Temperature Transmitters: Why They Matter for Signal Integrity

What a Temperature Transmitter Actually Does

Your RTD or ther­mo­cou­ple pro­duces a tiny elec­tri­cal sig­nal — a change in resis­tance for an RTD, a few mil­li­volts for a ther­mo­cou­ple. That sig­nal is fine over short dis­tances in a qui­et envi­ron­ment, but try run­ning it 50 or 100 metres across an indus­tri­al plant and you’ll pick up every bit of elec­tro­mag­net­ic inter­fer­ence along the way. Vari­able fre­quen­cy dri­ves, motors, switchgear, weld­ing equip­ment — they all inject noise into your measurement.

A tem­per­a­ture trans­mit­ter sits right at the sen­sor and con­verts that frag­ile sig­nal into a robust 4 – 20mA cur­rent loop out­put. Cur­rent sig­nals are inher­ent­ly resis­tant to elec­tri­cal noise and volt­age drops over dis­tance. Your PLC or DCS at the oth­er end receives a clean, sta­ble sig­nal that accu­rate­ly rep­re­sents the process temperature.

That’s real­ly all it is — a sig­nal con­di­tion­er that makes your tem­per­a­ture read­ing sur­vive the jour­ney from sen­sor to con­trol room. But get­ting the details right mat­ters more than most peo­ple realise.

Why 4 – 20mA? Because It Just Works

The 4 – 20mA cur­rent loop has been the indus­tri­al stan­dard for decades, and there’s a rea­son nobody’s man­aged to replace it. It’s sim­ple, reli­able, and has some clever engi­neer­ing baked in.

In a 4 – 20 mA loop, the bot­tom of the con­fig­ured mea­sure­ment range cor­re­sponds to 4 mA. A read­ing of 0 mA indi­cates loss of loop cur­rent, rather than a valid low­er-range mea­sure­ment. Detect­ed sen­sor faults use the transmitter’s con­fig­ured fail­ure response; for exam­ple, the HBS4400 pro­vides a con­fig­urable sen­sor-fail val­ue and out­put lim­its. Set and test the receiv­ing system’s alarms to match the trans­mit­ter con­fig­u­ra­tion. The cur­rent sig­nal alone does not guar­an­tee detec­tion of every mea­sure­ment fault.

The oth­er advan­tage is that many trans­mit­ters are loop-pow­ered — they draw their oper­at­ing pow­er from the same two wires that car­ry the sig­nal. No sep­a­rate pow­er sup­ply need­ed at the sen­sor end. That sim­pli­fies wiring enor­mous­ly, espe­cial­ly when you’re retro­fitting trans­mit­ters to exist­ing installations.

HART Protocol — Worth Having or Overkill?

Some of our trans­mit­ters sup­port HART (High­way Address­able Remote Trans­duc­er) pro­to­col. HART over­lays a dig­i­tal com­mu­ni­ca­tion sig­nal on top of the ana­logue 4 – 20mA loop, which means you can do things like remote­ly recon­fig­ure the trans­mit­ter’s range, read diag­nos­tic data, and check sen­sor health — all with­out inter­rupt­ing the live measurement.

Is it worth pay­ing for? It depends on your set­up. If you’ve got trans­mit­ters in hard-to-reach loca­tions — up a col­umn, inside a haz­ardous zone, behind three lay­ers of scaf­fold­ing — being able to inter­ro­gate and recon­fig­ure them from the con­trol room is gen­uine­ly valu­able. Remote access can sup­port diag­nos­tics and con­fig­u­ra­tion where the trans­mit­ter and con­trol sys­tem sup­port the required func­tions. It does not replace cal­i­bra­tion checks or estab­lish sen­sor accu­ra­cy by itself.

For a sin­gle, eas­i­ly acces­si­ble mea­sure­ment point in a benign envi­ron­ment? A basic 4 – 20mA trans­mit­ter with­out HART will do the job per­fect­ly well and cost you less.

Why Head-Mount Is the Way to Go

All our trans­mit­ters are head-mount designs — they fit inside the sen­sor’s con­nec­tion head, the hous­ing at the top of your ther­mow­ell or probe assem­bly. We made that design choice delib­er­ate­ly, and here’s why.

