the bridge and the body. A shunt reading that has fallen is telling you a real signal is falling too, which is exactly why the feature exists.
My bridge is 120 ohms. Any problem?
Worth checking with us before you order, because a 120 ohm bridge sits right at the edge of the excitation supply's factory current limit — the resistance is within the stated range but the current it draws at eight volts is very close to the limit. It may be entirely fine for your unit, and there may be a straightforward adjustment, but this is not something to discover during commissioning. Send us the transducer details and we will confirm it before anything ships. Higher resistance bridges have no such issue and 350 ohms and above is comfortable.
Can I use a 5 or 10 mV/V transducer?
Yes, and higher sensitivity is the easier case rather than the harder one. A more sensitive transducer produces a larger signal for the same excitation, so the amplifier simply uses less gain to reach the same output, and there is ample adjustment range for that. The awkward end is the opposite one: a very low sensitivity transducer asks for more gain than the amplifier provides and therefore falls short of full output. If your sensor is at either extreme of the sensitivity range, send us its figures and we will tell you exactly what output you will get before you commit.
I have the standard version and my test turns out to be faster than it.
Do not simply use it and accept the numbers, because the error runs in one direction only: a filter that is too narrow removes part of the event along with the noise, so every peak you record will be lower than the peak that actually occurred, and nothing downstream can recover it. There are three options. A wider standard version is available. The response can be extended much further by modification, which we should discuss rather than have you attempt blind. Or, if the event is genuinely fast, a different conditioner in the range may suit it better. Tell us how long your event lasts, in milliseconds, and the right answer is usually obvious.