The whole point of a trans­mit­ter is to con­vert a frag­ile sen­sor sig­nal into some­thing robust before it has to trav­el any dis­tance. A head-mount trans­mit­ter does that con­ver­sion right at the source. There’s vir­tu­al­ly no unpro­tect­ed cable between the sen­sor ele­ment and the trans­mit­ter elec­tron­ics, which means vir­tu­al­ly no oppor­tu­ni­ty for elec­tri­cal noise to creep into your measurement.

Com­pare that with a pan­el-mount or DIN rail trans­mit­ter sit­ting in a con­trol room 50 metres away. You’ve now got 50 metres of low-lev­el sen­sor cable run­ning through the plant — past VFDs, motor starters, pow­er cables — which is exact­ly the noise vul­ner­a­bil­i­ty a trans­mit­ter is sup­posed to elim­i­nate. You can mit­i­gate it with shield­ed cable and care­ful rout­ing, but you’re solv­ing a prob­lem you did­n’t need to create.

Head-mount also sim­pli­fies your instal­la­tion. One com­bined sen­sor-and-trans­mit­ter assem­bly, two wires back to the con­trol room car­ry­ing a clean 4 – 20mA sig­nal. No sep­a­rate trans­mit­ter enclo­sure to mount, no extra cable runs to manage.

The one thing to watch is ambi­ent tem­per­a­ture at the con­nec­tion head. The trans­mit­ter elec­tron­ics need to stay with­in their rat­ed range — typ­i­cal­ly ‑40°C to +85°C. If the sen­sor is mea­sur­ing very high tem­per­a­tures and the con­nec­tion head gets too hot, you may need an extend­ed neck on your ther­mow­ell to move the head fur­ther from the heat source. We design for this as stan­dard on high-tem­per­a­ture assemblies.

Signal Noise — The Problem Nobody Sees Until It’s Too Late

If read­ings are jit­tery, unsta­ble or incor­rect, check the com­plete mea­sure­ment chain, includ­ing the sen­sor, trans­mit­ter con­fig­u­ra­tion, wiring, pow­er sup­ply and installation.

Here’s what caus­es the most trouble:

Run­ning sig­nal cables along­side pow­er cables. We see this con­stant­ly. Some­one routes the 4 – 20mA out­put cable through the same cable tray as the 415V motor sup­ply, and then won­ders why the tem­per­a­ture read­ing bounces around. Keep sig­nal cables phys­i­cal­ly sep­a­rat­ed from pow­er cables — dif­fer­ent trays, dif­fer­ent con­duits, cross at right angles if they must cross at all.

Poor or miss­ing cable shield­ing. Even though the 4 – 20mA sig­nal from a head-mount trans­mit­ter is far more robust than a raw sen­sor sig­nal, it still ben­e­fits from shield­ed twist­ed-pair cable on longer runs. Ground the shield at one end only — the con­trol room end — to pre­vent ground loops.

VFDs (Vari­able Fre­quen­cy Dri­ves). These are the sin­gle biggest source of elec­tri­cal noise in mod­ern plants. If you’ve got VFDs any­where near your sig­nal cable routes, expect prob­lems unless you’ve tak­en prop­er pre­cau­tions with cable sep­a­ra­tion and shielding.

Loose or cor­rod­ed ter­mi­na­tions. A con­nec­tion that’s slight­ly loose or cor­rod­ed intro­duces resis­tance into the cir­cuit. At the trans­mit­ter end, this can cause sig­nal insta­bil­i­ty. We’ve diag­nosed ​“phan­tom” tem­per­a­ture fluc­tu­a­tions that turned out to be a ter­mi­nal screw that was half a turn loose.

Our Transmitter Range

Our trans­mit­ter range pairs with our RTD and ther­mo­cou­ple assemblies.

The HBS4100 is our ded­i­cat­ed RTD trans­mit­ter. If you’re using Pt100 sen­sors and don’t need haz­ardous area cer­ti­fi­ca­tion, this is the straight­for­ward, cost-effec­tive choice. It does one job and does it well.

The HBS4200 is the ther­mo­cou­ple equiv­a­lent — accepts all stan­dard ther­mo­cou­ple types and con­verts to 4 – 20mA. Again, no Ex approvals, so it’s suit­ed to gen­er­al indus­tri­al environments.

The HBS4400 accepts RTD and ther­mo­cou­ple inputs and pro­vides a 4 – 20 mA out­put. It is cer­ti­fied for intrin­si­cal­ly safe (Ex ia) appli­ca­tions under ATEX, IECEx and UKEX. Select and install it in accor­dance with the applic­a­ble cer­tifi­cate, prod­uct mark­ing and instal­la­tion instruc­tions, includ­ing the require­ments for an intrin­si­cal­ly safe sup­ply and suit­able enclosure.

The HBS4600 accepts RTD and ther­mo­cou­ple inputs and adds HART com­mu­ni­ca­tion to the 4 – 20 mA out­put. Like the HBS4400, it holds ATEX, IECEx and UKEX Ex ia cer­ti­fi­ca­tion. Haz­ardous-area suit­abil­i­ty depends on the com­plete instal­la­tion and the applic­a­ble cer­tifi­cate con­di­tions. We sup­ply the full cer­ti­fi­ca­tion doc­u­men­ta­tion with every unit.

And then there’s the HBS5600, which is a bit dif­fer­ent. It’s a com­bined trans­mit­ter and indi­ca­tor — so as well as out­putting a 4 – 20mA sig­nal, it has an inte­gral dis­play that shows the live tem­per­a­ture read­ing right at the sen­sor loca­tion. That’s use­ful for oper­a­tors doing local checks, or any­where you want a visu­al con­fir­ma­tion of process tem­per­a­ture with­out going back to the con­trol room. The HBS5600 also car­ries Ex approvals for haz­ardous areas.

Common Questions

Do I need a trans­mit­ter for every sen­sor?
Not nec­es­sar­i­ly. If your cable run is short (under about 5 metres), your envi­ron­ment is elec­tri­cal­ly qui­et, and your con­trol sys­tem can accept the raw sen­sor input direct­ly, you might not need one. But for any seri­ous indus­tri­al instal­la­tion, a trans­mit­ter pays for itself in sig­nal reliability.

Can I use a trans­mit­ter from one man­u­fac­tur­er with a sen­sor from anoth­er?
Yes — a Pt100 is a Pt100 regard­less of who made it, and the same applies to stan­dard ther­mo­cou­ple types. That said, when we sup­ply matched sen­sor-and-trans­mit­ter assem­blies, we cal­i­brate them togeth­er, which gives you the best pos­si­ble accu­ra­cy out of the box.

My read­ings are noisy — is the trans­mit­ter faulty?
Prob­a­bly not. Check the cable rout­ing first, then the ter­mi­na­tions, then the ground­ing. In our expe­ri­ence, the vast major­i­ty of ​“faulty trans­mit­ter” calls turn out to be instal­la­tion issues. If you’re stuck, call us and we’ll walk you through the diagnostics.

Which trans­mit­ter do I need for a haz­ardous area?
The HBS4400 and HBS4600 are Ex ia trans­mit­ters with ATEX, IECEx and UKEX cer­ti­fi­ca­tion; the HBS4600 adds HART com­mu­ni­ca­tion. The HBS5600 com­bines a trans­mit­ter with a local dis­play and is also avail­able with haz­ardous-area approvals. Select the mod­el and cer­ti­fied con­fig­u­ra­tion for the required input, com­mu­ni­ca­tion and dis­play, then check the prod­uct mark­ing, cer­tifi­cate con­di­tions and com­plete installation.

The Practical Summary

A tem­per­a­ture trans­mit­ter con­verts a frag­ile sen­sor sig­nal into some­thing your con­trol sys­tem can rely on. Head-mount­ing it right at the sen­sor elim­i­nates the biggest source of sig­nal noise — long, unpro­tect­ed cable runs — and gives you a clean 4 – 20mA out­put that’ll trav­el reli­ably back to your con­trol room. Choose the right mod­el for your sen­sor type and envi­ron­ment, run shield­ed cable with prop­er ground­ing, and keep sig­nal wiring away from pow­er cables.

If you’re not sure which trans­mit­ter suits your appli­ca­tion, or you’re chas­ing a noise prob­lem you can’t pin down, get in touch. We deal with this stuff every day and we’re hap­py to help — give us a call or send us a mes­sage.